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See detailUpscaling or a local model in a regional-scale model
Vandenbulcke, Luc; Barth, Alexander

in Ocean Science (2019), 15(2), 291-305

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See detailThe AlborEX dataset: sampling of sub-mesoscale features in the Alboran Sea
Troupin, Charles; Pascual, Ananda; Ruiz, Simon; Olita, Antonio; Casas, Benjamin; Margirier, Félix; Poulain, Pierre-Marie; Notarstefano, Giulio; Torner, Marc; Fernández, Juan Gabriel; Rújula, Miquel Àngel; Muñoz, Cristian; Alou, Eva; Ruiz, Inmaculada; Tovar-Sánchez, Antonio; Allen, John T.; Mahadevan, Amala; Tintoré, Joaquín

in Earth System Science Data (2019), 11(1), 129145

The AlborEX (Alboran Sea Experiment) consisted of a multi-platform, multi-disciplinary experiment carried out in the Alboran Sea (western Mediterranean Sea) between 25 and 31 May 2014. The observational component of AlborEx aimed to sample the physical and biogeochemical properties of oceanographic features present along an intense frontal zone, with a particular interest in the vertical motions in its vicinity. To this end, the mission included 1 research vessel (66 profiles), 2 underwater gliders (adding up 552 profiles), 3 profiling floats, and 25 surface drifters. Near real-time ADCP velocities were collected nightly and during the CTD sections. All of the profiling floats acquired temperature and conductivity profiles, while the Provor-bio float also measured oxygen and chlorophyll a concentrations, coloured dissolved organic matter, backscattering at 700nm, downwelling irradiance at 380, 410, and 490nm, as well as photo-synthetically active radiation (PAR). In the context of mesoscale and sub-mesoscale interactions, the AlborEX dataset constitutes a particularly valuable source of information to infer mechanisms, evaluate vertical transport, and establish relationships between the thermal and haline structures and the biogeochemical variable evolution, in a region characterised by strong horizontal gradients provoked by the confluence of Atlantic and Mediterranean waters, thanks to its multi-platform, multi-disciplinary nature. The dataset presented in this paper can be used for the validation of high-resolution numerical models or for data assimilation experiment, thanks to the various scales of processes sampled during the cruise. All the data files that make up the dataset are available in the SOCIB data catalog at https://doi.org/10.25704/z5y2-qpye (Pascual et al., 2018). The nutrient concentrations are available at https://repository.socib.es:8643/repository/entry/show?entryid=07ebf505-bd27-4ae5-aa43-c4d1c85dd500 (last access: 24 December 2018).

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See detailMediterranean Sea climatic indices: Monitoring long-term variability and climate changes
Iona, A.; Theodorou, A.; Sofianos, S.; Watelet, Sylvain; Troupin, Charles; Beckers, Jean-Marie

in Earth System Science Data (2018), 10(4), 1829-1842

We present a new product composed of a set of thermohaline climatic indices from 1950 to 2015 for the Mediterranean Sea such as decadal temperature and salinity anomalies, their mean values over selected depths, decadal ocean heat and salt content anomalies at selected depth layers as well as their long time series. It is produced from a new high-resolution climatology of temperature and salinity on a 1=8 regular grid based on historical high-quality in situ observations. Ocean heat and salt content differences between 1980-2015 and 1950-1979 are compared for evaluation of the climate shift in the Mediterranean Sea. The two successive periods are chosen according to the standard WMO climate normals. The spatial patterns of heat and salt content shifts demonstrate that the climate changes differently in the several regions of the basin. Long time series of heat and salt content for the period 1950 to 2015 are also provided which indicate that in the Mediterranean Sea there is a net mean volume warming and salinification since 1950 that has accelerated during the last two decades. The time series also show that the ocean heat content seems to fluctuate on a cycle of about 40 years and seems to follow the Atlantic Multidecadal Oscillation climate cycle, indicating that the natural large-scale atmospheric variability could be superimposed onto the warming trend. This product is an observation-based estimation of the Mediterranean climatic indices. It relies solely on spatially interpolated data produced from in situ observations averaged over decades in order to smooth the decadal variability and reveal the long-term trends. It can provide a valuable contribution to the modellers' community, next to the satellite-based products, and serve as a baseline for the evaluation of climate-change model simulations, thus contributing to a better understanding of the complex response of the Mediterranean Sea to the ongoing global climate change. The product is available in netCDF at the following sources: annual and seasonal T =S anomalies (https://doi.org/10.5281/zenodo.1408832), annual and seasonal T =S vertical averaged anomalies (https://doi.org/10.5281/zenodo.1408929), annual and seasonal areal density of OHC/OSC anomalies (https://doi.org/10.5281/zenodo.1408877), annual and seasonal linear trends of T =S, OHC/OSC anomalies (https://doi.org/10.5281/zenodo.1408917), annual and seasonal time series of T =S, OHC/OSC anomalies (https://doi.org/10.5281/zenodo.1411398), and differences of two 30-year averages of annual and seasonal T =S, OHC/OSC anomalies (https://doi.org/10.5281/zenodo.1408903). © Author(s) 2018.

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See detailThe AlborEX dataset: sampling of submesoscale features in the Alboran Sea
Troupin, Charles; Pascual, Ananda; Ruiz, Simón; Olita, Antonio; Casas, Benjamín; Margirier, Félix; Poulain, Pierre-Marie; Notarstefano, Giulio; Torner, Marc; Fernández, Juan Gabriel; Rújula, Miquel Àngel; Muñoz, Cristian; Allen, John T.; Mahadevan, Amala; Tintoré, Joaquín

in Earth System Science Data Discussions (2018)

AlborEX (Alboran Sea Experiment) consisted of a multi-platform, multi-disciplinary experiment carried out in the Alboran Sea (Western Mediterranean Sea) between May 25 and 31, 2014. The observational component of AlborEx aimed to sample the physical and biogeochemical properties of oceanographic features present along an intense frontal zone, with a particular interest in the vertical motions in its vicinity. To this end, the mission included 1 research vessel (66 profiles), 2 underwater gliders (adding up 554 profiles), 3 profiling floats and 25 surface drifters. Near real-time ADCP velocities were collected nightly and during the CTD sections. All of the profiling floats acquired temperature and conductivity profiles, while the Provor-bio float also measured oxygen and chlorophyll-a concentrations, colored dissolved organic matter, backscattering at 700 nm, downwelling irradiance at 380, 410, 490 nm, and photo-synthetically active radiation (PAR). In the context of mesoscale and submesoscale interactions, the AlborEX dataset constitutes a particularly valuable source of information to infer mechanisms, evaluate vertical transport and establish relationships between the thermal and haline structures and the biogeochemical variable evolution, in a region characterised by strong horizontal gradients provoked by the confluence of Atlantic and Mediterranean Waters, thanks to its multi-platform, multi-disciplinary nature. The most recent version of the dataset is available at http://doi.org/10.5281/zenodo.1328238.

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See detailMediterranean Sea Hydrographic Atlas: towards optimal data analysis by including time-dependent statistical parameters
Iona, Athanasia; Theodorou, Athanasios; Watelet, Sylvain; Troupin, Charles; Beckers, Jean-Marie; Simoncelli, Simona

in Earth System Science Data (2018), 10(3), 1281-1300

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See detailThe 48th Liege colloquium: submesoscale processes: mechanisms, implications, and new frontiers
Barth, Alexander; Mahadevan, Amala; Pascual, Ananda; Ruiz, Simón; Troupin, Charles

in Ocean Dynamics (2018), 68(8), 1067-1069

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See detailState-of-the-art stochastic data assimilation methods for high-dimensional non-Gaussian problems
Vetra-Carvalho, Sanita; van Leeuwen, Peter Jan; Nerger, Lars; Barth, Alexander; Altaf, M. Umer; Brasseur, Pierre; Kirchgessner, Paul; Beckers, Jean-Marie

in Tellus. Series A (2018), 70

This paper compares several commonly used state-of-the-art ensemble-based data assimilation methods in a coherent mathematical notation. The study encompasses different methods that are applicable to high-dimensional geophysical systems, like ocean and atmosphere and provide an uncertainty estimate. Most variants of Ensemble Kalman Filters, Particle Filters and second-order exact methods are discussed, including Gaussian Mixture Filters, while methods that require an adjoint model or a tangent linear formulation of the model are excluded. The detailed description of all the methods in a mathematically coherent way provides both novices and experienced researchers with a unique overview and new insight in the workings and relative advantages of each method, theoretically and algorithmically, even leading to new filters. Furthermore, the practical implementation details of all ensemble and particle filter methods are discussed to show similarities and differences in the filters aiding the users in what to use when. Finally, pseudo-codes are provided for all of the methods presented in this paper.

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See detailImproving SMOS Sea Surface Salinity in the Western Mediterranean Sea through Multivariate and Multifractal Analysis
Olmedo, Estrella; Taupier-Letage, Isabelle; Turiel, Antonio; Alvera Azcarate, Aida

in Remote Sensing (2018), 10(3), 485

A new methodology using a combination of debiased non-Bayesian retrieval, DINEOF (Data Interpolating Empirical Orthogonal Functions) and multifractal fusion has been used to obtain Soil Moisture and Ocean Salinity (SMOS) Sea Surface Salinity (SSS) fields over the North Atlantic Ocean and the Mediterranean Sea. The debiased non-Bayesian retrieval mitigates the systematic errors produced by the contamination of the land over the sea. In addition, this retrieval improves the coverage by means of multiyear statistical filtering criteria. This methodology allows obtaining SMOS SSS fields in the Mediterranean Sea. However, the resulting SSS suffers from a seasonal (and other time-dependent) bias. This time-dependent bias has been characterized by means of specific Empirical Orthogonal Functions (EOFs). Finally, high resolution Sea Surface Temperature (OSTIA SST) maps have been used for improving the spatial and temporal resolution of the SMOS SSS maps. The presented methodology practically reduces the error of the SMOS SSS in the Mediterranean Sea by half. As a result, the SSS dynamics described by the new SMOS maps in the Algerian Basin and the Balearic Front agrees with the one described by in situ SSS, and the mesoscale structures described by SMOS in the Alboran Sea and in the Gulf of Lion coincide with the ones described by the high resolution remotely-sensed SST images (AVHRR).

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See detailEMODnet Chemistry Spatial Data Infrastructure for marine observations and related information
Giorgetti, A.; Partescano, E.; Barth, Alexander; Buga, L.; Gatti, J.; Giorgi, G.; Iona, A.; Lipizer, M.; Holdsworth, N.; Larsen, M. M.; Schaap, D.; Vinci, M.; Wenzer, M.

in Ocean and Coastal Management (2018)

Scientific research as well as management of the marine environment, and sustainable blue growth are based on the availability of quality-assured observations, reliable data and solid scientific-based information. These represent three consecutive steps of Data-Information-Knowledge-Wisdom paradigm on the pyramid of wisdom, providing different layers of information. EMODnet (European Marine Observation and Data network) is one of the key infrastructures engaged in collecting, facilitating access and promoting use and re-use of marine observation and data products for both scientific research and marine environmental management. Its Spatial Data Infrastructure (SDI) represents a powerful mechanism to support the implementation of the Marine Strategy Framework Directive Article 19.3 in accordance with the INSPIRE Directive standards and implementing rules. Standardized, harmonized and validated chemical data collections are made available for water quality evaluation at a regional scale, establishing interoperability between the data sets from the many different providers (more than 60 in EMODnet Chemistry). Concentration maps of nutrients, chlorophyll-a and dissolved oxygen are computed on a standard grid, providing information at a regular time interval, per season and over several vertical layers, including the deepest one. Dedicated Open Geospatial Consortium standard services for browsing, viewing and downloading chemistry observation data and data products for the European waters have been developed, and are actively maintained and monitored. These results can provide knowledge layers and can also answer the needs of the directive on Maritime Spatial Planning (EU, 2014), which requires the integration of multidisciplinary data and information on the state of the marine environment with maritime and human activities. © 2018 Elsevier Ltd.

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See detailReconstruction of the Gulf Stream from 1940 to the present and correlation with the North Atlantic Oscillation
Watelet, Sylvain; Beckers, Jean-Marie; Barth, Alexander

in Journal of Physical Oceanography (2017)

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See detailA Multiplatform Experiment to Unravel Meso- and Submesoscale Processes in an Intense Front (AlborEx)
Pascual, Ananda; Ruiz, Simon; Olita, Antonio; Troupin, Charles; Claret, Mariona; Casa, Benjamin; Mourre, Baptiste; Poulain, Pierre-Marie; Tovar-Sanchez, Antonio; Capet, Arthur; Mason, Evan; Allen, John; Mahadevan, Amala; Tintoré, Joaqin

in Frontiers in Marine Science (2017)

The challenges associated with meso- and submesoscale variability (between 1 and 100 km) require high-resolution observations and integrated approaches. Here we describe a major oceanographic experiment designed to capture the intense but transient vertical motions in an area characterized by strong fronts. Finescale processes were studied in the eastern Alboran Sea (Western Mediterranean) about 400 km east of the Strait of Gibraltar, a relatively sparsely sampled area. In-situ systems were coordinated with satellite data and numerical simulations to provide a full description of the physical and biogeochemical variability. Hydrographic data confirmed the presence of an intense salinity front formed by the confluence of Atlantic Waters, entering from Gibraltar, with the local Mediterranean waters. The drifters coherently followed the northeastern limb of an anticyclonic gyre. Near real time data from acoustic current meter data profiler showed consistent patterns with currents of up to 1 m/s in the southern part of the sampled domain. High-resolution glider data revealed submesoscale structures with tongues of chlorophyll-a and oxygen associated with the frontal zone. Numerical results show large vertical excursions of tracers that could explain the subducted tongues and filaments captured by ocean gliders. A unique aspect of AlborEx is the combination of high-resolution synoptic measurements of vessel-based measurements, autonomous sampling, remote sensing and modeling, enabling the evaluation of the underlying mechanisms responsible for the observed distributions and biogeochemical patchiness. The main findings point to the importance of fine-scale processes enhancing the vertical exchanges between the upper ocean and the ocean interior.

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See detailNumerical study of Balearic meteotsunami generation and propagation under synthetic gravity wave forcing
Matjaž, Ličer; Mourre, Baptiste; Troupin, Charles; Krietemeyer, Andreas; Jansá, Agusti; Tintoré, Joaquín

in Ocean Modelling (2017), 111

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See detailObservations of asymmetric turbulent stirring in inner and marginal seas using satellite imagery
Karimova, Svetlana

in International Journal of Remote Sensing (2017), 38(6), 1642-1664

For the open ocean, it was already reported on the cyclonic (anticyclonic) asymmetry of appearance of eddies of a certain spatial scale. In this article, we scrutinize the ratios of mostly mesoscale cyclonic and anticyclonic eddies in a number of inner and marginal marine basins, namely in the Baltic, Black, North, and Western Mediterranean Seas. As research material, over 9700 eddy manifestations in the thermal infrared, visible-range, and radar satellite imagery were used. The analysis performed showed that in all the seas the typical and average values of the diameter of anticyclonic eddies were greater than those of cyclonic eddies. The main factor that defines the ratio between anticyclonic and cyclonic eddies in the basins under consideration was discovered to be the intensity of their surface currents. Thus, in the presence of a strong jet flow at the scales of about 2–4 baroclinic Rossby radii the cyclonic eddy dominance typical for smaller eddies was replaced by the anticyclonic one. If strong jet streams were missing, as that typical of the Baltic Sea, cyclonic eddies were prevailing over the entire spectrum of eddy diameters.

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See detailPoorly ventilated deep ocean at the Last Glacial Maximum inferred from carbon isotopes: a data-model comparison study
Menviel, L.; Yu, J.; Joos, F.; Mouchet, Anne; Meissner, K. J.; England, M. H.

in Paleoceanography (2017)

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See detailBiogeochemical protocols and diagnostics for the CMIP6 Ocean Model Intercomparison Project (OMIP)
Orr, J. C.; Najjar, R. G.; Aumont, O.; Bopp, L.; Bullister, J. L.; Danabasoglu, G.; Doney, S. C.; Dunne, J. P.; Dutay, J.-C.; Graven, H.; Griffies, S. M.; John, J. G.; Joos, F.; Levin, I.; Lindsay, K.; Matear, R. J.; McKinley, G. A.; Mouchet, Anne; Oschlies, A.; Romanou, A.; Schlitzer, R.; Tagliabue, A.; Tanhua, T.; Yool, A.

in Geoscientific Model Development (2017), 10(6), 2169-2199

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See detailHigh-resolution regional modelling of natural and anthropogenic radiocarbon in the Mediterranean Sea
Ayache, M.; Dutay, J.-C.; Mouchet, Anne; Tisnérat-Laborde, N.; Montagna, P.; Tanhua, T.; Siani, G.; Jean-Baptiste, P.

in Biogeosciences (2017), 14(5), 1197-1213

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See detailComparison of different incremental analysis update schemes in a realistic assimilation system with Ensemble Kalman Filter
Yan, Yajing; Barth, Alexander; Beckers, Jean-Marie; Brankart, J. M.; Brasseur, P.; Candille, G.

in Ocean Modelling (2017), 115

In this paper, three incremental analysis update schemes (IAU 0, IAU 50 and IAU 100) are compared in the same assimilation experiments with a realistic eddy permitting primitive equation model of the North Atlantic Ocean using the Ensemble Kalman Filter. The difference between the three IAU schemes lies on the position of the increment update window. The relevance of each IAU scheme is evaluated through analyses on both thermohaline and dynamical variables. The validation of the assimilation results is performed according to both deterministic and probabilistic metrics against different sources of observations. For deterministic validation, the ensemble mean and the ensemble spread are compared to the observations. For probabilistic validation, the continuous ranked probability score (CRPS) is used to evaluate the ensemble forecast system according to reliability and resolution. The reliability is further decomposed into bias and dispersion by the reduced centred random variable (RCRV) score. The obtained results show that 1) the IAU 50 scheme has the same performance as the IAU 100 scheme 2) the IAU 50/100 schemes outperform the IAU 0 scheme in error covariance propagation for thermohaline variables in relatively stable region, while the IAU 0 scheme outperforms the IAU 50/100 schemes in dynamical variables estimation in dynamically active region 3) in case with sufficient number of observations and good error specification, the impact of IAU schemes is negligible. The differences between the IAU 0 scheme and the IAU 50/100 schemes are mainly due to different model integration time and different instability (density inversion, large vertical velocity, etc.) induced by the increment update. The longer model integration time with the IAU 50/100 schemes, especially the free model integration, on one hand, allows for better re-establishment of the equilibrium model state, on the other hand, smooths the strong gradients in dynamically active region. © 2017 Elsevier Ltd

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See detailCorrecting circulation biases in a lower-resolution global general circulation model with data assimilation
Canter, Martin; Barth, Alexander; Beckers, Jean-Marie

in Ocean Dynamics (2016)

In this study, we aim at developing a new method of bias correction using data assimilation. This method is based on the stochastic forcing of a model to correct bias by directly adding an additional source term into the model equations. This method is presented and tested first with a twin experiment on a fully controlled Lorenz ’96 model. It is then applied to the lower-resolution global circulation NEMO-LIM2 model, with both a twin experiment and a real case experiment. Sea surface height observations are used to create a forcing to correct the poorly located and estimated currents. Validation is then performed throughout the use of other variables such as sea surface temperature and salinity. Results show that the method is able to consistently correct part of the model bias. The bias correction term is presented and is consistent with the limitations of the global circulation model causing bias on the oceanic currents.

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See detailCorrection of inertial oscillations by assimilation of HFradar data in a model of the Ligurian Sea
Vandenbulcke, Luc; Barth, Alexander; Beckers, Jean-Marie

in Ocean Dynamics (2016)

This article aims at analyzing if high-frequency radar observations of surface currents allow to improve model forecasts in the Ligurian Sea, where inertial oscillations are a dominant feature. An ensemble of ROMS models covering the Ligurian Sea, and nested in the Mediterranean Forecasting System, is coupled with two WERA high-frequency radars. A sensitivity study allows to determine optimal parameters for the ensemble filter. By assimilating observations in a single point, the obtained correction shows that the forecast error covariance matrix represents the inertial oscillations, as well as large- and meso-scale processes. Furthermore, it is shown that the velocity observations can correct the phase and amplitude of the inertial oscillations. Observations are shown to have a strong effect during approximately half a day, which confirms the importance of using a high temporal observation frequency. In general, data assimilation of HF radar observations leads to a skill score of about 30 % for the forecasts of surface velocity.

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See detailA new global interior ocean mapped climatology: the 1° × 1° GLODAP version 2
Lauvset, Siv K.; Key, Robert M.; Olsen, Are; van Heuven, Stephen; Velo, Anton; Xiaohua, Lin; Schirnick, Carsten; Kozyr, Alex; Tanhua, Toste; Hoppema, Mario; Jutterström, Sara; Steinfeldt, Reiner; Jeansson, Emil; Ishii, Masao; Perez, Fiz F.; Suzuki, Toru; Watelet, Sylvain

in Earth System Science Data (2016)

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See detailAnalysis of SMOS sea surface salinity data using DINEOF
Alvera Azcarate, Aïda; Barth, Alexander; Parard, Gaëlle; Beckers, Jean-Marie

in Remote Sensing of Environment (2016), 180

n analysis of daily Sea Surface Salinity (SSS) at 0.15 ° × 0.15° spatial resolution from the Soil Moisture and Ocean Salinity (SMOS) satellite mission using DINEOF (Data Interpolating Empirical Orthogonal Functions) is presented. DINEOF allows reconstructing missing data using a truncated EOF basis, while reducing the amount of noise and errors in geophysical datasets. This work represents a first application of DINEOF to SMOS SSS. Results show that a reduction of the error and the amount of noise is obtained in the DINEOF SSS data compared to the initial SMOS SSS data. Errors associated to the edge of the swath are detected in 2 EOFs and effectively removed from the final data, avoiding removing the data at the edges of the swath in the initial dataset. The final dataset presents a centered root mean square error of 0.2 in open waters when comparing with thermosalinograph data at their original spatial and temporal resolution. Constant biases present near land masses, large scale biases and latitudinal biases cannot be corrected with DINEOF because persistent signals are retained in high order EOFs, and therefore these need to be corrected separately. The signature of the Douro and Gironde rivers is detected in the DINEOF SSS. The minimum SSS observed in the Gironde plume corresponds to a flood event in June 2013, and the shape and size of the Douro river shows a good agreement with chlorophyll-a satellite data. These examples show the capacity of DINEOF to remove noise and provide a full SSS dataset at a high temporal and spatial resolution with reduced error, and the possibility to retrieve physical signals in zones with high initial errors.

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See detailReconstruction and analysis of long-term satellite-derived sea surface temperature for the South China Sea
Huynh, Thi Hong Ngu; Alvera Azcarate, Aïda; Barth, Alexander; Beckers, Jean-Marie

in Journal of Oceanography (2016)

Sea surface temperature (SST) is one of the key variables often used to investigate ocean dynamics, ocean-atmosphere interaction, and climate change. Unfortunately, the SST data sources in the South China Sea (SCS) are not abundant due to sparse measurements of in situ SST and a high percentage of missing data in the satellite-derived SST. Therefore, SST data sets with low resolution and/or a short-term period have often been used in previous researches. Here we used Data INterpolating Empirical Orthogonal Functions, a self-consistent and parameter-free method for filling in missing data, to reconstruct the daily nighttime 4-km AVHRR Pathfinder SST for the long-term period spanning from 1989 to 2009. In addition to the reconstructed field, we also estimated the local error map for each reconstructed image. Comparisons between the reconstructed and other data sets (satellite-derived microwave and in situ SSTs) show that the results are reliable for use in many different researches, such as validating numerical models, or identifying and tracking meso-scale oceanic features. Moreover, the Empirical Orthogonal Function (EOF) analysis of the reconstructed SST and the reconstructed SST anomalies clearly shows the subseasonal, seasonal, and interannual variability of SST under the influence of monsoon and El Niño-Southern Oscillation (ENSO), as well as reveals some oceanic features that could not be captured well in previous EOF analyses. The SCS SST often lags ENSO by about half a year. However, in this study, we see that the time lag changes with the frequencies of the SST variability, from 1 to 6 months.

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See detailLocal ensemble assimilation scheme with global constraints and conservation
Barth, Alexander; Yan, Yajing; Alvera Azcarate, Aida; Beckers, Jean-Marie

in Ocean Dynamics (2016), 66

Ensemble assimilation schemes applied in their original, global formulation respect linear conservation properties if the ensemble perturbations are set up accordingly. For realistic ocean systems, only a relatively small number of ensemble members can be calculated. A localization of the ensemble increment is therefore necessary to filter out spurious long-range correlations. The conservation of the global properties will be lost if the assimilation is performed locally, since the conservation requires a coupling between all model grid points which is removed by the localization. The distribution of ocean observations is often highly inhomogeneous. Systematic errors of the observed parts of the ocean state can lead to spurious adjustment of the non-observed parts via data assimilation and thus to a spurious increase or decrease in long-term simulations of global properties which should be conserved. In this paper, we propose a local assimilation scheme (with different variants and assumptions) which can satisfy global conservation properties. The proposed scheme can also be used for non-local observation operators. Different variants of the proposed scheme are tested in an idealized model and compared to the traditional covariance localization with an ad-hoc step enforcing conservation. It is shown that the inclusion of the conservation property reduces the total RMS error and that the presented stochastic and deterministic schemes avoiding error space rotation provide better results than the traditional covariance localization.

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See detailImproved statistics of submesoscale eddies in the Baltic Sea retrieved from SAR imagery
Karimova, Svetlana; Gade, Martin

in International Journal of Remote Sensing (2016), 37(10), 2394-2414

In the present paper the spatio-temporal distribution of submesoscale eddies seen in Envisat Advanced Synthetic Aperture Radar (ASAR) imagery of the Baltic Sea is discussed. A total of 1250 ASAR images acquired between 2009 and 2011 form the basis of our studies and show imprints of almost 7000 submesoscale eddies. Since the visibility of vortical structures in synthetic aperture radar (SAR) imagery significantly depends on the near-surface wind speed, wind data from a numerical model of the Baltic Sea were additionally used to get improved eddy statistics. Seasonally averaged fields of near-surface wind speed, surface currents, sea surface temperature (SST), and SST gradient were also analyzed in order to reveal the role of these hydrophysical parameters in the observed spatial and temporal variation of submesoscale eddies.

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See detailDecline of the Black Sea oxygen inventory
Capet, Arthur; Stanev, Emil; Beckers, Jean-Marie; Murray, Jim; Grégoire, Marilaure

in Biogeosciences (2016), 13

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See detailA stochastic operational forecasting system of the Black Sea: Technique and validation
Vandenbulcke, Luc; Barth, Alexander

in Ocean Modelling (2015), 93

In this article, we present the latest version of an ensemble forecasting system of the hydrodynamics of the Black Sea, based on the GHER model. The system includes the Weakly Constrained Ensembles algorithm to generate random, but physically balanced perturbations to initialize members of the ensemble. On top of initial conditions, the ensemble accounts also for uncertainty on the atmospheric forcing fields, and on some scalar parameters such as river flows or model diffusion coefficients. The forecasting system also includes the Ocean Assimilation Kit, a sequential data assimilation package implementing the SEEK and Ensemble Kalman filters. A novel aspect of the forecasting system is that not only our best estimate of the future ocean state is provided, but also the associated error estimated from the ensemble of models. The primary goal of this paper is to quantitatively show that the ensemble variability is a good estimation of the model error, regardless of the magnitude of the forecast errors themselves. In order for this estimation to be meaningful, the model itself should also be well validated. Therefore, we describe the model validation against general circulation patterns. Some particular aspects critical for the Black Sea circulation are validated as well: the mixed layer depth and the shelfopen sea exchanges. The model forecasts are also compared with observed sea surface temperature, and errors are compared to those of another operational model as well.

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See detailAssimilation of sea surface temperature, sea ice concentration and sea ice drift in a model of the Southern Ocean
Barth, Alexander; Canter, Martin; Van Schaeybroeck, Bert; Vannitsem, Stéphane; Massonnet, François; Zunz, Violette; Mathiot, Pierre; Alvera Azcarate, Aïda; Beckers, Jean-Marie

in Ocean Modelling (2015), 93

Current ocean models have relatively large errors and biases in the Southern Ocean. The aim of this study is to provide a reanalysis from 1985 to 2006 assimilating sea surface temperature, sea ice concentration and sea ice drift. In the following it is also shown how surface winds in the Southern Ocean can be improved using sea ice drift estimated from infrared radiometers. Such satellite observations are available since the late seventies and have the potential to improve the wind forcing before more direct measurements of winds over the ocean are available using scatterometry in the late nineties. The model results are compared to the assimilated data and to independent measurements (the World Ocean Database 2009 and the mean dynamic topography based on observations). The overall improvement of the assimilation is quantified, in particular the impact of the assimilation on the representation of the polar front is discussed. Finally a method to identify model errors in the Antarctic sea ice area is proposed based on Model Output Statistics techniques using a series of potential predictors. This approach provides new directions for model improvements.

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See detailEnsemble assimilation of ARGO temperature profile, sea surface temperature, and altimetric satellite data into an eddy permitting primitive equation model of the North Atlantic Ocean
Yan, Yajing; Barth, Alexander; Beckers, Jean-Marie; Candille, Guillem; Brankart, Jean-Michel; Brasseur, Pierre

in Journal of Geophysical Research. Oceans (2015), 120

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See detailCoastal Ocean Forecasting: science foundation and user benefits
Kourafalou, V. H.; Kourafalou, P.; Staneva, J.; Ayoub, N.; Barth, Alexander; Chao, Y.; Cirano, M.; Fiechter, J.; Herzfeld, M.; Kurapov, A.; Moore, A. M.; Oddo, P.; Pullen, J.; Van Der Westhuysen, A.; Weisberg, R. H.

in Journal of Operational Oceanography (2015), 8

The advancement of Coastal Ocean Forecasting Systems (COFS) requires the support of continuous scientific progress addressing: (a) the primary mechanisms driving coastal circulation; (b) methods to achieve fully integrated coastal systems (observations and models), that are dynamically embedded in larger scale systems; and (c) methods to adequately represent air-sea and biophysical interactions. Issues of downscaling, data assimilation, atmosphere-wave-ocean couplings and ecosystem dynamics in the coastal ocean are discussed. These science topics are fundamental for successful COFS, which are connected to evolving downstream applications, dictated by the socioeconomic needs of rapidly increasing coastal populations.

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See detailAnalysis of high frequency geostationary ocean colour data using DINEOF
Alvera Azcarate, Aïda; Vanhellemont, Quinten; Ruddick, Kevin; Barth, Alexander; Beckers, Jean-Marie

in Estuarine Coastal and Shelf Science (2015), 159

DINEOF (Data Interpolating Empirical Orthogonal Functions), a technique to reconstruct missing data, is applied to turbidity data obtained through the Spinning Enhanced Visible and Infrared Imager (SEVIRI) on board Meteosat Second Generation 2. The aim of this work is to assess if the tidal variability of the southern North Sea in 2008 can be accurately reproduced in the reconstructed dataset. Such high frequency data have not previously been analysed with DINEOF and present new challenges, like a strong tidal signal and long night-time gaps. An outlier detection approach that exploits the high temporal resolution (15 min) of the SEVIRI dataset is developed. After removal of outliers, the turbidity dataset is reconstructed with DINEOF. In situ Smartbuoy data are used to assess the accuracy of the reconstruction. Then, a series of tidal cycles are examined at various positions over the southern North Sea. These examples demonstrate the capability of DINEOF to reproduce tidal variability in the reconstructed dataset, and show the high temporal and spatial variability of turbidity in the southern North Sea. An analysis of the main harmonic constituents (annual cycle, daily cycle, M2 and S2 tidal components) is performed, to assess the contribution of each of these modes to the total variability of turbidity. The variability not explained by the harmonic fit, due to the natural processes and satellite processing errors as noise, is also assessed.

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See detailHydrological fronts seen in visible and infrared MODIS imagery of the Black Sea
Karimova, Svetlana

in International Journal of Remote Sensing (2014), 35(16), 6113-6134

In the paper, an examination of hydrological fronts in the Black Sea is presented based on visible and infrared Aqua Moderate Resolution Imaging Spectrometer (MODIS) imagery obtained in 2008-2013. First the images were preliminary treated (e.g., via applying a median filter) and then the fronts were detected by means of the Sobel operator. Further procession of the fields of the gradient norm thus obtained helped to get the generalized schemes of the front density for different years and seasons. On analysis of such schemes it was shown that the strongest thermal and optical fronts of the Black Sea are located in the near-coastal area. Nevertheless, analysis of the front density contrasts in the open sea was discovered to be very helpful as well, because it provided important information on the trajectories of the main macro- and mesoscale circulation patterns in the basin such as the Rim Current, Batumi eddy, and eddies of the Anatolian coast.

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See detailApproximate and Efficient Methods to Assess Error Fields in Spatial Gridding with Data Interpolating Variational Analysis (DIVA)
Beckers, Jean-Marie; Barth, Alexander; Troupin, Charles; Alvera Azcarate, Aïda

in Journal of Atmospheric and Oceanic Technology (2014), 31(2), 515-530

We present new approximate methods to provide error fields for the spatial analysis tool Diva. It is first shown how to replace the costly analysis of a large number of covariance functions by a single analysis for quick error computations. Then another method is presented where the error is only calculated in a small number of locations and from there the spatial error field itself interpolated by the analysis tool. The efficiency of the methods is illustrated on simple schematic test cases and a real application in the Mediterranean Sea. These examples show that with these methods one has the possibility for quick masking of regions void of sufficient data and the production of "exact" error fields at reasonable cost. The error-calculation methods can also be generalized for use with other analysis methods such as 3D-Var and are therefore potentially interesting for other implementations.

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See detailUntangling spatial and temporal trends in the variability of the Black Sea Cold Intermediate Layer and mixed Layer Depth using the DIVA detrending procedure
Capet, Arthur; Troupin, Charles; Cartensen, Jacob; Grégoire, Marilaure; Beckers, Jean-Marie

in Ocean Dynamics (2014), 64(3), 315-324

Current spatial interpolation products may be biased by uneven distribution of measurements in time. This manuscript presents a detrending method that recognizes and eliminates this bias. The method estimates temporal trend components in addition to the spatial structure and has been implemented within the Data Interpolating Variational Analysis (DIVA) analysis tool. The assets of this new detrending method are illustrated by producing monthly and annual climatologies of two vertical properties of the Black Sea while recognizing their seasonal and interannual variabilities : the mixed layer depth, and the cold content of its Cold Intermediate Layer (CIL). The temporal trends, given as by-products of the method, are used to analyze the seasonal and interannual variability of these variables over the past decades (1955-2011). In particular, the CIL interannual variability is related to the cumulated winter air temperature anomalies, explaining 88\% of its variation.

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See detailComparison of different assimilation schemes in a sequential Kalman filter assimilation system
Yan, Yajing; Barth, Alexander; Beckers, Jean-Marie

in Ocean Modelling (2014), 73

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See detailAssimilation of HF radar surface currents to optimize forcing in the northwestern Mediterranean Sea
Marmain, J.; Molcard, A.; Forget, P.; Barth, Alexander; Ourmières, Y.

in Nonlinear Processes in Geophysics (2014), 21

HF radar measurements are used to optimize surface wind forcing and baroclinic open boundary condition forcing in order to constrain model coastal surface currents. This method is applied to a northwestern Mediterranean (NWM) regional primitive equation model configuration. A new radar data set, provided by two radars deployed in the Toulon area (France), is used. To our knowledge, this is the first time that radar measurements of the NWM Sea are assimilated into a circulation model. Special attention has been paid to the improvement of the model coastal current in terms of speed and position. The data assimilation method uses an ensemble Kalman smoother to optimize forcing in order to improve the model trajectory. Twin experiments are initially performed to evaluate the method skills. Real measurements are then fed into the circulation model and significant improvements to the modeled surface currents, when compared to observations, are obtained.

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See detailMulti-scale optimal interpolation: application to DINEOF analysis spiced with a local optimal interpolation
Beckers, Jean-Marie; Barth, Alexander; Tomazic, Igor; Alvera Azcarate, Aïda

in Ocean Science Discussions (2014), 11

We present a method in which the optimal interpolation of multi-scale processes can be untangled into a succession of simpler interpolations. First, we prove how the optimal analysis of a superposition of two processes can be obtained by different mathematical formulations involving iterations and analysis focusing on a single process. From the 5 different mathematical equivalent formulations we then select the most efficient ones by analyzing the behavior of the different possibilities in a simple and well controlled test case. The clear guidelines deduced from this experiment are then applied in a real situation in which we combine large-scale analysis of hourly SEVIRI satellite images using DINEOF with a local optimal interpolation using a Gaussian covariance. It is 10 shown that the optimal combination indeed provides the best reconstruction and can therefore be exploited to extract the maximum amount of useful information from the original data

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See detaildivand-1.0: n-dimensional variational data analysis for ocean observations
Barth, Alexander; Beckers, Jean-Marie; Troupin, Charles; Alvera Azcarate, Aïda; Vandenbulcke, Luc

in Geoscientific Model Development (2014), 7

A tool for multidimensional variational analysis (divand) is presented. It allows the interpolation and analysis of observations on curvilinear orthogonal grids in an arbitrary high dimensional space by minimizing a cost function. This cost function penalizes the deviation from the observations, the deviation from a first guess and abruptly varying fields based on a given correlation length (potentially varying in space and time). Additional constraints can be added to this cost function such as an advection constraint which forces the analysed field to align with the ocean current. The method decouples naturally disconnected areas based on topography and topology. This is useful in oceanography where disconnected water masses often have different physical properties. Individual elements of the a priori and a posteriori error covariance matrix can also be computed, in particular expected error variances of the analysis. A multidimensional approach (as opposed to stacking 2-dimensional analysis) has the benefit of providing a smooth analysis in all dimensions, although the computational cost is increased. Primal (problem solved in the grid space) and dual formulations (problem solved in the observational space) are implemented using either direct solvers (based on Cholesky factorization) or iterative solvers (conjugate gradient method). In most applications the primal formulation with the direct solver is the fastest, especially if an a posteriori error estimate is needed. However, for correlated observation errors the dual formulation with an iterative solver is more efficient. The method is tested by using pseudo observations from a global model. The distribution of the observations is based on the position of the ARGO floats. The benefit of the 3-dimensional analysis (longitude, latitude and time) compared to 2-dimensional analysis (longitude and latitude) and the role of the advection constraint are highlighted. The tool divand is free software, and is distributed under the terms of the GPL license (http://modb.oce.ulg.ac.be/mediawiki/index.php/divand).

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See detailNon-stationary eddies in the Black Sea as seen by satellite infrared and visible imagery
Karimova, Svetlana

in International Journal of Remote Sensing (2013), 34(23), 8503-8517

Examination of non-stationary eddies in the Black Sea is presented based on satellite visible and infrared imagery. The images were obtained by the Advanced Very High Resolution Radiometer (AVHRR) and Moderate Resolution Imaging Spectrometer (MODIS) since September, 2004 to December, 2010. As a result of the analysis performed it was discovered that there are four main non-stationary eddy types which can be frequently observed in such imagery: near-shore anticyclonic eddies, mushroom-like currents, eddies of the Anatolian coast, and eddy chains. For each type of eddies, spatio-temporal parameters were retrieved such as areas of the most frequent generation and typical length scale as well as their seasonality. Every non-stationary eddy type showed to have its own area of location. Analysis of eddy spatial scale revealed that anticyclonic eddies of all types were greater in size than cyclonic ones (in average 52 km for anticyclones and 36 km for cyclones).

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See detailLow contribution of N2 fixation to new production and excess nitrogen in the subtropical northeast Atlantic margin
Benavides, Mar; Arístegui, Javier; Agawin, Nona S. R.; Álvarez-Salgado, Xosé Antón; Álvarez, Marta; Troupin, Charles

in Deep-Sea Research. Part I, Oceanographic Research Papers (2013), 81(0), 36-48

We used 15N-labeled substrates to measure dinitrogen (N2) fixation, nitrate and ammonium uptake, regeneration and associated dissolved organic nitrogen (DON) release in a coastal upwelling system (Cape Ghir, ∼31°N) and an open ocean grid (bounded between 25°–42°N and 20°W) in the Canary Current region during the summer of 2009. New production (Pnew= uptake+N2 fixation+DON released from uptake− regeneration) was higher in the upwelling than in the open ocean zone (0.126 and 0.014 µmol N L−1 h−1, respectively), while regenerated production (Preg= uptake+DON released from uptake+ regeneration) was similar in both zones (0.157 and 0.133 µmol N L−1 h−1, respectively). The resulting f-ratio (Pnew/Pnew+Preg) for the open ocean and upwelling zones was 0.08 and 0.48, respectively. The availability of nitrogen in excess of that expected from Redfield stoichiometry is generally attributed to N2 fixation. A previous study indicated that our open ocean grid zone had an excess nitrogen production rate of 40±22×1010 mol N yr−1. We revisited this budget including new dissolved organic matter and fluxes through the Strait of Gibraltar and estimated a revised nitrogen excess rate of 22±19×1010 mol N yr−1. The average volumetric rate of N2 fixation for this zone was only 1.3×10−3 nmol N L−1 d−1, indicating that its influence in Pnew and nitrogen excess production in this part of the Atlantic is negligible.

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See detailReconstruction of spatiotemporal capture data by means of orthogonal functions: the case of skipjack tuna (Katsuwonus pelamis) in the Central-east Atlantic
Ganzedo, Unai; Erdaide, Oihane; Trujillo-Santana, Aaron; Alvera Azcarate, Aïda; Castro, José J.

in Scientia Marina (2013), 77(4), 575-584

The information provided by the International Commission for the Conservation of Atlantic Tunas (ICCAT) about captures of skipjack tuna (Katsuwonus pelamis) in the Central-east Atlantic has a number of limitations, such as gaps in the statistics for certain fleets or the level of spatiotemporal detail at which catches are reported. As a result, the quality of such data and their effectiveness for providing management advice is limited. In order to reconstruct missing spatial-temporal data of catches, the present study uses Data INterpolating Empirical Orthogonal Functions (DINEOF), a technique for missing data reconstruction applied here for first time to fisheries data. DINEOF is based on an Empirical Orthogonal Functions (EOF) decomposition performed with a Lanczos method. DINEOF was tested with different amounts of missing data, intentionally removing values from 3.4% to 95.2% of data loss, and then compared to the same data set with no missing data. These validation analyses show that DINEOF is a reliable methodological approach of data reconstruction for the purposes of fishery management advice, even when the amount of missing data is very high.

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See detailDrivers, mechanisms and long-term variability of seasonal hypoxia on the Black Sea northwestern shelf – is there any recovery after eutrophication?
Capet, Arthur; Beckers, Jean-Marie; Grégoire, Marilaure

in Biogeosciences (2013), 10

The Black Sea northwestern shelf (NWS) is a shallow eutrophic area in which the seasonal stratification of the water column isolates the bottom waters from the atmosphere. This prevents ventilation from counterbalancing the large consumption of oxygen due to respiration in the bottom waters and in the sediments, and sets the stage for the development of seasonal hypoxia. A three-dimensional (3-D) coupled physical–biogeochemical model is used to investigate the dynamics of bottom hypoxia in the Black Sea NWS, first at seasonal and then at interannual scales (1981–2009), and to differentiate its driving factors (climatic versus eutrophication). Model skills are evaluated by a quantitative comparison of the model results to 14 123 in situ oxygen measurements available in the NOAA World Ocean and the Black Sea Commission databases, using different error metrics. This validation exercise shows that the model is able to represent the seasonal and interannual variability of the oxygen concentration and of the occurrence of hypoxia, as well as the spatial distribution of oxygen-depleted waters. During the period 1981–2009, each year exhibits seasonal bottom hypoxia at the end of summer. This phenomenon essentially covers the northern part of the NWS – which receives large inputs of nutrients from the Danube, Dniester and Dnieper rivers – and extends, during the years of severe hypoxia, towards the Romanian bay of Constanta. An index H which merges the aspects of the spatial and temporal extension of the hypoxic event is proposed to quantify, for each year, the intensity of hypoxia as an environmental stressor. In order to explain the interannual variability of H and to disentangle its drivers, we analyze the long time series of model results by means of a stepwise multiple linear regression. This statistical model gives a general relationship that links the intensity of hypoxia to eutrophication and climate-related variables. A total of 82% of the interannual variability of H is explained by the combination of four predictors: the annual riverine nitrate load (N), the sea surface temperature in the month preceding stratification (Ts), the amount of semi-labile organic matter accumulated in the sediments (C) and the sea surface temperature during late summer (Tf). Partial regression indicates that the climatic impact on hypoxia is almost as important as that of eutrophication. Accumulation of organic matter in the sediments introduces an important inertia in the recovery process after eutrophication, with a typical timescale of 9.3 yr. Seasonal fluctuations and the heterogeneous spatial distribution complicate the monitoring of bottom hypoxia, leading to contradictory conclusions when the interpretation is done from different sets of data. In particular, it appears that the recovery reported in the literature after 1995 was overestimated due to the use of observations concentrated in areas and months not typically affected by hypoxia. This stresses the urgent need for a dedicated monitoring effort in the Black Sea NWS focused on the areas and months concerned by recurrent hypoxic events.

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See detailAssessing the impact of brightness temperature simulation adjustment conditions in correcting Metop-A SST over the Mediterranean Sea
Tomazic, Igor; Roquet, Hervé; Le Borgne, Pierre

in Remote Sensing of Environment (2013)

Multispectral sea surface temperature (SST) algorithms applied to infrared (IR) radiometer data exhibit regional biases due to the intrinsic inability of the SST algorithm to cope with the vast range of atmospheric types, mainly influenced by water vapor and temperature profiles. Deriving a SST correction from simulated brightness temperatures (BTs), obtained by applying a Radiative Transfer Model (RTM) to Numerical Weather Prediction (NWP) atmospheric profiles and first guess SST, is one of the solutions to reduce regional biases. This solution is envisaged in the particular case of Metop-A Advanced Very High Resolution Radiometer (AVHRR) derived SST. Simulated BTs show errors, linked to RTM, atmospheric profiles or guess field errors. We investigated the conditions of adjusting simulated to observed BTs in the particular case of the Mediterranean Sea over almost one year. Our study led to define optimal spatio/temporal averaging parameters of the simulation observation differences, both during day and night, summer and colder season and for two simulation modes: operational (with reduced vertical resolution – 15 levels – NWP atmospheric profiles and two days old analysis used as first guess SST) and delayed (full vertical resolution – 91 levels – and concurrent analysis used as first guess SST). Each BT adjustment has been evaluated by comparing the corresponding corrected AVHRR SST to the AATSR SST that we adopted as validation reference. We obtained an optimized result across all defined conditions and modes for a spatial smoothing of 15 deg and a temporal averaging between 3 and 5 days. Specifically, analyses based on 10 day averages showed that a standard deviation based criterion favors spatial smoothing above 10 deg for all temporal averaging, while a bias based criterion favors shorter temporal averaging during daytime (< 5 days) and higher spatial smoothing (> 10 deg) for nighttime. This study has shown also the impact of diurnal warming both in deriving BT adjustment and in validation results.

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See detailExperimental in situ exposure of the seagrass Posidonia oceanica (L.) Delile to 15 trace elements
Richir, Jonathan; Luy, Nicolas; Lepoint, Gilles; Rozet, Eric; Alvera Azcarate, Aïda; Gobert, Sylvie

in Aquatic Toxicology (2013), 140-141

The Mediterranean seagrass Posidonia oceanica (L.) Delile has been used for trace element (TE) biomonitoring since decades ago. However, present informations for this bioindicator are limited mainly to plant TE levels, while virtually nothing is known about their fluxes through P. oceanica meadows. We therefore contaminated seagrass bed portions in situ at two experimental TE levels with a mix of 15 TEs (Al, V,Cr, Mn, Fe, Co, Ni, Cu, Zn, As, Mo, Ag, Cd, Pb and Bi) to study their uptake and loss kinetics in P. oceanica. Shoots immediately accumulated pollutants from the beginning of exposures. Once contaminations ended, TE concentrations came back to their original levels within two weeks, or at least showed a clear decrease. P. oceanica leaves exhibited different uptake kinetics depending on elements and leaf age: the younger growing leaves forming new tissues incorporated TEs more rapidly than the older senescent leaves. Leaf epiphytes also exhibited a net uptake of most TEs, partly similar to that of P. oceanica shoots. The principal route of TE uptake was through the water column, as no contamination of superficial sediments was observed. However, rhizomes indirectly accumulated many TEs during the overall experiments through leaf to rhizome translocation processes. This study thus experimentally confirmed that P.oceanica shoots are undoubtedly an excellent short-term bioindicator and that long-term accumulations could be recorded in P. oceanica rhizomes.

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See detailObservation of eddy structures in the Baltic Sea with the use of radiolocation and radiometric satellite data
Karimova, Svetlana; Lavrova, Olga; Solov'ev, Dmitriy

in Izvestiya, Atmospheric and Oceanic Physics (2012), 48(9), 10061013

The manifestations of mesoscale and submesoscale eddy elements in surface currents of the Baltic Sea in satellite images differing in physical nature and spatial resolution are considered. The investigation is based on the Envisat ASAR and ERS-2 SAR high-resolution radiolocation images (RLIs) obtained in 2008–2009 for different sites of the Baltic Sea water area and the use of the Envisat MERIS and Landsat ETM+ radiometric images of the visible spectrum. Possible mechanisms of the appearance of eddy structures in RLIs of the Baltic Sea water area are considered. Joint analysis of the mentioned data revealed specific features of the appearance of eddy structures in satellite images, taking into account the variability of optical characteristics of Baltic Sea surface waters during the summer blooming of cyanobacteria and the spring blooming of diatomic algae.

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See detailSpiral eddies in the Baltic, Black and Caspian seas as seen by satellite radar data
Karimova, Svetlana

in Advances in Space Research (2012), 50(8), 1107-1124

Despite spiral eddies were first seen on the sea surface more than 40 years ago, there is still a lot of uncertainty concerning these eddies. The present paper is aimed to provide the comprehensive results on the occurrence and statistics of small-scale eddies in the three inner seas (the Baltic, Black and Caspian) using satellite synthetic aperture radar (SAR) images. The dataset used includes over 2,000 medium resolution Envisat ASAR and ERS-2 SAR images obtained in 2009-2010 in the different parts of the seas mentioned. As a result of the analysis performed ~14,000 vortical structures were detected. 71% of them were visualized due to surfactant films (“black” eddies), while 29% due to wave/current interactions (“white” eddies). Practically all eddies detected were cyclonically rotating. Their diameter was within 1-20 km. Characteristic size of the “black” eddies in all the basins was discovered to be less than that of the “white” eddies. Characteristic eddy size for the Baltic, Black and Caspian seas proved to be strictly proportional to the values of the baroclinic Rossby radius of deformation typical for these basins. The “black” eddies did not demonstrate a significant connection with the basin- and meso-scale surface circulation of the seas. Most of the “white” eddies detected were attributed to the zones with the most intense drift currents, i.e. those along the western boundaries and (in the Baltic Sea only) in the elongated parts of the basin.

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See detailGeneration of analysis and consistent error fields using the Data Interpolating Variational Analysis (Diva)
Troupin, Charles; Barth, Alexander; Sirjacobs, Damien; Ouberdous, Mohamed; Brankart, Jean-Michel; Brasseur, Pierre; Rixen, Michel; Alvera Azcarate, Aïda; Belounis, Mahdia; Capet, Arthur; Lenartz, Fabian; Toussaint, Marie-Eve; Beckers, Jean-Marie

in Ocean Modelling (2012), 52-53

The Data Interpolating Variational Analysis (Diva) is a method designed to interpolate irregularly-spaced, noisy data onto any desired location, in most cases on regular grids. It is the combination of a particular methodology, based on the minimisation of a cost function, and a numerically efficient method, based on a finite-element solver. The cost function penalises the misfit between the observations and the reconstructed field, as well as the regularity or smoothness of the field. The intrinsic advantages of the method are its natural way to take into account topographic and dynamic constraints (coasts, advection, . . . ) and its capacity to handle large data sets, frequently encountered in oceanography. The method provides gridded fields in two dimensions, usually in horizontal layers. Three-dimension fields are obtained by stacking horizontal layers. In the present work, we summarize the background of the method and describe the possible methods to compute the error field associated to the analysis. In particular, we present new developments leading to a more consistent error estimation, by determining numerically the real covariance function in Diva, which is never formulated explicitly, contrarily to Optimal Interpolation. The real covariance function is obtained by two concurrent executions of Diva, the first providing the covariance for the second. With this improvement, the error field is now perfectly consistent with the inherent background covariance in all cases. A two-dimension application using salinity measurements in the Mediterranean Sea is presented. Applied on these measurements, Optimal Interpolation and Diva provided very similar gridded fields (correlation: 98.6%, RMS of the difference: 0.02). The method using the real covariance produces an error field similar to the one of OI, except in the coastal areas.

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See detailBio-ORACLE: a global environmental dataset for marine species distribution modelling
Tyberghein, Lennert; Verbruggen, Heroen; Klaas, Pauly; Troupin, Charles; Mineur, Frederic; De Clerck, Olivier

in Global Ecology and Biogeography (2012), 21(2), 272-281

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See detailComment on “A set of rapid-response models for pollutant dispersion assessments in southern Spain coastal waters” by R. Periañez and F. Caravaca, Marine Pollution Bulletin 60 (2010) 1412–1422
Djenidi, Salim

in Marine Pollution Bulletin (2012), 64

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See detailOutlier detection in satellite data using spatial coherence
Alvera Azcarate, Aïda; Sirjacobs, Damien; Barth, Alexander; Beckers, Jean-Marie

in Remote Sensing of Environment (2012), 119

Satellite data sets often contain outliers (i.e., anomalous values with respect to the surrounding pixels), mostly due to undetected clouds and rain or to atmospheric and land contamination. A methodology to detect outliers in satellite data sets is presented. The approach uses a truncated Empirical Orthogonal Function (EOF) basis. The information rejected by this EOF basis is used to identify suspect data. A proximity test and a local median test are also performed, and a weighted sum of these three tests is used to accurately detect outliers in a data set. Most satellite data undergo automated quality-check analyses. The approach presented exploits the spatial coherence of the geophysical fields, therefore detecting outliers that would otherwise pass such checks. The methodology is applied to infrared sea surface temperature (SST), microwave SST and chlorophyll-a concentration data over different domains, to show the applicability of the technique to a range of variables and temporal and spatial scales. A series of sensitivity tests and validation with independent data are also conducted.

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See detailInterannual variability of Black Sea’s hydrodynamics and connection to atmospheric patterns
Capet, Arthur; Barth, Alexander; Beckers, Jean-Marie; Grégoire, Marilaure

in Deep-Sea Research. Part II, Topical Studies in Oceanography (2012)

The long term variability (1962–2000) of the Black Sea physical processes (e.g. temperature, main circulation, cold intermediate layer, sea level) and its relation to atmospheric conditions and large scale climate patterns are investigated using an eddy-resolving tridimensional model in ombination with statistical tools (e.g. Empirical Orthogonal Functions, Self Organizing Maps). First, the ability of the model to represent the interannual dynamics of the system is assessed by comparing the modeled and satellite sea surface temperature (SST) and sea level anomaly (SLA) decomposed into their dominant Empirical Orthogonal Functions (EOFs). The correlation between the spatial and temporal EOFs modes derived from model and satellite data is usually satisfactory and this gives some confidence in using the model as a tool to investigate not only the SST and SLA dynamics but also the dynamics of connected variables. Then, the long term variability (1962–2000) of the Black Sea hydrodynamics is assessed by decomposing into their dominant EOFs modeled SST, SLA and selected key hydrodynamical variables associated to the main circulation and vertical structure of the water column. Significant correlations between the EOFs associated to these variables are investigated in order to link the variability of surface fields and the internal dynamics of the system. In particular, the intensity of the general cyclonic circulation (the Rim Current) is shown to impact strongly (1) the mean sea level, (2) the SST response to air temperature (AT), (3) the formation of the cold intermediate layer, (4) the meridional repartition of the SST anomaly and (5) the exchanges of heat between the north-western shelf and the open basin. In order to appraise the variability of atmospheric conditions over the Black Sea during 1962–2000 and their role in driving the hydrodynamics, a self-organizing maps technique is used to identify spatial recurrent patterns of atmospheric fields (i.e., AT, wind stress and curl). The impact on these patterns of large scale climatic variability over the north Atlantic, Eurasia and the Pacific Ocean (estimated by respectively the north Atlantic oscillation (NAO), the east Atlantic/west ̃Russia oscillation (EA/WR) and the El Nino southern oscillation (ENSO) indexes) is assessed. Distinct time scales of influence of the large scale teleconnection patterns on the AT are identified: EA/WR drives the short scale (1–5 years) variations of SST, while the long term (4-5 years) trends of the NAO drive the long term SST trends. The drastic changes that have occurred in the Black Sea deep sea ecosystem at the end of the 80s are connected to an intensification of the general circulation that has promoted an export of riverine materials from the eutrophicated north-western shelf to the deep sea. Finally, in the last two decades, we find an increased duration of persistent atmospheric anomalies regime that has the potential to drive the system away from its average state as occurred in the late 80s. If persistent in the future, such long lasting atmospheric anomalies may have a significant impact on the ecosystem functioning.

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See detailGeneration of the Cape Ghir upwelling filament: A numerical study
Troupin, Charles; Mason, Evan; Beckers, Jean-Marie; Sangrà, Pablo

in Ocean Modelling (2012), 41

Filaments are narrow, shallow structures of cool water originating from the coast. They are typical features of the four main eastern boundary upwelling systems (EBUS). In spite of their significant biological and chemical roles, through the offshore exportation of nutrient-rich waters, the physical processes that generate them are still not completely understood. This paper is a process-oriented study of filament generation mechanisms. Our goal is twofold: firstly, to obtain a numerical solution able to well represent the characteristics of the filament off Cape Ghir (30°38'N, northwestern Africa) in the Canary EBUS and secondly, to explain its formation by a simple mechanism based on the balance of potential vorticity. The first goal is achieved by the use of the ROMS model (Regional Ocean Modeling System) in embedded domains around Cape Ghir, with a horizontal resolution going up to 1.5 km for the finest domain. The latter gets its initial and boundary conditions from a parent solution and is forced by climatological, high-resolution atmospheric fields. The modeled filaments display spatial, temporal and physical characteristics in agreement with the available in situ and satellite observations. This model solution is used as a reference to compare the results with a set of process-oriented experiments. These experiments allow us to reach the second objective. Their respective solution serves to highlight the contribution of various processes in the filament generation. Since the study is focused on general processes present under climatological forcing conditions, inter-annual forcing is not necessary. The underlying idea for the filament generation is the balance of potential vorticity in the Canary EBUS: the upwelling jet is characterized by negative relative vorticity and flows southward along a narrow band of uniform potential vorticity. In the vicinity of the cape, an injection of relative vorticity induced by the wind breaks the existing vorticity balance. The upwelling jet is prevented from continuing its way southward and has to turn offshore to follow lines of equal potential vorticity. The model results highlight the essential role of wind, associated with the particular topography (coastline and bottom) around the cape. The mechanism presented here is general and thus can be applied to other EBUS.

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See detailSeasonal variability of the Canary Current: a numerical study
Mason, Evan; Colas, François; Molemaker, Jeroen; Shchepetkin, Alexander; Troupin, Charles; McWilliams, James; Sangrà, Pablo

in Journal of Geophysical Research. Oceans (2011), 116

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See detailThermocline characterisation in the Cariaco basin: A modelling study of the thermocline annual variation and its relation with winds and chlorophyll-a concentration
Alvera Azcarate, Aïda; Barth, Alexander; Weisberg, Robert H.; Castañeda, Julián J.; Vandenbulcke, Luc; Beckers, Jean-Marie

in Continental Shelf Research (2011), 31(1), 73-84

The spatial and temporal evolution of the thermocline depth and width of the Cariaco basin (Venezuela) is analysed by means of a three-dimensional hydrodynamic model. The thermocline depth and width are determined through the fitting of model temperature profiles to a sigmoid function. The use of whole profiles for the fitting allows for a robust estimation of the thermocline characteristics, mainly width and depth. The fitting method is compared to the maximum gradient approach, and it is shown that, under some circumstances, the method presented in this work leads to a better characterization of the thermocline. After assessing, through comparison with independent {\it in situ} data, the model capabilities to reproduce the Cariaco basin thermocline, the seasonal variability of this variable is analysed, and the relationship between the annual cycle of the thermocline depth, the wind field and the distribution of chlorophyll-a concentration in the basin is studied. The interior of the basin reacts to easterly winds intensification with a rising of the thermocline, resulting in a coastal upwelling response, with the consequent increase in chlorophyll-a concentration. Outside the Cariaco basin, where an open-ocean, oligothrophic regime predominates, wind intensification increases mixing of the surface layers and induces therefore a deepening of the thermocline. The seasonal cycle of the thermocline variability in the Cariaco basin is therefore related to changes in the wind field. At shorter time scales (i.e. days), it is shown that other processes, such as the influence of the meandering Caribbean Current, can also influence the thermocline variability. The model thermocline depth is shown to be in good agreement with the two main ventilation events that took place in the basin during the period of the simulation.

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See detailCloud filling of ocean colour and sea surface temperature remote sensing products over the Southern North Sea by the Data Interpolating Empirical Orthogonal Functions methodology.
Sirjacobs, Damien; Alvera Azcarate, Aïda; Barth, Alexander; Lacroix, Geneviève; Park, Youngje; Nechad, Bouchra; Ruddick, Kevin; Beckers, Jean-Marie

in Journal of Sea Research (2011), 65(1), 114-130

Optical remote sensing data is now being used systematically for marine ecosystem applications, such as the forcing of biological models and the operational detection of harmful algae blooms. However, applications are hampered by the incompleteness of imagery and by some quality problems. The Data Interpolating Empirical Orthogonal Functions methodology (DINEOF) allows calculation of missing data in geophysical datasets without requiring a priori knowledge about statistics of the full data set and has previously been applied to SST reconstructions. This study demonstrates the reconstruction of complete space-time information for 4 years of surface chlorophyll a (CHL), total suspended matter (TSM) and sea surface temperature (SST) over the Southern North Sea (SNS) and English Channel (EC). Optimal reconstructions were obtained when synthesising the original signal into 8 modes for MERIS CHL and into 18 modes for MERIS TSM. Despite the very high proportion of missing data (70%), the variability of original signals explained by the EOF synthesis reached 93.5 % for CHL and 97.2 % for TSM. For the MODIS TSM dataset, 97.5 % of the original variability of the signal was synthesised into 14 modes. The MODIS SST dataset could be synthesised into 13 modes explaining 98 % of the input signal variability. Validation of the method is achieved for 3 dates below 2 artificial clouds, by comparing reconstructed data with excluded input information. Complete weekly and monthly averaged climatologies, suitable for use with ecosystem models, were derived from regular daily reconstructions. Error maps associated with every reconstruction were produced according to Beckers et al. (2006) [6]. Embedded in this error calculation scheme, a methodology was implemented to produce maps of outliers, allowing identification of unusual or suspicious data points compared to the global dynamics of the dataset. Various algorithms artefacts were associated with high values in the outlier maps (undetected cloud edges, haze areas, contrails, cloud shadows). With the production of outlier maps, the data reconstruction technique becomes also a very efficient tool for quality control of optical remote sensing data and for change detection within large databases.

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See detailReconstruction of sea surface temperature by means of DINEOF: a case study during the fishing season in the Bay of Biscay
Ganzedo, Unai; Alvera Azcarate, Aïda; Esnaola, Ganix; Ezcurra, Agustin; Saenz, Jon

in International Journal of Remote Sensing (2011), 32(4), 933-950

The Spanish surface fishery operates mainly during the summer season in the waters of the Bay of Biscay. Sea surface temperature (SST) data recovered from satellite images are being used to improve the operational efficiency of fishing vessels (e.g. reduce search time and increase catch rate) and to improve the understanding of the variations in catch distribution and rate needed to properly manage fisheries. The images used for retrieval of SST often present gaps due to the existence of clouds or satellite malfunction periods. The data gaps can totally or partially affect the area of interest. Within this study, an application of a technique for the reconstruction of missing data called DINEOF (data interpolating empirical orthogonal functions) is analysed, with the aim of testing its applicability in operational SST retrieval during summer months. In this case study, the Bay of Biscay is used as the target area. Three months of SST Moderate Resolution Imaging Spectroradiometer (MODIS) images, ranging from 1 May 2006 to 31 July 2006, were used. The main objective of this work is to test the overall performance of this technique, under potential operational use for the support of the fleet during the summer fishing season. The study is designed to analyse the sensitivity of the results of this technique to several details of the methodology used in the reconstruction of SST, such as the number of empirical orthogonal functions (EOFs) retained, the handling of the seasonal cycle or the length (number of images) of the SST database used. The results are tested against independent SST data from International Comprehensive Ocean–Atmosphere Data Set (ICOADS) ship reports and standing buoys and estimations of the error of the reconstructed SST fields are given. Conclusions show that over this area three months of data are enough for efficient SST reconstruction, which yields four EOFs as the optimal number needed for this case study. An extended EOF experiment with SST and SST with a lag of one day was carried out to analyse whether the autocorrelation of the SST data allows better performance in the SST reconstruction, although theexperiment did not improve the results. The validation studies show that the reconstructed SSTs can be trusted, even when the amount of missing data is very high. The mean absolute deviation maps show that the error is greatest near to the coast and mainly in the upwelling areas close to the French and north-western Spanish coasts.

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See detailRealistic modelling of the exceptional inflows into the central Baltic Sea in 2003 using terrain-following coordinates
Hofmeister, R.; Beckers, Jean-Marie; Burchard, H.

in Ocean Modelling (2011), 39

Dense inflows into the permanently stratified Baltic Sea are renewing the deep waters in the central basins. The present study evaluates the performance of terrain-following coordinates, which are resolving the along-bottom flow, in an annual simulation covering several inflows reaching the deeper basins of the Baltic Sea in 2003. Therefor the simulations are carried out using sigma-coordinates and vertically adaptive coordinates for two different horizontal resolutions (2 NM and 1 NM). The simulations with the sigma coordinates could not reproduce the hydrography of the major Baltic inflows realistically due to discretisation errors such as numerical mixing and pressure gradient errors. It is shown that the adaptive coordinates improve the simulation, because numerical mixing is reduced and the model’s discretisation supports a more physically-justified representation of the physical processes. For the higher-resolution simulations, adding a parameterisation of internal mixing enhances the effective mixing in the simulation and induces a reduction of the numerical mixing. Additionally to the analysis of the model performance, the inflows’ hydrography as projected by the higher-resolution model using adaptive coordinates is presented. The characteristic cross-channel circulation of gravity currents in channelised bathymetry is found to be an essential feature of the inflow dynamics in the Baltic Sea. The usage of adaptive coordinates reduces the numerical mixing in the simulation as effective as the doubling of the horizontal resolution for sigma-coordinates. However, the numerical mixing accounts for at least 50 % of the salinity mixing in the simulations.

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See detailReconstruction of MODIS total suspended matter time series maps by DINEOF and validation with autonomous platform data
Nechad, Bouchra; Alvera Azcarate, Aïda; Ruddick, Kevin; Greenwood, Naomi

in Ocean Dynamics (2011)

In situ measurements of total suspended matter (TSM) over the period 2003–2006, collected with two autonomous platforms from the Centre for Environment, Fisheries and Aquatic Sciences (Cefas) measuring the optical backscatter (OBS) in the southern North Sea, are used to assess the accuracy of TSM time series extracted from satellite data. Since there are gaps in the remote sensing (RS) data, due mainly to cloud cover, the Data Interpolating Empirical Orthogonal Functions (DINEOF) is used to fill in the TSM time series and build a continuous daily “recoloured” dataset. The RS datasets consist of TSM maps derived from MODIS imagery using the bio-optical model of Nechad et al. (Rem Sens Environ 114: 854–866, 2010). In this study, the DINEOF time series are compared to the in situ OBS measured in moderately to very turbid waters respectively in West Gabbard and Warp Anchorage, in the southern North Sea. The discrepancies between instantaneous RS, DINEOF-filled RS data and Cefas data are analysed in terms of TSM algorithm uncertainties, space–time variability and DINEOF reconstruction uncertainty.

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See detailCorrecting surface winds by assimilating High-Frequency Radar surface currents in the German Bight
Barth, Alexander; Alvera Azcarate, Aïda; Beckers, Jean-Marie; Staneva, Joanna; Emil V., Stanev; Johannes, Schulz-Stellenfleth

in Ocean Dynamics (2011), 61(5), 599-610

Surface winds are crucial for accurately modeling the surface circulation in the coastal ocean. In the present work, high-frequency (HF) radar surface currents are assimilated using an ensemble scheme which aims to obtain improved surface winds taking into account ECMWF (European Centre for Medium-Range Weather Forecasts) winds as a first guess and surface current measurements. The objective of this study is to show that wind forcing can be improved using an approach similar to parameter estimation in ensemble data assimilation. Like variational assimilation schemes, the method provides an improved wind field based on surface current measurements. However, the technique does not require an adjoint and it is thus easier to implement. In addition, it does not rely on a linearization of the model dynamics. The method is validated directly by comparing the analyzed wind speed to independent in situ measurements and indirectly by assessing the impact of the corrected winds on model sea surface temperature (SST) relative to satellite SST.

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See detailComparison between satellite and in situ sea surface temperature data in the Western Mediterranean Sea
Alvera Azcarate, Aïda; Troupin, Charles; Barth, Alexander; Beckers, Jean-Marie

in Ocean Dynamics (2011), 61(6), 767-778

A comparison between in situ and satellite sea surface temperature (SST) is presented for the western Mediterranean Sea during 1999. Several international databases are used to extract in situ data (World Ocean Database (WOD), MEDAR/Medatlas, Coriolis Data Center, International Council for the Exploration of the Sea (ICES) and International Comprehensive Ocean-Atmosphere Data Set (ICOADS)). The in situ data are classified into different platforms or sensors (CTD, XBT, drifters, bottles, ships), in order to assess the relative accuracy of these type of data respect to AVHRR (Advanced Very High Resolution Radiometer) SST satellite data. It is shown that the results of the error assessment vary with the sensor type, the depth of the in situ measurements, and the database used. Ship data are the most heterogeneous data set, and therefore present the largest differences with respect to in situ data. A cold bias is detected in drifter data. The differences between satellite and in situ data are not normally distributed. However, several analysis techniques, as merging and data assimilation, usually require Gaussian-distributed errors. The statistics obtained during this study will be used in future work to merge the in situ and satellite data sets into one unique estimation of the SST.

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See detailData Interpolating Empirical Orthogonal Functions (DINEOF): a tool for geophysical data analyses
Alvera Azcarate, Aïda; Barth, Alexander; Sirjacobs, Damien; Lenartz, Fabian; Beckers, Jean-Marie

in Mediterranean Marine Science (2011), 12(3), 5-11

An overview of the technique called DINEOF (Data Interpolating Empirical Orthog- onal Functions) is presented. DINEOF reconstructs missing information in geophys- ical data sets, such as satellite imagery or time series. A summary of the technique is given, with its main characteristics, recent developments and future research di- rections. DINEOF has been applied to a large variety of oceanographic variables in various domains of different sizes. This technique can be applied to a single variable (monovariate approach), or to several variables together (multivariate approach), with no complexity increase in the application of the technique. Error fields can be computed to establish the accuracy of the reconstruction. Examples are given to illustrate the capabilities of the technique. DINEOF is freely offered to download, and help is provided to users in the form of a wiki and through a discussion email list.

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See detailResponses of European forest ecosystems to 21(st) century climate: assessing changes in interannual variability and fire intensity
Dury, Marie; Hambuckers, Alain; Warnant, P.; Henrot, Alexandra; Favre, E.; Ouberdous, Mohamed; François, Louis

in iForest: Biogeosciences and Forestry (2011), 4

Significant climatic changes are currently observed and, according to projections, will be strengthened over the 21(st) century throughout the world with the continuing increase of the atmospheric CO2 concentration. Climate will be generally warmer with notably changes in the seasonality and in the precipitation regime. These changes will have major impacts on the biodiversity and the functioning of natural ecosystems. The CARAIB dynamic vegetation model driven by the ARPEGE/Climate model under forcing from the A2 IPCC emission scenario is used to illustrate and analyse the potential impacts of climate change on forest productivity and distribution as well as fire intensity over Europe. The potential CO2 fertilizing effect is studied throughout transient runs of the vegetation model over the 1961-2100 period assuming constant and increasing atmospheric CO2 concentration. Without fertilisation effect, the net primary productivity (NPP) might increase in high latitudes and altitudes (by up to 40 % or even 60-100 %) while it might decrease in temperate (by up to 50 %) and in warmer regions, e.g., Mediterranean area (by up to 80 %). This strong decrease in NPP is associated with recurrent drought events occurring mostly in summer time. Under rising CO2 concentration, NPP increases all over Europe by as much as 25-75%, but it is not clear whether or not soils might sustain such an increase. The model indicates also that interannual NPP variability might strongly increase in the areas which will undergo recurrent water stress in the future. During the years exhibiting summer drought, the NPP might decrease to values much lower than present-day average NPP even when CO2 fertilization is included. Moreover, years with such events will happen much more frequently than today. Regions with more severe droughts might also be affected by an increase of wildfire frequency and intensity, which may have large impacts on vegetation density and distribution. For instance, in the Mediterranean basin, the area burned by wildfire can be expected to increase by a factor of 3-5 at the end of the 21(st) century compared to present.

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See detailEnhanced ocean temperature forecast skills through 3-D super-ensemble multi-model fusion
Lenartz, Fabian; Mourre, B.; Barth, Alexander; Beckers, Jean-Marie; Vandenbulcke, Luc; Rixen, M.

in Geophysical Research Letters (2010), 37(L19606),

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See detailA web interface for griding arbitrarily distributed in situ data based on Data-Interpolating Variational Analysis (DIVA)
Barth, Alexander; Alvera Azcarate, Aïda; Troupin, Charles; Ouberdous, Mohamed; Beckers, Jean-Marie

in Advances in Geosciences (2010), 28(28), 29-37

Spatial interpolation of observations on a regular grid is a common task in many oceanographic disciplines (and geosciences in general). It is often used to create climatological maps for physical, biological or chemical parameters representing e.g. monthly or seasonally averaged fields. Since instantaneous observations can not be directly related to a field representing an average, simple spatial interpolation of observations is in general not acceptable. DIVA (Data-Interpolating Variational Analysis) is an analysis tool which takes the error in the observations and the typical spatial scale of the underlying field into account. Barriers due to the coastline and the topography in general and also currents estimates (if available) are used to propagate the information of a given observation spatially. DIVA is a command-line driven application written in Fortran and Shell Scripts. To make DIVA easier to use, a web interface has been developed (http://gher-diva.phys.ulg.ac.be). Installation and compilation of DIVA is therefore not required. The user can directly upload the data in ASCII format and enter several parameters for the analysis. The analyzed field, location of the observations, and the error mask are presented as different layers using the Web Map Service protocol. They are visualized in the browser using the Javascript library OpenLayers allowing the user to interact with layers (for example zooming and panning). Finally, the results can be downloaded as a NetCDF file, Matlab/Octave file and Keyhole Markup Language (KML) file for visualization in applications such as Google Earth.

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See detailHigh-resolution Climatology of the northeast Atlantic using Data-Interpolating Variational Analysis (DIVA)
Troupin, Charles; Machin, Francis; Ouberdous, Mohamed; Sirjacobs, Damien; Barth, Alexander; Beckers, Jean-Marie

in Journal of Geophysical Research (2010), 115(C08005), 20

Numerous climatologies are available at different resolutions and cover various parts of the global ocean. Most of them have a resolution too low to represent suitably regional processes and the methods for their construction are not able to take into account the influence of physical effects (topographic constraints, boundary conditions, advection, etc.). A high-resolution atlas for temperature and salinity is developed for the northeast Atlantic Ocean on 33 depth levels. The originality of this climatology is twofold: (1) For the data set, data are collected on all major databases and aggregated to lead to an original data collection without duplicates, richer than the World Ocean Database 2005, for the same region of interest. (2) For the method, climatological fields are constructed using the variational method Data-Interpolating Variational Analysis. The formulation of the latter allows the consideration of coastlines and bottom topography, and has a numerical cost almost independent on the number of observations. Moreover, only a few parameters, determined in an objective way, are necessary to perform an analysis. The results show overall good agreement with the widely used World Ocean Atlas, but also reveal significant improvements in coastal areas. Error maps are generated according to different theories and emphasize the importance of data coverage for the creation of such climatological fields. Automatic outlier detection is performed, and its effects on the analysis are examined. The method presented here is very general and not dependent on the region, hence it may be applied for creating other regional atlas in different zones of the global ocean.

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See detailOnboard implementation of the GHER model for the Black Sea, with SST and CTD data assimilation
Vandenbulcke, Luc; Capet, Arthur; Beckers, Jean-Marie; Grégoire, Marilaure

in Journal of Operational Oceanography (2010), 3(2), 47-54

The first operational implementation of the GHER hydrodynamic model is described. It took place onboard the research vessel Alliance with all computation and sharing of forecasts being realised from the vessel in near-real time. The forecasts were realised in the context of the Turkish Straits System 2008 campaign, which aimed at the real-time characterisation of the Marmara Sea and (south-western) Black Sea. The model performed badly at first, mainly because of poor initial conditions. Hence, as the model includes a reduced-rank extended Kalman filter assimilation scheme, after a hindcast where sea surface temperature and temperature and salinity profiles were assimilated, the model yielded realistic forecasts. Furthermore, the time required to run a one-day simulation (about 300 seconds of simulation, or 500 with pre-processing and data transfers included) was very limited and thus operational use of the model is possible.

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See detailSuper-ensemble techniques applied to wave forecast: performance and limitations
Lenartz, Fabian; Beckers, Jean-Marie; Chiggiato, Jacopo; Mourre, Baptiste; Troupin, Charles; Vandenbulcke, Luc; Rixen, Michel

in Ocean Science (2010), 6(2), 595-604

Nowadays, several operational ocean wave forecasts are available for a same region. These predictions may considerably differ, and to choose the best one is generally a difficult task. The super-ensemble approach, which consists in merging different forecasts and past observations into a single multi-model prediction system, is evaluated in this study. During the DART06 campaigns organized by the NATO Undersea Research Centre, four wave forecasting systems were simultaneously run in the Adriatic Sea, and significant wave height was measured at six stations as well as along the tracks of two remote sensors. This effort provided the necessary data set to compare the skills of various multi-model combination techniques. Our results indicate that a super-ensemble based on the Kalman Filter improves the forecast skills: The bias during both the hindcast and forecast periods is reduced, and the correlation coefficient is similar to that of the best individual model. The spatial extrapolation of local results is not straightforward and requires further investigation to be properly implemented.

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See detailSeasonal variability of the oceanic upper layer and its modulation of biological cycles in the Canary Island region
Troupin, Charles; Sangrà, Pablo; Arístegui, Javier

in Journal of Marine Systems (2010), 80(3-4), 172-183

The Canary Island region is rich in mesoscale phenomena that affect cycles of physical and biological processes. A 1D version of the Regional Oceanic Modeling System (ROMS) is used south of the Gran Canaria Island to simulate seasonal climatologies of these cycles. The model is forced with monthly air–sea fluxes averaged from 1993 to 2002 and initialized with mean in situ profiles of temperature, salinity, oxygen and nitrate concentrations. The K-Profile Parameterization (KPP) mixed layer submodel is compared with other submodels using idealized numerical experiments. When forced with realistic air–sea fluxes, the model correctly reproduces the annual cycle of temperature (mixed layer depth), with minimum surface values of 18 °C (maximal depth > 105 m) in February during convective mixing resulting from a negative heat flux. Maximum temperatures above 23 °C (minimal depth < 20 m) are simulated from September to October after strong summer heating and a decrease in Trade Winds intensity. A simple ecosystem model is coupled to the physical model, which provides simulated biological cycles that are in agreement with regional observations. A phytoplankton bloom develops in late winter, driven by the injection of new nutrients into the euphotic layer. Simulated chlorophyll shows a deep maximum fluctuating around 100 m with concentrations around 1 mg Chla /m³, while surface values are low (around 0.1 mg Chla /m³) during most of the year. The physical and biological model results are validated by comparisons with data from regional studies, climatological fields and time-series from the ESTOC station.

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See detailEnsemble perturbation smoother for optimizing tidal boundary conditions by assimilation of High-Frequency radar surface currents - application to the German Bight
Barth, Alexander; Alvera Azcarate, Aïda; Gurgel, Klaus-Werner; Staneva, Joanna; Port, Alex; Beckers, Jean-Marie; Stanev, Emil V

in Ocean Science (2010), 6(1), 161-178

High-Frequency (HF) radars measure the ocean surface currents at various spatial and temporal scales. These include tidal currents, wind-driven circulation, density-driven circulation and Stokes drift. Sequential assimilation methods updating the model state have been proven successful to correct the density-driven currents by assimilation of observations such as sea surface height, sea surface temperature and in-situ profiles. However, the situation is different for tides in coastal models since these are not generated within the domain, but are rather propagated inside the domain through the boundary conditions. For improving the modeled tidal variability it is therefore not sufficient to update the model state via data assimilation without updating the boundary conditions. The optimization of boundary conditions to match observations inside the domain is traditionally achieved through variational assimilation methods. In this work we present an ensemble smoother to improve the tidal boundary values so that the model represents more closely the observed currents. To create an ensemble of dynamically realistic boundary conditions, a cost function is formulated which is directly related to the probability of each boundary condition perturbation. This cost function ensures that the boundary condition perturbations are spatially smooth and that the structure of the perturbations satisfies approximately the harmonic linearized shallow water equations. Based on those perturbations an ensemble simulation is carried out using the full three-dimensional General Estuarine Ocean Model (GETM). Optimized boundary values are obtained by assimilating all observations using the covariances of the ensemble simulation.

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See detailNon-uniform adaptive vertical grids for 3D numerical ocean models
Hofmeister, R.; Burchard, H.; Beckers, Jean-Marie

in Ocean Modelling (2010), 33

A new strategy for the vertical gridding in terrain-following 3D ocean models is presented here. The vertical grid adaptivity is partially given by a vertical diffusion equation for the vertical layer positions, with diffusivities being proportional to shear, stratification and distance from the boundaries. In the horizontal, the grid can be smoothed with respect to z-levels, grid layer slope and density. Lagrangian tendency of the grid movement is supported. The adaptive terrain-following grid can be set to be an Eulerian–Lagrangian grid, a hybrid r–q or r–z grid and combinations of these with great flexibility. With this, internal flow structures such as thermoclines can be well resolved and followed by the grid. A set of idealised examples is presented in the paper, which show that the introduced adaptive grid strategy reduces pressure gradient errors and numerical mixing significantly. The grid adaption strategy is easy to implement in various types of terrain-following ocean models. The idealised examples give evidence that the adaptive grids can improve realistic, long-term simulations of stratified seas while keeping the advantages of terrain-following coordinates.

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See detailMulti-sensor survey of seasonal variability in coastal eddy and internal wave signatures in the north-eastern Black Sea
Mityagina, Marina; Lavrova, Olga; Karimova, Svetlana

in International Journal of Remote Sensing (2010), 31(17-18), 47794790

In this paper the remote sensing satellite sensor data (obtained by Envisat Advanced Synthetic Aperture Radar (ASAR), Terra and Aqua Moderate Resolution Imaging Spectroradiometer (MODIS), and National Oceanic and Atmospheric Administration (NOAA) Advanced Very High Resolution Radiometer (AVHRR) instruments) is used to study coastal dynamics of the north-eastern Black Sea. Occurrence, evolution and drift of small-scale eddies in coastal waters are investigated. Seasonal variability of their manifestations is established. Instances of surface manifestations of non-tidal internal waves (IW) are discovered. The main finding was that practically all cases of IW manifestations were observed in the vicinity of mesoscale sea eddy structures or hydrological fronts. The joint analysis of data from different sensors was performed to reveal specific conditions leading to the intensification of wave processes and to their manifestation in radar imagery as well as to determine possible sources of the IW generation.

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See detailEffects of CO2, continental distribution, topography and vegetation changes on the climate at the Middle Miocene: a model study
Henrot, Alexandra; François, Louis; Favre, Eric; Butzin, Martin; Ouberdous, Mohamed; Munhoven, Guy

in Climate of the Past (2010), 6

The Middle Miocene was one of the last warm periods of the Neogene, culminating with the Middle Miocene Climatic Optimum (MMCO, approximatively 17–15 Ma). Several proxy-based reconstructions support warmer and more humid climate during the MMCO. The mechanisms responsible for the warmer climate at the MMCO and particularly the role of the atmospheric carbon dioxide are still highly debated. Here we carried out a series of sensitivity experiments with the model of intermediate complexity Planet Simulator, investigating the contributions of the absence of ice on the continents, the opening of the Central American and Eastern Tethys Seaways, the lowering of the topography on land, the effect of various atmospheric CO2 concentrations and the vegetation feedback. Our results show that a higher than present-day CO2 concentration is necessary to generate a warmer climate at all latitudes at the Middle Miocene, in agreement with the terrestrial proxy reconstructions which suggest high atmospheric CO2 concentrations at the MMCO. Nevertheless, the changes in sea-surface conditions, the lowering of the topography on land and the vegetation feedback also produce significant local warming that may, locally, even be stronger than the CO2 induced temperature increases. The lowering of the topography leads to a more zonal atmospheric circulation and allows the westerly flow to continue over the lowered Plateaus at mid-latitudes. The reduced height of the Tibetan Plateau notably prevents the development of a monsoon-like circulation, whereas the reduction of elevations of the North American and European reliefs strongly increases precipitation from northwestern to eastern Europe. The changes in vegetation cover contribute to maintain and even to intensify the warm and humid conditions produced by the other factors, suggesting that the vegetation-climate interactions could help to improve the model-data comparison.

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See detailSuper-Ensemble techniques: application to surface drift prediction
Vandenbulcke, Luc; Beckers, Jean-Marie; Lenartz, Fabian; Barth, Alexander; Poulain, P. M.; Aidonidis, M.; Meyrat, J.; Ardhuin, F.; Tonani, M.; Fratianni, C.; Torrisi, L.; Pallela, D.; Chiggiato, J.; Tudor, M.; Book, J. W.; Martin, P.; Peggion, G.; Rixen, Michel

in Progress in Oceanography (2009), 82(3), 149-167

The prediction of surface drift of floating objects is an important task, with applications such as marine transport, pollutant dispersion, and search-and-rescue activities. But forecasting even the drift of surface waters is very challenging, because it depends on complex interactions of currents driven by the wind, the wave field and the general prevailing circulation. Furthermore, although each of those can be forecasted by deterministic models, the latter all suffer from limitations, resulting in imperfect predictions. In the present study, we try and predict the drift of two buoys launched during the DART06 (Dynamics of the Adriatic sea in Real-Time 2006) and MREA07 (Maritime Rapid Environmental Assessment 2007) sea trials, using the so-called hyper-ensemble technique: different models are combined in order to minimize departure from independent observations during a training period; the obtained combination is then used in forecasting mode. We review and try out different hyper-ensemble techniques, such as the simple ensemble mean, least-squares weighted linear combinations, and techniques based on data assimilation, which dynamically update the model’s weights in the combination when new observations become available. We show that the latter methods alleviate the need of fixing the training length a priori, as older information is automatically discarded. When the forecast period is relatively short (12 h), the discussed methods lead to much smaller forecasting errors compared with individual models (at least three times smaller), with the dynamic methods leading to the best results. When many models are available, errors can be further reduced by removing colinearities between them by performing a principal component analysis. At the same time, this reduces the amount of weights to be determined. In complex environments when meso- and smaller scale eddy activity is strong, such as the Ligurian Sea, the skill of individual models may vary over time periods smaller than the forecasting period (e.g. when the latter is 36 h). In these cases, a simpler method such as a fixed linear combination or a simple ensemble mean may lead to the smallest forecast errors. In environments where surface currents have strong mean-kinetic energies (e.g. the Western Adriatic Current), dynamic methods can be particularly successful in predicting the drift of surface waters. In any case, the dynamic hyper-ensemble methods allow to estimate a characteristic time during which the model weights are more or less stable, which allows predicting how long the obtained combination will be valid in forecasting mode, and hence to choose which hyper-ensemble method one should use.

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See detailUS GODAE: Global Ocean Prediction with the HYbrid Coordinate Ocean Model (HYCOM)
Chassignet, E. P.; Hurlburt, H. E.; Metzger, E. J.; Smedstad, O. M.; Cummings, J. A.; Halliwell, G. R.; Bleck, R.; Baraille, R.; Wallcraft, A. J.; Lozano, C.; Tolman, H. L.; Srinivasan, A.; Hankin, S.; Cornillon, P.; Weisberg, R.; Barth, Alexander; He, R.; Werner, F.; Wilkin, J.

in Oceanography (2009), 22(2), 64-75

During the past five to ten years, a broad partnership of institutions under NOPP sponsorship has collaborated in developing and demonstrating the performance and application of eddy-resolving, real-time global- and basin-scale ocean prediction systems using the HYbrid Coordinate Ocean Model (HYCOM). The partnership represents a broad spectrum of the oceanographic community, bringing together academia, federal agencies, and industry/commercial entities, and spanning modeling, data assimilation, data management and serving, observational capabilities, and application of HYCOM prediction system outputs. In addition to providing real-time, eddy-resolving global- and basin-scale ocean prediction systems for the US Navy and NOAA, this project also offered an outstanding opportunity for NOAA-Navy collaboration and cooperation, ranging from research to the operational level. This paper provides an overview of the global HYCOM ocean prediction system and highlights some of its achievements. An important outcome of this effort is the capability of the global system to provide boundary conditions to even higherresolution regional and coastal models.

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See detailModelling the Gibraltar Strait/Western Alboran Sea ecohydrodynamics
Skliris, Nikolaos; Beckers, Jean-Marie

in Ocean Dynamics (2009), 59(3), 489-508

The ecohydrodynamics of the Gibraltar Strait and the Western Alboran Sea is investigated using a 3-D, two-way nested, coupled hydrodynamic/plankton ecosystem model, exploiting the MEDATLAS climatological database. A high-resolution model (~1 km) of the Gibraltar/Western Alboran region embedded within a coarse-resolution model of the West Mediterranean (~5 km) is implemented. The model seasonal climatology of the 3-D circulation and the flow characteristics at the Gibraltar Strait and the Alboran Sea are discussed, and their impact on the plankton ecosystem evolution is explored. An important ecohydrodynamic feature produced by the model is a permanent upwelling zone in the northwestern part of the Alboran Sea in agreement with observations. Model results show that both horizontal and vertical current intensity of the Atlantic Jet increases progressively at the strait to obtain maximum values in the northeastern Mediterranean entrance, inducing an upward displacement of the nitracline. The nutrient-rich water transport through the strait along with the generation of cyclonic vorticity in the northwestern Alboran Sea result in the accumulation of nutrients there and thus induce a permanent fertilisation of this area.

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See detailThe Surface Circulation of the Caribbean Sea and the Gulf of Mexico as Inferred from Satellite Altimetry
Alvera Azcarate, Aïda; Barth, Alexander; Weisberg, Robert H.

in Journal of Physical Oceanography (2009), 39(3), 640657

The surface circulation of the Caribbean Sea and Gulf of Mexico is studied using 13 years of satellite altimetry data. Variability in the Caribbean Sea is evident over several time scales. At the annual scale, sea surface height (SSH) varies mainly by a seasonal steric effect. Interannually, a longer cycle affects the SSH slope across the current and hence the intensity of the Caribbean Current. This cycle is found to be related to changes in the wind intensity, the wind stress curl, and El Niño–Southern Oscillation. At shorter time scales, eddies and meanders are observed in the Caribbean Current, and their propagation speed is explained by baroclinic instabilities under the combined effect of vertical shear and the β effect. Then the Loop Current (LC) is considered, focusing on the anticyclonic eddies shed by it and the intrusion of the LC into the Gulf of Mexico through time. Twelve of the 21 anticyclonic eddies observed to detach from the LC are shed from July to September, suggesting a seasonality in the timing of these events. Also, a relation is found between the intrusion of the LC into the Gulf of Mexico and the size of the eddies shed from it: larger intrusions trigger smaller eddies. A series of extreme LC intrusions into the Gulf of Mexico, when the LC is observed as far as 92°W, are described. The analyses herein suggest that the frequency of such events has increased in recent years, with only one event occurring in 1993 versus three from 2002 to 2006. Transport through the Straits of Florida appears to decrease during these extreme intrusions.

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See detailA nested model of the Cariaco Basin (Venezuela): description of the basin’s interior hydrography and interactions with the open ocean
Alvera Azcarate, Aïda; Barth, Alexander; Weisberg, Robert H.

in Ocean Dynamics (2009), 59(1), 97-120

A high-resolution (1/60°), three-dimensional numerical circulation model of the Cariaco Basin (Venezuela) is constructed by nesting the Regional Ocean Modeling System (ROMS) in the 1/12° global Hybrid Coordinate Ocean Model (HYCOM). A new bathymetry, computed by merging DBDB2 data and in situ depth measurements using optimal interpolation, is described. This new bathymetry corrects the depth of the channels that connect the Cariaco Basin with the open ocean and which play a very important role in the basin circulation. Results from a 2004 ROMS hindcast are presented. Observations (temperature, salinity, and currents) are used to validate the model results before using the model to describe the annual cycle of the Cariaco Basin and the interactions between the basin and the open ocean. Two modes of interaction are described, the first being the meanders and eddies that travel westward with the Caribbean Current, and the second being a subsurface eastward current that flows along the north coast of South America. The circulation path within the basin is directly related to the intensity of this current. Both mechanisms described play a role in the ventilation of the basin. The present study is also an example of the feasibility of one of the objectives of GODAE (Global Ocean Data Assimilation Experiment): downscaling from a large-scale model to a regional model. In particular, the nesting ratio of 5 used in this work demonstrates that a high-resolution model can be successfully nested in HYCOM.

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See detailImpact of GODAE products on nested HYCOM simulations of the West Florida Shelf
Halliwell, G. R.; Barth, Alexander; Weiberg, R. H.; Hogan, P.; Smedstad, O. M.; Cummings, J.

in Ocean Dynamics (2009), 59(1), 139-155

Nested non-assimilative simulations of the West Florida Shelf for 2004-2005 are used to quantify the impact of initial and boundary conditions provided by Global Ocean Data Assimilation Experiment ocean products. Simulations are nested within an optimum interpolation hindcast of the Atlantic Ocean, the initial test of the US Navy Coupled Ocean Data Assimilation system for the Gulf of Mexico, and a global ocean hindcast that used the latter assimilation system. These simulations are compared to one that is nested in a non-assimilative Gulf of Mexico model to document the importance of assimilation in the outer model. Simulations are evaluated by comparing model results to moored Acoustic Doppler Current Profiler measurements and moored sea surface temperature time series. The choice of outer model has little influence on simulated velocity fluctuations over the inner and middle shelf where fluctuations are dominated by the deterministic wind-driven response. Improvement is documented in the representation of alongshore flow variability over the outer shelf, driven in part by the intrusion of the Loop Current and associated cyclones at the shelf edge near the Dry Tortugas. This improvement was realized in the simulation nested in the global ocean hindcast, the only outer model choice that contained a realistic representation of Loop Current transport associated with basin-scale wind-driven gyre circulation and the Atlantic Meridional Overturning Circulation. For temperature, the non-assimilative outer model had a cold bias in the upper ocean that was substantially corrected in the data-assimilative outer models, leading to improved temperature representation in the simulations nested in the assimilative outer models.

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See detailLimnological variability and pelagic fish abundance (Stolothrissa tanganicae and Lates stappersii) in Lake Tanganyika
Plisnier, P. D.; Mgana, H.; Kimirei, I.; Chande, A.; Makasa, L.; Chimanga, J.; Zulu, F.; Cocquyt, C.; Horion, Stéphanie; Bergamino, N.; Naithani, Jaya; Deleersnijder, Eric; André, L.; Descy, J. P.; Cornet, Yves

in Hydrobiologia (2009), 625(1), 117-134

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See detailA coordinated coastal ocean observing and modeling system for the West Florida Continental Shelf
Weisberg, R. H.; Barth, Alexander; Alvera Azcarate, Aïda; Zheng, L. Y.

in Harmful Algae (2009), 8(4), 585-597

The evolution of harmful algal blooms, while dependent upon complex biological interactions, is equally dependent upon the ocean circulation since the circulation provides the basis for the biological interactions by uniting nutrients with light and distributing water properties. For the coastal ocean, the circulation and the resultant water properties, in turn, depend on interactions between both the continental shelf and the deep-ocean and the continental shelf and the estuaries since the deep-ocean and the estuaries are primary nutrient sources. Here we consider a coordinated program of observations and models for the West Florida Continental Shelf (WFS) intended to provide a supportive framework for K. brevis red-tide prediction as well as for other coastal ocean matters of societal concern. Predicated on lessons learned, the goal is to achieve a system complete enough to support data assimilative modeling and prediction. Examples of the observations and models are presented and application is made to aspects of the 2005 red-tide. From an observational perspective, no single set of measurements is adequate. Required are a broad mix of sensors and sensor delivery systems capable of describing the three-dimensional structure of the velocity and density fields. Similarly, models must be complete enough to include the relevant physical processes, and data assimilation provides the integrative framework for maximizing the joint utility of the observations and models. While we are still in the exploratory stages of development, the lessons learned and application examples may be useful to similar programs under development elsewhere. One scientific finding is that the key to understanding K. brevis red-tide on the WFS lies not at the surface, but at depth

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See detailEnhancing temporal correlations in EOF expansions for the reconstruction of missing data using DINEOF
Alvera Azcarate, Aïda; Barth, Alexander; Sirjacobs, Damien; Beckers, Jean-Marie

in Ocean Science (2009), 5(4), 475-485

DINEOF (Data Interpolating Empirical Orthogonal Functions) is an EOF-based technique for the reconstruction of missing data in geophysical fields, such as those produced by clouds in sea surface temperature satellite images. A technique to reduce spurious time variability in DINEOF reconstructions is presented. The reconstruction of these images within a long time series using DINEOF can lead to large discontinuities in the reconstruction. Filtering the temporal covariance matrix allows to reduce this spurious variability and therefore more realistic reconstructions are obtained. The approach is tested in a three years sea surface temperature data set over the Black Sea. The effect of the filter in the temporal EOFs is presented, as well as some examples of the improvement achieved with the filtering in the SST reconstruction, both compared to the DINEOF approach without filtering.

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See detailDynamically constrained ensemble perturbations - application to tides on the West Florida Shelf
Barth, Alexander; Alvera Azcarate, Aïda; Beckers, Jean-Marie; Weisberg, R. H.; Vandenbulcke, Luc; Lenartz, Fabian; Rixen, Michel

in Ocean Science (2009), 5(3), 259-270

A method is presented to create an ensemble of perturbations that satisfies linear dynamical constraints. A cost function is formulated defining the probability of each perturbation. It is shown that the perturbations created with this approach take the land-sea mask into account in a similar way as variational analysis techniques. The impact of the land-sea mask is illustrated with an idealized configuration of a barrier island. Perturbations with a spatially variable correlation length can be also created by this approach. The method is applied to a realistic configuration of the West Florida Shelf to create perturbations of the M2 tidal parameters for elevation and depth-averaged currents. The perturbations are weakly constrained to satisfy the linear shallow-water equations. Despite that the constraint is derived from an idealized assumption, it is shown that this approach is applicable to a non-linear and baroclinic model. The amplitude of spurious transient motions created by constrained perturbations of initial and boundary conditions is significantly lower compared to perturbing the variables independently or to using only the momentum equation to compute the velocity perturbations from the elevation.

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See detailAssimilation of high-frequency radar currents in a nested model of the West Florida Shelf
Barth, Alexander; Alvera Azcarate, Aïda; Weisberg, R. H.

in Journal of Geophysical Research. Oceans (2008), 113(C8),

High-frequency radar currents are assimilated in a West Florida Shelf (WFS) model based on the Regional Ocean Model System (ROMS), which is nested in the Atlantic Hybrid Coordinate Ocean Model (HYCOM) for the purpose of including both local and deep-ocean forcing, particularly the Gulf of Mexico Loop Current. Tides are not included in this model. An ensemble simulation of the WFS model is carried out under different wind-forcings in order to estimate the error covariance of the model state vector and the covariance between ocean currents and winds. Radial currents measured by high-frequency radar antennas near Saint Petersburg and Venice, Florida, USA, are assimilated using this ensemble-based error covariance. Different assimilation techniques using a time-average ensemble, a filter to reduce surface-gravity waves and an extended state vector including wind stress were tested. Results of the WFS model assimilating surface currents show an improvement of the model currents not only at the surface but also at depth.

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See detailA nested model study of the Loop Current generated variability and its impact on the West Florida Shelf
Barth, Alexander; Alvera Azcarate, Aïda; Weisberg, R. H.

in Journal of Geophysical Research. Oceans (2008), 113(C5),

A West Florida Shelf model based on the Regional Ocean Modeling System (ROMS) is nested in the North Atlantic Hybrid Coordinate Ocean Model (NAT HYCOM). The focus of this work is the study of the impact of the Loop Current on the West Florida Shelf. In order to assess the model's accuracy, it is compared quantitatively to in situ temperature and velocity measurements on the shelf. A series of sensitivity experiments are conducted to determine the appropriate wind forcing, sea surface temperature relaxation, and mixing scheme. By the inclusion of the Loop Current, we are able to study the propagation of an anticyclonic vortex detaching from the Loop Current. We found that the ambient gradient of potential vorticity is able to explain the vortex path and speed. The statistics of such Loop Current generated flow features were examined by including a tracer marking Loop Current water. This allows to track the Loop Current water on the West Florida Shelf and to quantify the amount of Loop Current water reaching the shelf.

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See detailBenefit of nesting a regional model into a large-scale ocean model instead of climatology. Application to the West Florida Shelf
Barth, Alexander; Alvera Azcarate, Aïda; Weiberg, R. H.

in Continental Shelf Research (2008), 28(4-5), 561-573

The impact of open boundary conditions on the dynamics and accuracy of a regional West Florida Shelf model is addressed. A ROMS-based model nested in monthly climatological temperature and salinity and in the North Atlantic HYCOM model is implemented. The model results of these nesting implementations are compared to altimetry, in situ temperature time series, and ADCP and high-frequency (HF) radar currents. A significant improvement of the model results is found using the boundary conditions of the HYCOM model over the climatology. The ageostrophic nature of the LC is studied and the benefit using the velocity and surface elevation boundary conditions is shown. (C) 2007 Elsevier Ltd. All rights reserved.

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See detailAn analysis of the error space of a high-resolution implementation of the GHER hydrodynamic model in the Mediterranean Sea
Vandenbulcke, Luc; Rixen, M.; Alvera Azcarate, Aïda; Barth, Alexander; Beckers, Jean-Marie

in Ocean Modelling (2008), 24(1-2), 46-64

An ensemble of 250 model setups covering the Mediterranean Sea is built by perturbing various parameters: the bathymetry, the initial conditions, atmospheric forcing fields (air temperature, cloud coverage, wind), and internal model parameters (diffusion coefficients). The ensemble is then forwarded in time using the GHER hydrodynamic model, allowing to obtain information about the expected error associated with the forecast in a natural way. The evolution of this error is analyzed. In particular, we examine the time evolution and stationarity of its spatial average, and the spatial distribution of the error at different instants, by means of its first to fourth order moments, and of empirical orthogonal functions. We verify whether the a posteriori error distribution is Gaussian using the Anderson-Darling test. From these results, we are able to assess what parameters and forcing fields are most critical for the forecast. Qualitative conclusions are obtained throughout the text, in accordance with our expectations. Moreover, quantitative estimations of the expected error are also given. (C) 2008 Elsevier Ltd. All rights reserved.

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See detailMultisensor satellite monitoring of seawater state and oil pollution in the north-eastern coastal zone of the Black Sea
Karimova, Svetlana; Lavrova, Olga; Mityagina, Marina

in International Journal of Remote Sensing (2008), 29(21), 6331-6345

A new approach aimed at a better understanding of the state of pollution of the Black Sea coastal zone is suggested. It consists of the combined use of all available quasi‐concurrent satellite information (NOAA AVHRR, TOPEX/Poseidon, Jason‐1, Terra/Aqua MODIS, Envisat ASAR, ERS‐2 SAR and QuikSCAT) and was first applied during an operational seawater monitoring campaign in the coastal zone of the northeastern Black Sea conducted in 2006. The monitoring is based on daily receiving, processing and analysis of data different in nature (microwave radar images, optical and infrared data), resolution and surface coverage. These data allow us to retrieve information on seawater pollution, sea surface and air–sea boundary layer conditions, seawater temperature and suspended matter distributions, chlorophyll‐a concentration, mesoscale water dynamics, near‐surface wind, and surface wave fields. Such an approach helps in oil spill detection with synthetic aperture radar (SAR), especially in distinguishing oil slicks from look‐alikes. The focus is on coastal seawater circulation mechanisms and their impact on the evolution of pollutants.

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See detailForecast verification of a 3D model of the Mediterranean Sea. The use of discrete wavelet transforms and EOFs in the skill assessment of spatial forecasts
Alvera Azcarate, Aïda; Barth, Alexander; Ben Bouallegue, Zied; Rixen, Michel; Beckers, Jean-Marie

in Journal of Marine Systems (2007), 65(1-4), 460-483

The quality assessment of a nested model system of the Mediterranean Sea is realised. The model has two zooms in the Provencal Basin and in the Ligurian Sea, realised with a two-way nesting approach. The experiment lasts for nine weeks, and at each week sea surface temperature (SST) and sea level anomaly are assimilated. The quality assessment of the surface temperature is done in a spatio-temporal approach, to take into account the high complexity of the SST distribution. We focus on the multi-scale nature of oceanic processes using two powerful tools for spatio-temporal analysis, wavelets and Empirical Orthogonal Functions (EOFs). We apply two-dimensional wavelets to decompose the high-resolution model and observed SST into different spatial scales. The Ligurian Sea model results are compared to observations at each of those spatial scales, with special attention on how the assimilation affects the model behaviour. We also use EOFs to assess the similarities between the Mediterranean Sea model and the observed SST. The results show that the assimilation mainly affects the model large-scale features, whereas the small scales show little or no improvement and sometimes, even a decrease in their skill. The multiresolution analysis reveals the connection between large- and small-scale errors, and how the choice of the maximum correlation length of the assimilation scheme affects the distribution of the model error among the different spatial scales. (c) 2006 Elsevier B.V. All rights reserved.

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See detailApplication of a SEEK filter to a 1D biogeochemical model of the Ligurian Sea: Twin experiments and real in-situ data assimilation
Raick, Caroline; Alvera Azcarate, Aïda; Barth, Alexander; Brankart, J. M.; Soetaert, K.; Grégoire, Marilaure

in Journal of Marine Systems (2007), 65(1-4), 561-583

The Singular Evolutive Extended Kalman (SEEK) filter has been implemented to assimilate in-situ data in a 1D coupled physical-ecosystem model of the Ligurian Sea. The biogeochemical model describes the partly decoupled nitrogen and carbon cycles of the pelagic food web. The GHER hydrodynamic model (1D version) is used to represent the physical forcings. The data assimilation scheme (SEEK filter) parameterizes the error statistics by means of a set of empirical orthogonal functions (EOFs). Twin experiments are first performed with the aim to choose the suitable experimental protocol (observation and estimation vectors, number of EOFs, frequency of the assimilation,...) and to assess the SEEK filter performances. This protocol is then applied to perform real data assimilation experiments using the DYFAMED data base. By assimilating phytoplankton observations, the method has allowed to improve not only the representation of the phytoplankton community, but also of other variables such as zooplankton and bacteria that evolve with model dynamics and that are not corrected by the data assimilation scheme. The validation of the assimilation method and the improvement of model results are studied by means of suitable error measurements. (c) 2006 Elsevier B.V. All rights reserved.

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See detailMultigrid state vector for data assimilation in a two-way nested model of the Ligurian Sea
Barth, Alexander; Alvera Azcarate, Aïda; Beckers, Jean-Marie; Rixen, Michel; Vandenbulcke, Luc

in Journal of Marine Systems (2007), 65(1-4), 41-59

A system of two nested models composed by a coarse resolution model of the Mediterranean Sea, an intermediate resolution model of the Provencal Basin and a high resolution model of the Ligurian Sea is coupled with a Kalman-filter based assimilation method. The state vector for the data assimilation is composed by the temperature, salinity and elevation of the three models. The forecast error is estimated by an ensemble run of 200 members by perturbing initial condition and atmospheric forcings. The 50 dominant empirical orthogonal functions (EOF) are taken as the error covariance of the model forecast. This error covariance is assumed to be constant in time. Sea surface temperature (SST) and sea surface height (SSH) are assimilated in this system. (c) 2006 Elsevier B.V. All rights reserved.

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See detailFiltering inertia-gravity waves from the initial conditions of the linear shallow water equations
Barth, Alexander; Beckers, Jean-Marie; Alvera Azcarate, Aïda; Weisberg, R.

in Ocean Modelling (2007), 19(3-4), 204-218

A method for filtering inertia-gravity waves from elevation and depth-averaged velocity is described. This filtering scheme is derived from the linear shallow water equations for constant depth and constant Coriolis frequency. The filtered solution is obtained by retaining only the eigenvectors corresponding to the geostrophic equilibrium and by discarding explicitly the eigenvectors corresponding to the fast moving inertia-gravity waves. An alternative formulation is derived using a variational approach. Both filtering methods are tested numerically for a periodic domain with constant depth and the variational approach is implemented for a closed domain with large topographic variations. The filtering methods significantly reduce the amplitudes of the inertia-gravity waves while preserving the mean flow. The variational method is compared to the Incremental Analysis Update technique and the benefits of the variational filter are presented. (C) 2007 Elsevier Ltd. All rights reserved.

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See detailMultivariate reconstruction of missing data in sea surface temperature, chlorophyll, and wind satellite fields
Alvera Azcarate, Aïda; Barth, Alexander; Beckers, Jean-Marie; Weisberg, Robert H

in Journal of Geophysical Research. Oceans (2007), 112(C3), 03008

An empirical orthogonal function–based technique called Data Interpolating Empirical Orthogonal Functions (DINEOF) is used in a multivariate approach to reconstruct missing data. Sea surface temperature (SST), chlorophyll a concentration, and QuikSCAT winds are used to assess the benefit of a multivariate reconstruction. In particular, the combination of SST plus chlorophyll, SST plus lagged SST plus chlorophyll, and SST plus lagged winds have been studied. To assess the quality of the reconstructions, the reconstructed SST and winds have been compared to in situ data. The combination of SST plus chlorophyll, as well as SST plus lagged SST plus chlorophyll, significantly improves the results obtained by the reconstruction of SST alone. All the experiments correctly represent the SST, and an upwelling/downwelling event in the West Florida Shelf reproduced by the reconstructed data is studied.

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See detailRelationship between the evolution of the shoreline and the Posidonia oceanica meadow limit in a Sardinian coastal zone
Tigny, V.; Ozer, André; De Falco, G.; Baroli, M.; Djenidi, Salim

in Journal of Coastal Research (2007), 23(3), 787-793

Important environmental changes have been observed for some coastal processes in the Gulf of Oristano (west coast of Sardinia, Italy). With remote sensing as the principal tool, this study aims to assess littoral evolution over time (1977-2000) and to evaluate whether there is a relationship between the evolutionary trend of the shoreline and that of the upper limit of the Posidonia oceanica (L.) Delile meadow. Results show that some portions of the coastline, mainly located on the sandy part of the littoral, have encountered significant variations that can be partially related to the evolution of the upper limit of the P. oceanica meadow. Mainly of natural origins, this evolutionary trend has also been related to anthropogenic pressures put on the seagrass meadow. This finding confirms that Posidonia meadows significantly affect the littoral geomorphology, providing biogenic sediments, controlling beach slope, and acting as a '' brake '' on coastal water masses.

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See detailStudy of the combined effects of data assimilation and grid nesting in ocean models – application to the Gulf of Lions
Vandenbulcke, Luc; Barth, Alexander; Rixen, Michel; Alvera Azcarate, Aïda; Ben Bouallegue, Zied; Beckers, Jean-Marie

in Ocean Science (2006), 2

Modern operational ocean forecasting systems routinely use data assimilation techniques in order to take observations into account in the hydrodynamic model. Moreover, as end users require higher and higher resolution predictions, especially in coastal zones, it is now common to run nested models, where the coastal model gets its open-sea boundary conditions from a low-resolution global model. This configuration is used in the "Mediterranean Forecasting System: Towards environmental predictions" (MFSTEP) project. A global model covering the whole Mediterranean Sea is run weekly, performing 1 week of hindcast and a 10-day forecast. Regional models, using different codes and covering different areas, then use this forecast to implement boundary conditions. Local models in turn use the regional model forecasts for their own boundary conditions. This nested system has proven to be a viable and efficient system to achieve high-resolution weekly forecasts. However, when observations are available in some coastal zone, it remains unclear whether it is better to assimilate them in the global or local model. We perform twin experiments and assimilate observations in the global or in the local model, or in both of them together. We show that, when interested in the local models forecast and provided the global model fields are approximately correct, the best results are obtained when assimilating observations in the local model.

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See detailCoupling a two-way nested primitive equation model and a statistical SST predictor of the Ligurian Sea via data assimilation
Barth, Alexander; Alvera Azcarate, Aïda; Beckers, Jean-Marie; Rixen, Michel

in Ocean Modelling (2006), 13(3-4), 255-270

A primitive equation model and a statistical predictor are coupled by data assimilation in order to combine the strength of both approaches. In this work, the system of two-way nested models centred in the Ligurian Sea and the satellite-based ocean forecasting (SOFT) system predicting the sea surface temperature (SST) are used. The data assimilation scheme is a simplified reduced order Kalman filter based on a constant error space. The assimilation of predicted SST improves the forecast of the hydrodynamic model compared to the forecast obtained by assimilating past SST observations used by the statistical predictor. This study shows that the SST of the SOFT predictor can be used to correct atmospheric heat fluxes. Traditionally this is done by relaxing the model SST towards the climatological SST. Therefore, the assimilation of SOFT SST and climatological SST are also compared. (c) 2006 Elsevier Ltd. All rights reserved.

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See detailDINEOF reconstruction of clouded images including error maps. Application to the Sea-Surface Temperature around Corsican Island
Beckers, Jean-Marie; Barth, Alexander; Alvera Azcarate, Aïda

in Ocean Science (2006), 2

We present an extension to the Data INterpolating Empirical Orthogonal Functions (DINEOF) technique which allows not only to fill in clouded images but also to provide an estimation of the error covariance of the reconstruction. This additional information is obtained by an analogy with optimal interpolation. It is shown that the error fields can be obtained with a clever rearrangement of calculations at a cost comparable to that of the interpolation itself. The method is presented on the reconstruction of sea-surface temperature in the Ligurian Sea and around the Corsican Island (Mediterranean Sea), including the calculation of inter-annual variability of average surface values and their expected errors. The application shows that the error fields are not only able to reflect the data-coverage structure but also the covariances of the physical fields.

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See detailPlankton dynamics controlled by hydrodynamic processes near a submarine canyon off NW corsican coast: A numerical modelling study
Skliris, Nikolaos; Djenidi, Salim

in Continental Shelf Research (2006), 26(11), 1336-1358

A three-dimensional (3D) non-linear high-resolution hydrodynamic model coupled to a coastal plankton ecosystem model is used to estimate the impact of hydrodynamic processes on the evolution of the spring phytoplankton bloom in the vicinity of a submarine canyon. Model results for the plankton distribution showed a clear 3D character around and in the canyon, with large horizontal and vertical gradients, induced by the hydrodynamic constraints. Phytoplankton concentrations were significantly larger all along the slope domain with maximum values obtained over the canyon. Upwelling of deep water rich in nitrate takes place both upstream (with respect to the current direction normal to the central axis of the canyon) and downstream of the canyon enhancing primary production. As phytoplankton-rich water enters into the western part of the canyon it is downwelled and trapped by the cyclonic circulation leading to accumulation of phytoplankton biomass there. The effect of wind events was to induce an upward nitrate flux into the upper layer through vertical turbulent diffusion, allowing the start of a short-live phytoplankton bloom. Maximum surface nitrate concentrations were found along the slope and particularly upstream and downstream of the canyon just after the wind stopped. Enhanced turbulent diffusion combined with upwelling motion in these areas resulted in larger upward nitrate transports, further enhancing primary production. (c) 2006 Elsevier Ltd. All rights reserved.

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See detailOn the behaviour of the residence time at the bottom of the mixed layer
Deleersnijder, Eric; Beckers, Jean-Marie; Delhez, Eric

in Environmental Fluid Mechanics (2006), 6(6), 541-547

To understand why the findings of Deleersnijder et al. [(2006), Environ Fluid Mech 6: 25-42]-the residence time in the mixed layer in not necessarily zero at the pycnocline-are consistent with those of Delhez and Deleersnijder [(2006), Ocean Dyn 56:139-150]-the residence time in a control domain vanishes at the open boundaries of this control domain-, it is necessary to consider a control domain that includes part of the pycnocline, in which the eddy diffusivity is assumed to be zero. Then, depending on the behaviour of the eddy diffusivity near the bottom of the mixed layer, the residence time may be seen to exhibit a discontinuity at the interface between the mixed layer and the pycnocline. If such a discontinuity exists, the residence time is non-zero in the former and zero in the latter. This is illustrated by analytical solutions obtained under the assumption that the eddy diffusivity is constant in the mixed layer.

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See detailThe residence time of settling particles in the surface mixed layer
Deleersnijder, Eric; Beckers, Jean-Marie; Delhez, Eric

in Environmental Fluid Mechanics (2006), 6(1), 25-42

The transport from the upper mixed layer into the pycnocline of particles with negative buoyancy is considered. Assuming the hydrodynamic parameters to be time-independent, an adjoint model is resorted to that provides a general expression of the residence time in the mixed layer of the constituent under study. It is seen that the residence time decreases as the settling velocity increases or the diffusivity decreases. Furthermore, it is demonstrated that the residence time must be larger than z/w and smaller than h/w, where z, h and w denote the distance to the pycnocline, the thickness of the mixed layer and the sinking velocity. In the vicinity of the pycnocline, the residence time is not necessarily zero; its behaviour critically depends on the eddy diffusivity pro. le in this region. Closed-form solutions are obtained for constant and quadratic diffusivity profiles, which allows for an analysis of the sensitivity of the residence time to the Peclet number. Finally, an approximate value is suggested of the depth-averaged value of the residence time.

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See detailReconstruction of incomplete oceanographic data sets using empirical orthogonal functions: application to the Adriatic Sea surface temperature
Alvera Azcarate, Aïda; Barth, Alexander; Rixen, Michel; Beckers, Jean-Marie

in Ocean Modelling (2005), 9(4), 325-346

A method for the reconstruction of missing data based on an EOF decomposition has been applied to a large data set, a test case of Sea Surface Temperature satellite images of the Adriatic Sea. The EOF decomposition is realised with a Lanczos method, which allows optimising computational time for large matrices. The results show that the reconstruction method leads to accurate reconstructions as well as a low cpu time when dealing with realistic cases. The method has been tested with different amounts of missing data, artificially adding clouds ranging from 40% to 80% of data loss, and then compared to the same data set with no missing data. A comparison with in situ data has also been made. These validation studies show that results are robust, even when the amount of missing data is very high. The reconstruction of the data from the Adriatic Sea shows realistic features and a reliable temperature distribution. In addition, the method is compared to an Optimal Interpolation reconstruction. The results obtained with both methods are very similar. The main difference is the computational time, which is reduced nearly 30 times with the method presented here. Once the reconstruction has been performed, the EOF decomposition is analysed to show the method's reliability, and a cold event on the Albanian coast is studied. The reconstructed data reflect the effect of wind on the Albanian coast, that led to a cold-water episode in this zone for a 6-day period. (c) 2004 Elsevier Ltd. All rights reserved.

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See detailTwo-way nested model of mesoscale circulation features in the Ligurian Sea
Barth, Alexander; Alvera Azcarate, Aïda; Rixen, Michel; Beckers, Jean-Marie

in Progress in Oceanography (2005), 66(2-4), 171-189

A coarse resolution primitive equation model of 1/4 degrees resolution is implemented covering the whole Mediterranea Sea. Within this grid a 1/20 degrees resolution model of the Liguro-Provencal basin and the northern part of the Tyrrhenian Sea is embedded. A third fine resolution model of 1/60 degrees is nested in the latter one and simulates the dynamics of the Ligurian Sea. Comparisons between one-way and two-way nesting in simulating the Northern Current (NC) are made. The properties of the Eastern and Western Corsican Current and the Northern Current are investigated with this nesting system. Special attention is given to the variability of the NC. Meanders and interactions with Winter Intermediate Water lenses are shown. Topographic features also lead to a highly variable NC. (c) 2005 Elsevier Ltd. All rights reserved.

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See detailA hydrographic and bio-chemical climatology of the Mediterranean and the Black Sea: a technical note on the use of coastal data
Rixen, M.; Beckers, Jean-Marie; Maillard, C.; MEDAR Group

in Bollettino di Geofisica Teorica ed Applicata (2005), 46

The aim of the MEDAR/MEDATLAS II project was to archive and rescue multidisciplinary in-situ hydrographic and bio-chemical data of the Mediterranean and the Black Sea through a wide cooperation of countries and to produce a climatological atlas of 12 core parameters, which include temperature and salinity, dissolved oxygen, hydrogen sulphur, alkalinity, phosphate, ammonium, nitrite, nitrate, silicate, chlorophyll and pH. Gridded fields have been computed using the Variational Inverse Model and calibrated by Generalized Cross Validation, by making usual assumptions on the statistical distribution of data and errors. They have been produced for both the Mediterranean and the Black Sea and several additional sub-basins including the Alboran Sea, the Balearic Sea, the Gulf of Lyons and the Ligurian Sea, the Sicily Strait, the Adriatic Sea, the Aegean Sea, the Marmara Sea and the Danube shelf area at climatic, seasonal and monthly scale when relevant. Inter-annual and decadal variability of T/S for both basins have been computed as well. The resulting atlas is made available, free of charge, at "http://modb.oce.ulg.ac.be/Medar" and on CD-Rom. We discuss here some technical issues relating to the use of coastal data in the objective analysis and provide a few examples of features appearing in the climatology. We finally suggest further possible improvements to the analysis method.

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See detailImpact implantation of a transmitter on Sarpa salpa behaviour: study with a computerized video tracking system
Jadot, Catherine; Donnay, Annick; Ylieff, Marc; Poncin, Pascal

in Journal of Fish Biology (2005), 67(2), 589-595

Two transmitter masses (2 and 6% of the fish's mass) were selected to examine the interference of tags with the behaviour of Sarpa salpa using a computerized video tracking system based on digital imaging techniques. The study demonstrated that light transmitters had no effect on the behavioural variables studied, and a substantial bias in behaviour is introduced if heavier (6%) tags are used. (c) 2005 The Fisheries Society of the British Isles.

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See detailWind driven upwelling along the African coast of the Strait of Gibraltar
Stanichny, Sergey; Tigny, V.; Stanichnaya, R.; Djenidi, Salim

in Geophysical Research Letters (2005), 32(L04604),

Regular remote sensing data from various sensors are used here for the study of the wind driven upwelling phenomenon along the African coast of the Strait of Gibraltar. It is shown for an extended summer period (May 15 till September 15, 2003) that sea surface temperature (SST) data in the strait are correlated with NCEP winds, each westward wind increase being followed by a clear surface temperature decrease. Local surface temperature of about 22degreesC at that time drops down to 15degreesC, value corresponding to the 80 - 120 m depth conditions. The analysis of subsequent images indicates that the cold upwelling plume typically moves first to the Atlantic during wind forcing, and then to the Mediterranean after the wind event. The presence of the northern coast of the strait is taken as responsible for a rise of a cross-strait sea level gradient and the enhancement of the associated westward geostrophic current that explains the first stage of the plume deployment. Sea level difference measured between Tarifa (European coast) and Ceuta (African coast), well described by a linear equation in term of the westward wind component, supports this idea as well as the subsequent remotely sensed SST distributions.

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See detailInfluence of the Aral Sea negative water balance on its seasonal circulation patterns: use of a 3D hydrodynamic model
Sirjacobs, Damien; Grégoire, Marilaure; Delhez, Eric; Nihoul, J. C. J.

in Journal of Marine Systems (2004), 47(1-4), 51-66

A 3D hydrodynamic model of the Aral Sea was successfully implemented to address the complex hydrodynamic changes induced by the combined effect of hydrologic and climatic change in the Aral region. The first barotropic numerical experiments allowed us to produce a comparative description of the mean general seasonal circulation patterns corresponding to the original situation (1956-1960) and of the average situation for the period from 1981 to 1985, a very low river flow period. The dominant anticyclonic circulation suggested by our seasonal simulation is in good agreement with previous investigations. In addition. this main anticyclonic gyre was shown to be stable and clearly established from February to September, while winter winds led to another circulation scenario. In winter, the main anticyclonic gyre was considerably limited, and cyclonic circulations appeared in the deep western basin and in the northeast of the shallow basin. In contrast, stronger anticyclonic circulation was observed in the Small Aral Sea during winter. As a consequence of the 10-m sea level drop observed between the two periods considered, the 1981-1985 simulation suggests an intensification of seasonal variability. Total water transport of the main gyre was reduced with sea level drop by a minimum of 30% in May and up to 54% in September. Before 1960, the study of the net flows through Berg and Kokaral Straits allowed us to evaluate the component of water exchange between the Small and the Large Seas linked with the general anticyclonic circulation around Kokaral Island. This exchange was lowest in summer (with a mean anticyclonic exchange of 222 m(3)/s for July and August), highest in fall and winter (with a mean value of 1356 m(3)/s from September to February) and briefly reversed in the spring (mean cyclonic circulation of 316 m(3)/s for April and May). In summer, the water exchange due to local circulation at the scale of each strait was comparatively more important because net flows through the straits were low. After about 20 years of negative water balance, the western Kokaral Strait was dried up and the depth of Berg Strait was reduced from 15 to 5 m. Simulation indicated a quasi-null net transport, except during the seasonal modification of the circulation pattern, in February and October. A limited, but stable, water exchange of about 100 m(3)/s remained throughout the year, as a result of the permanent superposition of opposite currents. (C) 2004 Elsevier B.V. All rights reserved.

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See detailEffects of extreme meteorological conditions on coastal dynamics near a submarine canyon
Skliris, Nikolaos; Lacroix, Geneviève; Djenidi, Salim

in Continental Shelf Research (2004), 24(9), 1033-1045

A 3-D hydrodynamic model is applied to assess shelf/slope exchanges in the Calvi Canyon region (Corsica, NW Mediterranean) during the violent storm that affected the Western Europe in December 1999. Simulations are carried out using high-frequency sampling meteorological data to take into account the short-term variability of the atmospheric conditions. It is shown that the combined effects of canyon topography and of the wind forcing during the storm are responsible for a large increase of both cross-shore and vertical transports in the area. Strong downwelling motion is simulated all along the continental slope with vertical velocities up to 2cm s(-1) within the canyon. High turbulent diffusion levels are obtained leading to the complete mixing of the water column within the canyon. Results suggest that increased turbulent diffusion and downwelling circulation in the canyon during the storm should result in a large transport of coastal water towards the abyssal plain. (C) 2004 Elsevier Ltd. All rights reserved.

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See detailRemotely sensed seasonal dynamics of phytoplankton in the Ligurian Sea in 1997-1999
Nezlin, N. P.; Lacroix, G.; Kostianoy, A. G.; Djenidi, Salim

in Journal of Geophysical Research. Oceans (2004), 109(C07013),

[1] Remotely sensed data and a one-dimensional hydrophysical model were used to study the seasonal dynamics of surface plant pigments concentration in the Ligurian-Provencal basin. The variations of phytoplankton biomass were estimated from the observations of the Coastal Zone Color Scanner ( 1978 - 1986) and Sea-viewing Wide Field-of-view Sensor (SeaWiFS) ( September 1997 to October 1999) radiometers. The factors of physical environment analyzed included remotely sensed sea surface temperature ( from advanced very high resolution radiometers), wind, air temperature, and atmospheric precipitation. The Geohydrodynamics and Environment Research (GHER) model was used to explain the observed correlations between the physical forcing and the response of phytoplankton biomass. The general pattern of phytoplankton seasonal dynamics was typical to subtropical areas: maximum biomass during cold season from October to April and low biomass during summer months. The intensity of winter/spring bloom significantly varied during different years. The correlation was revealed between the summer/autumn air temperature contrast ( expressed as the difference between the air temperatures in August and in November) and the maximum monthly averaged surface chlorophyll concentration during the subsequent winter/spring bloom. The features of seasonal dynamics of phytoplankton are regulated by the physical impacts influencing water stratification. The difference between two seasonal cycles ( from September 1997 to October 1999) illustrates the response of phytoplankton growth to local meteorological conditions. In March - April 1999 the vernal bloom was much more pronounced; it resulted from deeper winter cooling and more intensive winter convection. Heating of surface water layer, wind mixing, and freshwater load with rains and river discharge either stimulate or depress the development of phytoplankton, depending on what limiting environmental factor ( light or nutrient limitation) prevailed.

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See detailAtmospheric CO2 flux from mangrove surrounding waters
Borges, Alberto; Djenidi, Salim; Lacroix, Geneviève; Theate, Jean-Marie; Delille, Bruno; Frankignoulle, Michel

in Geophysical Research Letters (2003), 30(11),

[1] The partial pressure of CO2 (pCO(2)) was measured at daily and weekly time scales in the waters surrounding mangrove forests in Papua New Guinea, the Bahamas and India. The pCO(2) values range from 380 to 4800 muatm. These data, together with previously published data, suggest that overall oversaturation of CO2 with respect to atmospheric equilibrium in surface waters is a general feature of mangrove forests, though the entire ecosystems (sediment, water and vegetation) are probably sinks for atmospheric CO2. The computed CO2 fluxes converge to about +50 mmolC m(-2) day(-1). If this conservative value is extrapolated for worldwide mangrove ecosystems, the global emission of CO2 to the atmosphere is about 50 10(6) tC year(-1). Based on this tentative estimate, mangrove waters appear to be regionally a significant source of CO2 to the atmosphere and should be more thoroughly investigated, especially at seasonal time scale.

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See detailAlong or across front survey strategy? An operational example at an unstable front
Rixen, Michel; Allen, J. T.; Alderson, S.; Cornell, V.; Crisp, N.; Fielding, S.; Mustard, A. T.; Pollard, R. T.; Popova, E. E.; Smeed, D. A.; Srokosz, M. A.; Barth, Alexander; Beckers, Jean-Marie

in Geophysical Research Letters (2003), 30(1),

[1] We present results of the optimization of near-real time on-board sampling strategy in the Iceland-Faroes oceanic frontal area, based on the outputs of a mesoscale 3D operational data assimilation forecasting experiment. By minimizing a root mean square error cost function, we show that in this example an along-front sampling strategy, i.e. with transects parallel to the front, produces smaller errors in temperature, salinity, nitrate, phytoplankton, and zooplankton fields, as a result of a combination of the direction of the sampling of the front and errors associated with the asynopticy of observations (Doppler effect). This is contrary to the classic across-front sampling strategies that are used in most field experiments reported in the literature, i.e. where transects are perpendicular to the front. A control model shows that at these spatio-temporal scales, the along front sampling strategy is optimal when the frontal instability has sufficiently developed.

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See detailAlong or across front ocean survey strategy? The estimation of quasi-geostrophic vertical velocities and temperature fluxes
Rixen, Michel; Allen, J. T.; Pollard, R. T.; Beckers, Jean-Marie

in Geophysical Research Letters (2003), 30(5), 61-68

In a companion paper we have shown that at the mesoscale, the unusual across front sampling strategy (AL) is more accurate than the usual across front sampling strategy (AC) on hydrographic and bio-chemical properties when the front is sufficiently developed. The cruise design based on the optimal reconstruction of the density fields does however not imply that derived variables like quasi-geostrophic (QG) vertical velocities and temperature fluxes also exhibit minimal associated errors. Here we present results of optimized sampling strategies for diagnostic QG vertical velocities (wQG) and temperature fluxes (θQG) derived from the omega equation. Results are illustrated in the same framework, for the Iceland-Faroes oceanic frontal area and for a control model and reveal that at these spatio-temporal scales, the unusual AL may also provide better estimations of vertical velocities and temperature fluxes compared to the classic AC, especially when the front is sufficiently developed.

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See detailModelling eutrophication in mesotidal and macrotidal estuaries. The role of intertidal seaweeds.
Alvera Azcarate, Aïda; Ferreira, Joao; Nunes, Joao

in Estuarine Coastal and Shelf Science (2003), 57

The role of intertidal seaweeds in the primary production of mesotidal and macrotidal estuaries has been examined by means of a model, applied to the Tagus Estuary (Portugal). Special attention was paid to the description of the underwater light climate in intertidal areas, and to the importance of the formation of tidal pools. Two approaches were compared for the simulation of suspended particulate matter (SPM) in the pool areas, using three algal species. The use of an erosion–deposition approach to simulate the distribution of SPM in tidal pools gives an increase in net primary productivity per unit area of between 130 and 1300%, when compared to the more conventional approach where the suspended matter in the overlying water in intertidal areas is considered identical to that in the channels. The upscaled erosion–deposition model was applied to tidal pool areas and combined with the more conventional model for other intertidal areas. Results show that annual carbon fixation by intertidal seaweeds in the estuary exceeds 13,500 t C yr−1, and accounts for 21% of the total carbon fixed by all primary producers. The corresponding nitrogen removal by seaweeds corresponds to the annual nutrient loading of a population of 490,000 inhabitants.

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See detailA model of the seasonal dynamics of biomass and production of the seagrass Posidonia oceanica in the Bay of Calvi (Northwestern Mediterranean)
Elkalay, Khalid; Frangoulis, Constantin; Skliris, Nikolaos; Goffart, Anne; Gobert, Sylvie; Lepoint, Gilles; Hecq, Jean-Henri

in Ecological Modelling (2003), 167(1-2), 1-18

Modelling of seagrasses can be an effective tool to assess factors regulating their growth. Growth and production model of Posidonia oceanica, the dominant submerged aquatic macrophyte occurring in the Bay of Calvi (Corsica, Ligurian Sea, Northwestern (NW) Mediterranean) was developed. The state variables are the above- and below-ground biomass of P oceanica, the epiphyte biomass, and the internal nitrogen concentration of the whole plant. Light intensity and water temperature are the forcing variables. The model reproduces successfully seasonal growth and production for each variable at various depths (10, 20 and 30 m). The model can simulate also a number of consecutive years. Sensitivity analysis of model's parameters showed that the maximum nitrogen quota n(max) rate is the most sensitive parameter in this model. The results simulations imply that light intensity is one of the most important abiotic factors, the diminution of which can cause an important reduction in seagrass density. (C) 2003 Elsevier B.V. All rights reserved.

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See detailA note on the age of radioactive tracers
Delhez, Eric; Deleersnijder, Eric; Mouchet, Anne; Beckers, Jean-Marie

in Journal of Marine Systems (2003), 38(3-4), 277-286

wThe age of a water mass is often estimated experimentally using the radio-age computed from the distribution of a radioactive tracer (radiocarbon, helium-tritium). Deleersnijder et al. [J. Mar. Syst. 28 (2001) 229.] have shown that the radio-age underestimates the age of the water and is larger than the age of the radioactive tracer used for its evaluation. This result is generalized here to radio-ages computed from the ratio of two radioactive tracers. The differences between the different ages are also studied analytically and numerically as functions of the decay rate of the radioactive tracers. For small decay rates, the difference between the age of the water mass and the radio-age is shown to be proportional to the decay rate. It depends also on the level of mixing in the system; even radioactive tracers with small decay rates can provide poor estimates of the age of the water mass in a strongly diffusive flow. For small half lives, both the radio-age and the age of radioactive tracers decrease as the inverse of the square root of the decay rate. The same analysis applies to some extent to the estimates of the age of a water mass from stable tracers with known time dependent sources (e.g. chloroflurocarbons). (C) 2002 Elsevier Science B.V. All rights reserved.

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See detailWater fluxes at an ocean margin in the presence of a submarine canyon
Skliris, Nikolaos; Hecq, Jean-Henri; Djenidi, Salim

in Journal of Marine Systems (2002), 32(1-3), 239-251

A 3-D, unsteady, nonlinear, high-resolution model is used to estimate shelf/slope exchanges through Calvi Canyon (NW Corsica, Mediterranean Sea) in various regimes of stratification and wind patterns. To evaluate the alongshore and cross-shore fluxes within the canyon area as well as the water exchanges between the canyon and Calvi Bay, volume transports are computed at the sides of two closed, interconnected boxes encompassing the canyon on the shelf and slope domains. Model results show that water transports between Calvi Bay and the open sea are determined by flow modifications in the canyon area. The mean horizontal flow deviates southwestward upstream of the canyon, generating an onshore transport in the western part of Calvi Bay. Within the canyon, the circulation is cyclonic and is responsible for an offshore transport downstream of the canyon and in the eastern part of the bay. The effect of stratification is shown to limit the vertical extent of the influence of canyon topography so that the alongshore flow above the canyon is quasi-undisturbed in strong stratified conditions, resulting in weak cross-shore exchange. Wind events are shown to be responsible for a strong increase of cross-shore transports between the bay and the canyon area. (C) 2002 Elsevier Science B.V All rights reserved.

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See detailTransient behaviour of water ages in the World Ocean
Deleersnijder, Eric; Mouchet, Anne; Delhez, Eric; Beckers, Jean-Marie

in Mathematical and Computer Modelling (2002), 36(1-2), 121-127

The transient behaviour of the age of the water and that of the surface water is studied in the World Ocean. At any time and position, the age of the water, the age and the concentration of the surface water are seen to obey a simple algebraic relation. The latter is illustrated by means of results of a three-dimensional World Ocean model and the analytical solution of an idealised, purely-diffusive, one-dimensional problem. (C) 2002 Elsevier Science Ltd. All rights reserved.

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See detailNon-synoptic versus pseudo-synoptic data sets: an assimilation experiment
Rixen, Michel; Allen, J. T.; Beckers, Jean-Marie

in Journal of Marine Systems (2001), 29(1-4), 313-333

Several first-order correction methods are implemented to compute pseudo-synoptic data sets from non-synoptic raw data sets. These include a geostrophic relocation method, a linear and a quadratic interpolation method, and a method using spatio-temporal correlation functions. The relocation method involves analyses and geostrophic velocity computations to allow the relocation of stations in time and space to a particular analysis time. Interpolation methods involve several almost identical and consecutive surveys interpolated in time. Temporal weighting methods are based upon a spatio-temporal function modifying the weight on data with respect to the time at which they have been sampled. These techniques are tested on the OMEGA data set and are validated by simple nudging assimilation into a 3D primitive equation model (PE). It is shown that, under certain hypothesis, these methods are able to correct the lack of synopticity in hydrographic data sets, and improve the diagnosis of vertical velocities computed from the Omega equation. These methods are of particular interest for the scientific community. They might be used together with diagnostic models. They might provide suitable pseudo-synoptic fields required by 3D PE models as initial conditions, boundary conditions or assimilation data sets. They may also be useful in the design of mesoscale samplings. (C) 2001 Elsevier Science B.V. All rights reserved.

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See detailSolar Photocatalytic Destruction of p-Nitrophenol: A Pedagogical Use of Lab Wastes
Herrera-Melián, J. A.; Doña-Rodríguez, J. M.; Tello Rendón, E.; Soler Vila, A.; Brunet Quetglas, M.; Alvera Azcarate, Aïda; Pascual Pariente, L.

in Journal of Chemical Education (2001), 78(6), 775-777

In this article we propose the destruction of p-nitrophenol wastes obtained in a previous lab experiment, to generate a new lab experiment. The recommended destruction technique is solar TiO2-photocatalysis. When the effects of TiO2 and sunlight are tested separately, a slight decrease of p-nitrophenol is observed; but when TiO2 and sunlight are employed together p-nitrophenol disappears in 1 or 2 h. These experiments do not require sophisticated equipment or special lab training for the students. Groups of students tested different experimental conditions, such as the effect of different light intensities and sources (800-W UV-lamp, sunlight on sunny days, and sunlight on a cloudy day) or the addition of H2O2. p-Nitrophenol is degraded under the three light sources but at different rates. The addition of H2O2 to the TiO2-plus-sunlight system enhances p-nitrophenol degradation kinetics when compared with the TiO2 plus sunlight and H2O2 plus sunlight combinations.

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See detailOn the symmetry of the age field of a passive tracer released into a one-dimensional fluid flow by a point-source
Deleersnijder, E.; Delhez, Eric; Crucifix, M.; Beckers, Jean-Marie

in Bulletin de la Société Royale de Liège (2001), 70

Tracer is released from a point-source into an incompressible, one-dimensional fluid flow, with constant velocity and diffusivity. The age of a tracer parcel, which is defined as the time elapsed since leaving the source, may be evaluated as the ratio of the age concentration to the tracer concentration. The latter are governed by two partial differential equations. Time-dependent analytical solutions are derived, which show that the age is symmetric with respect to the source. This is astonishing, since it could have been expected that the age would reflect somehow the strong asymmetry of the tracer concentration, which tends to be much larger on the downstream side of the source than on the upstream side. Some finite-difference counterparts of this problem are seen to lead to age fields which, in their steady-state limit, are also symmetric with respect to the source. This is believed to be helpful to interpret the results of numerical models of complex fluid flows in which the age is introduced as a diagnostic variable.

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See detailMetal fluxes through the Strait of Gibraltar: the influence of the Tinto and Odiel rivers (SW Spain)
Elbaz-Poulichet, F.; Morley, N. H.; Beckers, Jean-Marie; Nomerange, P.

in Marine Chemistry (2001), 73(3-4), 193-213

A large set of new data concerning dissolved metal concentrations has been acquired in the Gulf of Cadiz and in the Strait of Gibraltar from 1996 to 1999. These data, associated with models (hydrodynamic, tracer advection-dispersion and mixing), have been used to assess the influence of rivers draining the South Iberian Pyrite Belt on the Gulf of Cadiz and on the Atlantic inflow in the Strait of Gibraltar. Metal concentrations in surface waters from the Gulf of Cadiz are maximal near the mouth of the Tinto/Odiel rivers with values exceeding 50 nmol/kg (Mn), 5 nmol/kg (Ni), 30 nmol/kg (Cu), 100 nmol/kg (Zn), 0.9 nmol/kg (Cd) and 45 nmol/kg (As). From the Tinto/Odiel river, a plume of contamination follows the coast in the direction of the Strait of Gibraltar. The computation of a tracer advection-dispersion model confirms that the coastal currents carry the metals discharged from the Tinto and Odiel to the Strait of Gibraltar. From temperature-salinity and metal-salinity plots, four water masses can be recognised in the Gulf of Cadiz and in the Strait of Gibraltar: North Atlantic Surface Water (NASW), North Atlantic Central Water (NACW) and metal-enriched Spanish Shelf Waters from the Gulf of Cadiz (SSW). The Mediterranean Outflow Water (MOW) is also clearly seen at depths greater than 300 m. The chemical characteristics of these various water masses have been used in a mixing model to evaluate their relative contribution to the Atlantic inflow through the Strait of Gibraltar. These contributions are seasonally variable. in June 1997, the contribution was: 80 +/- 20%, 5 +/- 5% and 15 +/- 10% for NASW, NACW and SSW, respectively. In September, the SSW contribution was apparently negligible. Finally, these relative contributions allow the evaluation of the metal fluxes in the Strait of Gibraltar. The presence of SSW in the Strait increases the metal flux to the Mediterranean Sea by a factor of 2.3 (Cu), 2.4 (Cd), 3 (Zn) and 7 (Mn). It does not modify significantly As and Ni fluxes. (C) 2001 Published by Elsevier Science B.V.

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See detailDiagnosis of vertical velocities with the QG Omega equation: a relocation method to obtain pseudo-synoptic data sets
Rixen, Michel; Beckers, Jean-Marie; Allen, J. T.

in Deep-Sea Research. Part I, Oceanographic Research Papers (2001), 48(6), 1347-1373

The quantification of vertical motion and vertical fluxes is essential in our ability to predict and tolerate climate change. However, diagnostic estimations might be affected by the errors arising from the necessary compromise between spatio-temporal resolution and cost of hydrographic surveys. Observations of a numerical ocean model have been made in order to test the accuracy of different sampling strategies and their possible a-posteriori corrections. A simple first-order correction method, computing a pseudo-synoptic data set from a non-synoptic data set and involving a geostrophic relocation of the stations is shown to correct significantly the synopticity error in hydrographic data, derived QG vertical motion and vertical temperature fluxes. Sensitivity analyses also show that the lack of synopticity is more critical than other factors, including the sampling resolution, the level of no-motion and the analysis. (C) 2001 Elsevier Science Ltd. All rights reserved.

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See detailSome properties of generalized age-distribution equations in fluid dynamics
Beckers, Jean-Marie; Delhez, Eric; Deleersnijder, Eric

in SIAM Journal on Applied Mathematics (2001), 61(5), 1526-1544

The concept of age in fluid dynamics is analyzed in the case of a tracer advection-diffusion equation. From the general solution in a uniform velocity field, it is shown that unexpected symmetry properties arise for the age field. In particular, for a point release, the age field is isotropic, regardless of the direction of the ow and the value of the diffusion coefficient. The analysis is then extended to situations with time-varying currents, where the symmetry can be broken under some circumstances. Finally, we show a method by which a time-dependent problem can be used to assess a stationary concentration distribution function, providing details about the propagation of younger and older material at a given location.

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See detailShelf-slope exchanges associated with a steep submarine canyon off Calvi (Corsica, NW Mediterranean Sea): A modelling approach
Skliris, Nikos; Goffart, Anne; Hecq, Jean-Henri; Djenidi, Salim

in Journal of Geophysical Research (2001), 106(C9), 19883-19901

A three-dimensional, unsteady, nonlinear, high-resolution model is used to investigate the impact of the Calvi Canyon (NW Corsica) steep topography on the shelf-slope exchanges as well as on the circulation in the Calvi Bay in homogeneous winter and early spring conditions. A double σ coordinate system is considered in order to represent adequately the high depth gradients within the canyon. The studied region is under the influence of the West Corsica Current flowing northeastward along the NW Corsican coast (right-bounded flow). Model results show that the circulation in the Calvi Bay is determined by flow modifications in the canyon area. The mean horizontal flow is deviated southwestward upstream of the canyon to form an anticyclonic gyre in the western part of the Calvi Bay. Within the canyon the circulation is cyclonic leading to an offshore flow downstream of the canyon. Around the canyon rim, the cross-shelf currents become important, indicating that this region acts as a transition zone of high exchange between nearshore and offshore areas. Furthermore, the canyon topography generates high downwelling (upwelling) and downsloping (upsloping) velocities responsible for an intense vertical transport of material in the area. Numerical runs are performed for typical prevailing wind conditions. The wind is responsible for a drastic increase of cross-shore transports between the bay and the canyon area (3–4 times larger than in the no-wind case). SW winds induce a further enhancement of cross-shelf exchanges, whereas the effect of N-NE winds is to reduce exchange at the shelf break apart from the canyon head where an intense offshore flow occurs. Within the canyon, high vertical velocities are shown to be associated with high cyclonic vorticity which is enhanced (reduced) by the N-NE (SW) wind event. A comparison between model results and measured distributions of nitrate and chlorophyll a concentrations in the area shows the role played by this specific hydrodynamics as a strong constraint on the coastal pelagic ecosystem.

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See detailA numerically efficient data analysis method with error map generation
Rixen, M.; Beckers, Jean-Marie; Brankart, J. M.; Brasseur, P.

in Ocean Modelling (2000), 2

The variational inverse model (VIM) for data analysis was already shown to be statistically equivalent to objective analysis (OA) provided the covariance function for the OA and the VIM reproducing kernel are identical. The VIM, however does not allow a direct derivation of the error field associated with the analysis. The purpose of the paper is to extend the one to one correspondance between the two analysis shemes by proposing a heuristic statistical error expression for the VIM. The numerical efficiency on analysis and error map generation of both methods is compared on quasi-synoptic and climatological data sets. It is shown that the VIM analysis and error map generation offers interesting numerical skills in both case studies.

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See detailNumerical discretization of rotated diffusion operators in ocean models
Beckers, Jean-Marie; Burchard, Hans; Deleersnijder, Eric; Mathieu, P. P.

in Monthly Weather Review (2000), 128(8), 2711-2733

A method to improve the behavior of the numerical discretization of a rotated diffusion operator such as, for example, the isopycnal diffusion parameterization used in large-scale ocean models based on the so-called z-coordinate system is presented. The authors then focus exclusively on the dynamically passive tracers and analyze some different approaches to the numerical discretization. Monotonic schemes are designed but are found to be rather complex, while simpler, linear schemes are shown to produce unphysical undershooting and overshooting. It is suggested that the choice of an appropriate discretization method depends on the importance of the rotated diffusion in a given simulation, whether the field to be diffused is dynamically active or not.

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See detailOn some stability properties of the discretization of damped propagation of shallow-water inertia–gravity waves on the Arakawa B-grid
Beckers, Jean-Marie

in Ocean Modelling (1999), 1(2-4), 53-69

We investigate a problem mentioned by Deleersnijder and Campin (Deleersnijder, E., & Campin, J. -M. (1993). Ocean Modelling 97, 2), who looked at the problem of the discretization of inertia–gravity waves in the staggered B-grid. We generalize this study by adding diffusion. General stability conditions are found with the help of Miller's theorem, and the paradox found in Deleersnijder and Campin (Deleersnijder, E., & Campin, J. -M. (1993). Ocean Modelling 97, 2) is discussed. It is argued that it stems from inappropriate application of boundary conditions in conjunction with a Coriolis force treatment which could produce mechanical work.

See detailAccuracy and stability of the discretised isopycnal mixing equation
Mathieu, P. P.; Deleersnijder, E.; Beckers, Jean-Marie

in Applied Mathematics Letters (1999), 12(4), 81-88

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See detailApplication of Miller's theorem to the stability analysis of numerical schemes; some useful tools for rapid inspection of discretisations in ocean modelling
Beckers, Jean-Marie

in Ocean Modelling (1999), 1

An old theorem about the roots of a polynomial is resurfaced and examined, focusing on the practical question of determining stability criteria for numerical schemes. Is is demonstrated that the theorem can be useful both for analytical studies of the stability limits, as well as numerical searches for stability regions. It is particularly important that when deciding whether the scheme is stable or not, it is not necessary to search for the roots of the polynomial. Indeed, such a decision may be reached through a finite and small number of arithmetic operations and verifications of inequalities. In addition to the analysis of the theorem, some practical conditions for polynomial of different orders are presented, as well as some useful tips on how simply necessary or sufficient stability conditions can be obtained.

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See detailNumerical simulations of seasonal and interannual variability of the Black Sea thermohaline circulation
Stanev, Emil; Beckers, Jean-Marie

in Journal of Marine Systems (1999), 22(4), 241-267

The Black Sea general circulation is simulated by a primitive equation model with active free surface. The forcing is seasonally variable and is based on available climatic data. The model reproduces the main features of the Black Sea circulation, including the river discharge effects on the mean sea level and the Bosphorus outflow. Model results show that the simulated sea surface elevation increases in spring over the whole sea, reaching a maximum in the Danube delta area. In the same region, a minimum is observed in winter. The amplitude of the seasonal oscillations (about 8-12 cm over the whole basin) is of the same order of magnitude as the maximum horizontal variations (about 15-18 cm between the coastal areas and the basin interior). This strong signal formed mostly by river discharges, along with the seasonal variability in the other forcing functions and the local dynamics creates a well-pronounced interannual variability. The performance of the model in simulating the seasonal and interannual variability is critically analyzed, with a special attention on the cold intermediate water formation and the circulation in the upper 150 m. The simulations demonstrate that the source of intermediate waters is on the shelf, and that the water mass in the core of cold intermediate layer changes with time as a response to the periodic forcing at sea surface. This type of variability is characterized by pronounced interannual changes, proving that important differences could exist between water mass structure in different years, even when using identical atmospheric forcings each year. (C) 1999 Elsevier Science B.V. All rights reserved.

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See detailBarotropic and baroclinic oscillations in strongly stratified ocean basins - Numerical study of the Black Sea
Stanev, Emil; Beckers, Jean-Marie

in Journal of Marine Systems (1999), 19(1-3), 65-112

We investigate barotropic and baroclinic oscillations in strongly stratified basins using the Black Sea as a test case. The GHER 3D-model, which has active free surface and temperature and salinity fields as scalar state variables produces model results which are then analysed with a focus on barotropic and baroclinic waves at different scales. The model is forced by seasonally variable climatic data and river run-off. High frequency oscillations of the sea level simulated by the model are compared against observations. It is found that phases and amplitudes are simulated realistically. For the density field, long internal gravity waves dominate the solution in the sub-inertial range, The amplitudes of these oscillations increase over the continental slope, which provides an efficient mechanism for mixing in the western Black Sea. It is found that the sea surface oscillations interact with the oscillations in the pycnocline. This interaction could contribute to a modification of the vertical stratification in a long run. The vertical stratification, on its side, jointly with the bottom relief causes different appearances of oscillations over the continental slope and in the basin interior. Changes in the stability of stratification, caused by the seasonal cycle, are thus an important factor modifying wave processes and the resulting internal mixing. (C) 1999 Elsevier Science B.V. All rights reserved.

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See detailDissection of the GHER turbulence closure scheme
Delhez, Eric; Grégoire, Marilaure; Nihoul, Jacques; Beckers, Jean-Marie

in Journal of Marine Systems (1999), 21(1-4), 379-397

In this paper, the turbulence closure scheme implemented in the GHER hydrodynamic model is described in detail. Two case studies carried out in two contrasting conditions-one in the shallow, tide dominated, north-western European continental shelf, and the other in the deep Mediterranean Sea-are used to identify the dominant terms of the equation for the turbulent kinetic energy, first theoretically, secondly from the results of the simulations. In both domains, the dominant terms are the local destruction and production terms, the vertical diffusion term and to a smaller degree, the time derivative. Advection and horizontal diffusion turn out to be negligible in most of the relevant cases for such large scale studies. This opens the way to simplifications and optimisations of the numerical models. (C) 1999 Elsevier Science B.V. All rights reserved.

See detailComment on "Stratification dynamics in wastewater stabilization ponds", by R. Gu and H.G. Stefan
Djenidi, Salim

in Water Research (1998), 32(1), 261-262

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See detailReconnaissance of the main Black Sea's ecohydrodynamics by means of a 3D interdisciplinary model
Grégoire, Marilaure; Beckers, Jean-Marie; Nihoul, Jacques; Stanev, Emil

in Journal of Marine Systems (1998), 16(1-2), 85-105

A 3D interdisciplinary model has been used to test the sensitivity of the Black Sea’s ecosystem to physical processes. The hydrodynamical model of the general circulation has been built up, using the GHER primitive equation model. A model with 15 km horizontal resolution and 25 vertical levels is used to compute the typical seasonal cycle. The model is forced by climatological monthly mean fields of temperature, salinity and wind stress at the air–sea interface; the river discharges of the Danube, Dnestr and Dnepr are taken into account. An ecosystem model at basin scale is then defined by a nitrogen cycle considering several phytoplankton and zooplankton sizes and including the microbial loop. The ecosystem model is embedded on-line into the 3D hydrodynamical model with a superimposed cycle for the light intensity. This model must be regarded rather as a first tool for testing the coupling of hydrodynamic and ecosystem submodels, while acquiring some preparatory assessment of the effect of physical processes on the ecodynamics. The results display a highly three-dimensional aspect with important horizontal and vertical variations, obviously imparted to the system by the physical processes horizontal and vertical advection, vertical mixing and diffusion, upwelling associated with light limitation at depth and sinking of dead organisms. In this paper, the results are described emphasizing the effects of the hydrodynamic constraints on the space–time distribution of the primary and secondary production.

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See detailAnother reason why simple discretizations of rotated diffusion operators cause problems in ocean models: Comments on "Isoneutral diffusion in a z-coordinate ocean model"
Beckers, Jean-Marie; Burchard, H.; Campin, J. M.; Deleersnijder, Eric; Mathieu, P.

in Journal of Physical Oceanography (1998), 28(7), 1552-1559

A method to improve the behavior of the numerical discretization of a rotated diffusion operator such as, for example, the isopycnal diffusion parameterization used in large-scale ocean models based on the so-called z-coordinate system is presented. The authors then focus exclusively on the dynamically passive tracers and analyze some different approaches to the numerical discretization. Monotonic schemes are designed but are found to be rather complex, while simpler, linear schemes are shown to produce unphysical undershooting and vershooting. It is suggested that the choice of an appropriate discretization method depends on the importance of the rotated diffusion in a given simulation, whether the field to be diffused is dynamically active or not.

See detailSimulation of the annual plankton productivity cycle in the Black Sea with a 3D high resolution interdisciplinary model.
Grégoire, Marilaure; Beckers, Jean-Marie; Nihoul, J.; Stanev, E.

in Journal of Marine Systems (1998), 16

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See detailSome mathematical problems associated withe the development and use of marine models
Deleersnijder, E.; Beckers, Jean-Marie; Campin, J. M.; El Mohajir, Mohammed; Fichefet, T.; Luyten, P.

in NATO ASI Series (1997), I(48),

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See detailCirculation of the western Mediterranean: From global to regional scales
Beckers, Jean-Marie; Brasseur, P.; Nihoul, Jacques

in Deep-Sea Research. Part II, Topical Studies in Oceanography (1997), 44(3-4), 531-549

A free-surface, three-dimensional, primitive equation model has been implemented with a horizontal resolution of 4.6 km to study the ocean circulation in the Gulf of Lions at time scales ranging from weeks to seasons. Numerical experiments have been conducted, in which the regional model is nested into a basin-scale model of the whole western Mediterranean. The global model is operated with a relatively coarse resolution (16 km) and provides boundary conditions at the open-sea boundaries of the regional domain. There is, however, no Feedback loop from the regional to the global model. The simulations are consistently driven with atmospheric fluxes computed from the output of the French PERIDOT meteorological forecasting system, between August 1988 and 1989. In addition to the initial conditions, in situ measurements of temperature and salinity are assimilated in the simulation of the general circulation, adopting a simple nudging technique to prevent an excessive drift of the model against climatology. The response of the regional model below and above the thermocline is discussed in the context of the prevailing meteorological situations. Some experiments give indications that a double-gyre system may develop from wind regimes that exhibit a cyclonic/anticyclonic wind stress curl. Advection-diffusion of passive tracers are also examined on the basis of the local hydrodynamic features, because this work has been conceived with the aim of determining the physical conditions in which ecological and biochemical processes develop at the interface between river mouths and the open ocean. (C) 1997 Elsevier Science Ltd.

See detailStorm surges in the Arabian/Persian Gulf
Djenidi, Salim; Neuberg, Luc

in Bulletin de la Société Royale des Sciences de Liège (1996), 65

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See detailA numerical investigation in the transition zone of the NW African upwelling
Elmoussaoui, A.; Djenidi, Salim; Kostianoy, A.; Beckers, Jean-Marie

in Bulletin de la Société Royale de Liège (1996), 65

See detailModelling the competition between two phytoplanktonic species
Elkafazi, Abderahman; Djenidi, Salim; Nihoul, Jacques

in Bulletin de la Société Royale des Sciences de Liège (1996), 65

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See detailSeasonal temperature and salinity fields in the Mediterranean Sea: Climatological analyses of a historical data set
Brasseur, P.; Beckers, Jean-Marie; Brankart, J. M.; Schoenauen, R.

in Deep-Sea Research. Part I, Oceanographic Research Papers (1996), 43(2), 159-192

Climatological analyses of a historical data base have been carried out with the aim of reconstructing the three-dimensional temperature and salinity fields in the Mediterranean Sea. Seasonal and monthly distributions of hydrographic properties have been computed by a variational inverse method as an alternate to the standard Gandin (1969; Objective analysis of meterological fields, Israeli Program for Scientific Translation, Jerusalem) procedure. The spline solutions of the minimization problem are demonstrated to be numerically and theoretically equivalent to field estimates obtained by conventional objective analysis. The application of a finite-element technique allows analysis to be performed in the model space rather than in the observational space, which substantially improves the numerical efficiency of the procedure. The parameters of the scheme are adjusted according to the statistics of the climatological data. The results,realized as gridded data sets (horizontal resolution of 0.25 degrees), show some trends in seasonal variability affecting the properties of water masses. As expected, the upper layer is subject to a well-defined seasonal signal affecting both the temperature and salinity fields. Error maps, reflecting the degree of uncertainty in the analyses, have been systematically produced. The present work is conceived as a basic support to more advanced studies such as diagnostic calculations, initialization of dynamical models, assimilation of hydrological data into primitive equation models, or planning of experimental surveys. New versions of the climatological fields will be released as data are added to the historical data base. Copyright (C) 1996 Elsevier Science Ltd.

See detailPreliminary study of the Black Sea's plankton bloom
Grégoire, Marilaure; Beckers, Jean-Marie; Nihoul, Jacques; Stanev, Emil

in Bulletin de la Société Royale des Sciences de Liège (1996), 65(1), 91-94

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See detailThermohydrodynamical modelling of a power plant implementation in the Zeebrugge harbour
Beckers, Jean-Marie; Vanormelingen, J.-J.

in Journal of Hydraulic Research (1995), 33(2), 163-180

A mathematical model and numerical simulation has been used to test the validity of a water and thermal circulation scheme induced by the implementation of a new power plant in the Zeebrugge harbour. The aim of the study was to: a) verify that in the interior basin of the inner harbour, a sufficient stratification is present to allow for the capture of cold bottom waters, whereas the ejected heated water remains at the surface and is evacuated to the main harbour, b) study the way the water cools in the main harbour and to quantify the amount of thermal energy which is pumped back into the inner harbour, c) determine if the global heat excess is evacuated and if the resulting temperature increase remains within reasonable limits at the power plant pump inlet. The study, accomplished in three steps, determined the parameters that control the recirculation (e.g. atmospheric conditions) and what their relative importance is. It was shown that a stratification is created in the interior basin. In the main harbour the heat evacuation is done primarily by evaporation in the harbour and only for 20% by the outflow to the open sea. Extreme temperature increases at the pump inlet are expected to be 1.6 degrees C and 6.6 degrees C,but generally the temperature increase lies between 3-4 degrees C. The actual value was found to be most sensitive to the atmospheric conditions.

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See detailProgress from 1989 to 1992 in understanding the circulation of the Western Mediterranean Sea
Lehucher, P. M.; Beautier, L.; Chartier, M.; Martel, F.; Mortier, L.; Brehmer, P.; Millot, C.; Alberola, C.; Benzhora, M.; Taupierletage, I.; Dhieres, G.; Didelle, H.; Gleizon, P.; Obaton, D.; Crepon, M.; Herbaut, C.; Madec, G.; Speich, S.; Nihoul, Jacques; Beckers, Jean-Marie; Brasseur, P.; Deleersnijder, Eric; Djenidi, Salim; Fontaine, J.; Castellon, A.; Garcialadona, E.; Lopezgarcia, M.; Manriquez, M.; Maso, M.; Salat, J.; Tintore, J.; Alonso, S.; Gomis, D.; Viudez, A.; Astraldi, M.; Bacciola, D.; Borghini, M.; Dellamico, F.; Galli, C.; Lazzoni, E.; Gasparini, G.; Sparnocchia, S.; Harzallah, A.

in Oceanologica Acta (1995), 18(2), 255-271

The present paper describes the major results obtained from 1989 to 1992 by the EUROMODEL group in studying the circulation in the Western Mediterranean Sea. Particular emphasis has been given to the physical processes responsible for seasonal and mesoscale variabilities. Observations (in situ and satellite), together with theoretical, physical and numerical models, have been widely used in the course of these studies. Attention has been focused on the dynamics of the northern basin (deep water formation, dynamics of the Northern Mediterranean Current, circulation and shelf/slope interaction in the Balearic Sea) and of the southern basin (Alboran Sea circulation, instabilities of the Algerian Current). The straits dynamics have been studied with particular reference to the Corsica channel. Preliminary runs of a basin-scale circulation model of the Western Mediterranean Sea forced by the straits are also presented. They are shown to be capable of generating a cyclonic circulation in the western basin.

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See detailA simple two species ecological model exhibiting chaotic behaviour
Beckers, Jean-Marie; Nihoul, Jacques

in Mathematical and Computer Modelling (1995), 21(6), 3-11

The classical prey-predator model of Lotka-Volterra is revisited. Instead of constant growth rates, a modulated growth rate for the prey is used. It is shown that in this very simple case, the resulting system's evolution may become nonlinearly resonant. This shows that complex behaviour is not necessarily related to complex systems.

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See detailEffect of topographic stress on circulation in the Western Mediterranean
Alvarez, A.; Tintore, J.; Holloway, G.; Eby, M.; Beckers, Jean-Marie

in Journal of Geophysical Research (1994), 99(C8), 16053-16064

The interaction of eddies with seafloor topography can exert large systematic forces on ocean circulation. Using a statistical mechanics approach it is possible to obtain a parametrization of this effect (the Neptune effect) for application in large-scale ocean circulation models. Circulation of the western Mediterranean has been observed to follow a definite cyclonic path. Numerical models usually show good qualitative agreement for the large-scale circulation but show systematic deficiencies at a subbasin scale. We have tested the importance of the Neptune effect on the circulation of the western Mediterranean Sea. To perform this test, different numerical experiments on western Mediterranean circulation were done with and without eddy-topography interaction. As a first step we analyze the influence of the Neptune effect in an ideal western Mediterranean with closed straits. After these experiments the more realistic case of open straits is studied. All the experiments show that the Neptune effect may be a significant factor in the basin and subbasin scale circulation in the western Mediterranean Sea.

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See detailTopographic forcing of the circulation of LIW in the Western Mediterranean
Alvarez, A.; Tintoré, J.; Holloway, G.; Eby, M.; Beckers, Jean-Marie

in Coastal Estuarine Studies (1994), 46

Circulation of the Western Mediterranean has been inferred from tracer properties and is the object of numerical modelling studies (eg., Beckers, 1991). The path of the Levantine Intermediate Water (LIW), thought o be a buoyancy forced flow, provides an indicator of mid-depth circulation (Millot, 1987b). However, modelling studies which include buoyancy forcing appear to misrepresent the flow of LIW. We have used the 'GFDL Modular Ocean Model' (MOM) to explore the suggestion of Holloway (1992) that subgridscale eddy-topography interaction may be responsible for such systematic defects. We have executed MOM with and without the parametrization of the eddytopography interaction. The different results are compared with circulation inferred from observational data.

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See detailInvestigation of the Western Mediterranean's hydrodynamics with the GHER three-dimensional primitive equation model
Beckers, Jean-Marie; Brasseur, P.; Djenidi, Salim; Nihoul, Jacques

in Coastal and Estuarine Studies (1994), 46

A three-dimensional, non-linear, primitive equation model with turbulent closure developed at the GeoHydrodynamics and Environment Research Laboratory (GHER), University of Liège, is designed to define marine weather-like processes ranging from mesoscale tides and storm surges, to synoptic frontal and eddy structures, to macroscale slowly-varying currents characteristic of the so-called “general circulation”. The model is applied here to the study of the hydrodynamics of the western Mediterranean as both a metagnostic (i.e. system-oriented) model and a diagnostic (i.e. process-oriented) model with both models allowed to run simultaneously and interactively. Critical processes such as deep water formation and the instabilities of the Algerian Current, are studied to provide a basic understanding of the physics needed to adequately parameterize sub-grid scale processes in the basin wide metagnostic model. This general circulation model provides boundary conditions for the finer grid diagnostic studies as well as the general overview of the basin's hydrodynamics. Further improvement is achieved by concurrent use of a variational inverse model to provide initial data fields and boundary conditions for the direct model. In turn, the metagnostic model provides hydrodynamic constraints which are imposed to the variational principle to ensure an interpolation/extrapolation of data compatible with the system's hydrodynamics. Results viz (i) a diagnostic study of the Algerian Current, (ii) a metagnostic study of the seasonal variability of the western Mediterranean Sea, are presented and compared with observations and with results of simpler (quasi-geostrophic, reduced gravity, etc.) models. Possible improved hydrodynamic forecasts of seasonal and interannual variability with the 3D primitive equation model is discussed with particular attention to the forecast's sensitivity to initial and boundary conditions.

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See detailOn destabilizing implicit factors in discrete advection-diffusion equations
Beckers, Jean-Marie

in Journal of Computational Physics (1994), 111(2), 260-265

In the present paper, we find necessary and sufficient stability conditions for a simple one-time step finite difference discretization of an N-dimensional advection-diffusion equation. Furthermore, it is shown that when the implicit factors differ in each direction, a strange behavior occurs: By increasing one implicit factor in only one direction, a stable scheme can become unstable. It is thus suggested to use a single implicit direction (for efficient computing), or the same implicit factor in each direction. (C) 1994 Academic Press, Inc.

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See detailStability of a FBTCS Scheme Applied to the Propagation of Shallow-Water Inertia-Gravity Waves on Various Space Grids
Beckers, Jean-Marie; Deleersnijder, Eric

in Journal of Computational Physics (1993), 108(1), 94-104

The equations governing the propagation of inertia-gravity waves in geophysical fluid flows are discretized on the A, B, C, and D grids according to the classical forward-backward on time and centered on space (FBTCS) numerical scheme. The von Neumann stability analysis is performed and it is shown that the stability condition of the inertia-gravity waves scheme is more restrictive, at least by a factor of , than that concerning pure gravity waves, whatever the magnitude of the Coriolis parameter. Finally, the general necessary and sufficient stability condition is derived for the A, B and C grids, while, on the D grid, the stability condition has been obtained only in particular cases.

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See detailResults of metagnostic (system-oriented) and diagnostic (process-oriented) models
Beckers, Jean-Marie; Haus, J.; Nihoul, Jacques; Schoenhauen, R.

in Progress in Belgian Oceanographic Research (1993)

After a review of the Mediterranean system, and the justification of the scientific interest, we examine the type of model (scope, purview and resolution) used for the description and simulation of the Mediterranean circulation from a system-oriented view and a process-oriented view. These two aspects of mathematical simulations are then illustrated by two examples: a) The simulation of the month to month variability of the general circulation in the Western Mediterranean Sea shows the main physical features, but the choice of initial conditions is crucial for a realistic simulation, and an inverse model is proven to be absolutely necessary for a valuable prognostic model. b) Secondly, a high resolution simulation of the Algerian Current instability is performed. It is well known that the Atlantic water flows along the Algerian coast as a light water intrusion. This current is unstable, and mesoscale activities generate cyclones and anticyclones, but only the latter grow enough to get separated from the mean flow. Numerical simulations, in an idealized case, and reality show strong instabilities that are analyzed by energy budgets. It is suggested that the instabilities are primarily baroclinic, which is consistent with initial vertical movements at the front.

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See detailApplying the Meridional Streamfunction Visualization Technique to Open Ocean Domains
Deleersnijder, E.; Beckers, Jean-Marie

in Ocean Modelling (1993), 101

It is common practice to represent by streamfunction the transport field ensuing from the integration - over one horizontal coordinate - of the velocity field predicted by ocean models. This very useful visualization technique requires that the transport field be divergenceless. If this is not the case, it is suggested to extract the divergenceless transport field that is closest to that stemming from the model while verifying the impermeability conditions of the ocean surface and bottom. A variational approach is examined.

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See detailMonth-to-month variability of the general circulation fields in the western Mediterranean and the Gulf of Lions
Beckers, Jean-Marie; Brasseur, P.

in Water Pollution Research (1993), 30

See detailThree-dimensional General Circulation Model of the Northern Bering Sea’s Summer Ecohydrodynamics
Nihoul, Jacques; Adam, Paul; Brasseur, Pierre; Deleersnijder, Eric; Djenidi, Salim; Haus, Jacques

in Continental Shelf Research (1993), 13(5-6), 509-542

The main features of the northern Bering Sea's summer ecohydrodynamics are investigated with the help of two three-dimensional—direct and inverse—models developed at theGeoHydrodynamics andEnvironmentResearch Laboratory of the University of Liège (GHER). Each model consists of two interacting sectorial submodels for (i) the general circulation hydrodynamics and synoptic structures, and (ii) the associated plankton ecosystem dynamics. The direct model is used to simulate, from an initial state compatible with historical, climatological and all available data pertinent to the summer season, a typical overview of the northern Bering Sea's ecohydrodynamics during the summer. The inverse model is applied in a two-fold perspective: (i) the reconstruction of typical summer distributions of temperature and salinity by using more than 1500 CTD profiles measured during the months of July, August and September, in the course of the ISHTAR program; (ii) considering the observations from specific ISHTAR surveys as quasi-synoptic, the reconstruction of individual data fields in order to provide additional information to assess the variability of the system. The model's predictions indicate that the summer dynamics are dominated by a few cogent semi-permanent and reproducible mechanisms which govern the main water mass transports, the upwellings, the fronts and the subsequent seasonal patterns of primary and secondary productions. The general circulation fields calculated by the direct model are considered as a standard of reference to give a coherent interpretation of—local and often instantaneous—observations, process studies and related results, in the context of the natural variability of the system. The simulated flow pattern has been validated, using the set of current measurements provided by 1985 and 1986 ISHTAR moorings. The contribution of the Anadyr Stream to the northward transport is reproduced qualitatively and quantitatively. The vertical motions—undetectable from direct experiments—are computed by the model, and represent one of the most efficient constraints on the ecohydrodynamics. For instance, the strong upwelling located along the Siberian coast—the existence of which was only presumed until recently—is now correctly estimated in position and intensity. The exceptionally high concentrations of nutrients found in the upwelled water turn this hydrodynamic structure into a catalyst element for the development of biological species in the region. The pattern of primary production shows two successive maxima: the first appears as a direct consequence of the frontal conditions associated with the Anadyr Stream, whereas the second develops further north, in the Chukchi Sea. The results display a fairly good agreement with the classical descriptions induced from observations, and suggest that the advection-growth coupling is the main physical conditioning factor for biological processes.

See detailSub-mesoscale Air-sea Interactions
Nihoul, Jacques; Djenidi, Salim

in Journal of Marine Systems (1993), 4(2-3),

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See detailDe la stabilité d'un écoulement uniforme côtier dans un modèle à gravité réduite
Beckers, Jean-Marie

in Comptes Rendus de l'Académie des Sciences. Série 2, Mécanique, Physique, Chimie, Sciences de l'Univers, Sciences de la Terre (1992), 315(Série II), 783-789

The stability of uniform coastal current is analyzed in the framework of a reduced gravity model. The general solution of the eigenfunction problem is presented. Using an integral method, the stability of the flow to small pertubations is demonstrated for both exponential and resonant perturbations.

See detailDynamique sédimentaire en mer à marée
Djenidi, Salim; Ronday, François

in Bulletin de la Société Royale des Sciences de Liège (1992), 61(1-2), 53-61

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See detailDiagnostic/metagnostic modelling of the western Mediterranean's general circulation with a 3D primitive equations K-ε model
Nihoul, Jacques; Beckers, Jean-Marie

in Bulletin de l'Institut Océanographique (1992), NS11

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See detailAnalytical linear numerical stability conditions for an anisotropic 3 D advection-diffusion equation
Beckers, Jean-Marie

in SIAM Journal on Numerical Analysis (1992), 29(3), 701-713

A one-timestep scheme for advective-diffusive problems in three dimensions is analysed from a numerical stability point of view. Choosing a realizable general seven-point centred discretization scheme, the amplification factor of the von Neumann method is calculated, and necessary and sufficient stability conditions for the general one-dimensional problem are retrieved. A similar analysis then leads to necessary conditions for the three-dimensional case. It is proved that the conditions obtained are also sufficient for an explicit N-dimensional case. Generalization is made to uncentered schemes and some classical results are recovered or corrected. For practical use, some miminum implicit factors necessary for stability are calculated and it is shown that the inspection of one-dimensional problems to get stability conditions can be tricky.

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See detailOn the use of the sigma-coordinate system in regions of large bathymetric variations
Deleersnijder, Eric; Beckers, Jean-Marie

in Journal of Marine Systems (1992), 3(4-5), 381-390

The sigma-transformation is a widely used coordinate change that maps the actual depth-varying sea onto a computational domain, the depth of which is constant. The advantages of this technique are numerous. It permits an efficient use of computer resources, a simple treatment of the surface and bottom boundary conditions, and an accurate representation of the bathymetry. However, if the range of the depth is too large, or when the depth varies too rapidly, as in the shelf break region, it may be shown that the sigma-transformation leads to severe numerical errors. In the application of GHER's three-dimensional model to the Western Mediterranean, the occurrence of those numerical errors is avoided by the introduction of a two-fold sigma-coordinate system in the deep sea.

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See detailModel of the algerian currents instability
Beckers, Jean-Marie; Nihoul, Jacques

in Journal of Marine Systems (1992), 3(4-5), 441-451

A preliminary ''small-perturbation'' analytical study of a non-dissipative idealized model of the Algerian Current is made, with the aim of identifying major characteristic scales and investigating the reality of possible classical instability mechanisms. The simple configuration is shown to be stable, from a linear perturbation point of view. The GHER 3D Primitive Equation Model is then applied to a more realistic investigation of the Algerian Current. The GHER model is non-linear, three-dimensional, with turbulent closure, mode splitting and a sigma-Coordinate formulation. The numerical simulation shows the development and mature pattern of the instability in excellent agreement with the observations. The relative importance of the existence of a coastal boundary layer, the nature of the initial perturbation, baroclinic and non-linear effects on the instability mechanism is discussed.

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See detailModelling the Western Mediterranean : from hydrology to hydrodynamics
Beckers, Jean-Marie; Djenidi, Salim; Nihoul, Jacques

in Water Pollution Research (1991), 28

See detailA reduced-gravity model for the Catalan Sea
Garcia-Ladona, Emilio; Djenidi, Salim

in Journal of Marine Systems (1991), 4

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See detailMean Fluxes Across Sections in the Mediterranean Sea Derived from GCM results
Stanev, E.; Beckers, Jean-Marie

in Journal of Marine Systems (1991), 1

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See detailApplication of the GHER 3D general circulation model to the Western Mediterranean
Beckers, Jean-Marie

in Journal of Marine Systems (1991), 1(4), 315-332

The GHER 3D k− OGCM is briefly described and its application to the Western Mediterranean Sea's specifities is discussed. The simulation of the Western Mediterranean General Circulation in typical winter conditions shows that the model is able to reproduce the main physical processes and the main trends of the general circulation. However, the detailed current patterns appears too sensitive to the choice of initial conditions and a better knowledge of these conditions is required for further routine forecasts.

See detailHierarchy and scales in marine ecohydrodynamics
Nihoul, Jacques; Djenidi, Salim

in Earth-Science Reviews (1991), 31(3-4), 255-277

Recent investigations reveal, in marine ecosystems, an ecohydrodynamic hierarchical organization resulting from the different rates of ecological processes confronted to a multi-scale physical environment. Major marine hydrodynamic processes are briefly analyzed here in an ecohydrodynamic perspective, emphasizing the effects they have on ecosystems at different levels of hierarchy and identifying appropriate “spectral windows” for modelling. A case study application to the Northern Bering Sea's Summer Ecohydrodynamics is given in illustration.

See detailFronts tidaux sur le Plateau Continental Nord Ouest Européen
Djenidi, Salim

in Bulletin de la Société Royale des Sciences de Liège (1990), 59(2), 205-216

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See detailAn anomalous nonlinear heat-conduction problem (improvement)
Beckers, Jean-Marie

in Mathematical and Computer Modelling (1990), 13(5), 89-92

A linear anomalous heat-flux problem is analysed from a stability point of view and a previous [Mathl Comput. Modelling 12(6), 671–672 (1989)] is improved. Then a non-linear problem proposed in the same paper is partly resolved and numerically verified, which shows that only one stationary solution is stable to perturbations.

See detailCarbon and nitrogen cycling within the Bering/Chukchi Seas: source regions for organic matter affecting AOU demands of the Artic Ocean
Walsh, J. J.; McRoy, C. P.; Coachman, L. K.; Goering, J. J.; Nihoul, Jacques; Whitledge, T. E.; Blackburn, T. H.; Parker, P. L.; Wirick, C. D.; Shuert, P. G.; Grebmeier, J. M.; Springer, A. M.; Tripp, R. D.; Hansell, D. A.; Djenidi, Salim; Deleersnijder, Eric; Henriksen, K.; Lund, B. A.; Andersen, P.; Müller-Karger, F. E.; Dean, K.

in Progress in Oceanography (1989), 22(4), 277-359

See detailModelling the general circulation of shelf seas by 3Dk-ε models
Nihoul, Jacques; Deleersnijder, Eric; Djenidi, Salim

in Earth-Science Reviews (1989), 26(1-3), 163-189

One examines the modifications which must be made-and the limitations which must be set-to classicalk-ε models to extend their application to the simulation of marine mesoscale, synopticscale and macroscale processes which compose the weather-like and general circulations of the sea. The case of the general circulation—for which sub-grid scale fluctuations include such semi-organized motions as tides and storm surges-is discussed in more detail. A 3Dk-ε model appropriate to the study of the general circulation in a shallow stratified sea is presented and illustrated with the results of a simulation of the general summer circulation in the Northern Bering Sea, made in the scope of the NSF ISHTAR (“Inner Shelf Transfer and Recycling”) Program.

See detailModelling coastal/shelf systems with emphasis on long term trends
Nihoul, Jacques; Djenidi, Salim; Hecq, Jean-Henri

in International Journal for Numerical Methods in Engineering (1989), 27(1), 113-127

Hydrodynamic studies of continental seas have been primarily concerned with tides and storm surges and the associated currents which can have velocities as high as several metres per second. However, the period of the dominant tide is only about half a day and the characteristic life time of a synoptic weather pattern is of the order of a few days. The very strong currents which are produced by the tides and the atmospheric forcing are thus relatively transitory and, over time scales of biological interest, they change and reverse so many times that they more or less cancel out, leaving only a small residual contribution to the net water circulation. Mathematical modelling appears at present as the most reliable approach to the determination of the residual circulation and of the long term transport of nutrients and pollutants in the sea. The residual circulation model developed at the GeoHydrodynamics and Environment Research Laboratory of Liège University (GHER) is described and illustrated by its application to the West-European Continental Shelf. Residual flow patterns on the shelf, and in particular in the Irish Sea and the North Sea, are presented and shown to be in excellent agreement with the observations. The results are exploited to estimate the typical routes and times of residence of nutrients and pollutants and the subsequent long term changes in shelf ecosystems and in the Belgian coastal zone.

See detailSeasonal variability of the general circulation in the Western Mediterranean
Arnould, Claude; Djenidi, Salim; Guglielmacci, Daniel

in Bulletin de la Société Royale des Sciences de Liège (1988), 57(4-5), 463-473

Full Text
See detailApplication d'un modèle 3-D à la modélisation de la Méditerranée Occidentale
Beckers, Jean-Marie

in Bulletin de la Société Royale des Sciences de Liège (1988), 57

En commençant par une description qualitative d'une circulation typique dans la Méditerranée occidentale, on montre quelques caractéristiques physiques à représenter par le modèle 3D. Ce modèle mathématique est alors présenté et une reformulation à l'aide d'un nouveau changement de variables, est effectuée afin d'adapter les équations au calcul numérique. On explicite une méthode numérique de "mode splitting" qui apporte un gain en temps de calcul non négligeable. On analyse finalement les résultats d'une simulation d'une tempête hivernale.

See detailThe Modem contribution to Medalpex
Djenidi, Salim; Nihoul, Jacques; Clément, Francis; Salas de Leon, David

in Annales Geophysicae. Series B, Terrestrial and Planetary Physics (1987), 5B(1), 3-12

See detailA 3D numerical model of the Northern Bering Sea
Nihoul, Jacques; Waleffe, Fabian; Djenidi, Salim

in Environmental Software (1986), 2

See detailRemote sensing and near surface structures and circulation in the Ligurian Sea off Corsica
Djenidi, Salim

in Bulletin de la Société Royale des Sciences de Liège (1985), 54(4-5), 277-284

See detailObservations au large de Calvi en régime d'été
Djenidi, Salim

in Bulletin de la Société Royale des Sciences de Liège (1985), 54(6), 287-300