Doctoral thesis (Dissertations and theses)
Reconstruction of the Gulf Stream variability since 1940 using a variational inverse method and study of its interaction with the North Atlantic Oscillation
Watelet, Sylvain
2021
 

Files


Full Text
Sylvain_Watelet_Thesis_v2_elec.pdf
Author postprint (11.55 MB)
Download

All documents in ORBi are protected by a user license.

Send to



Details



Keywords :
Gulf Stream; NAO; Numerical modelling; DIVA; Rossby waves; Barents Sea
Abstract :
[en] In this study, the Gulf Stream’s (GS) response to the North Atlantic oscillation (NAO) is investigated by generating an observation-based reconstruction of the GS path between 70° and 50°W since 1940. Using in situ data from WOD, SeaDataNet, ICES, Hydrobase3 and ARGO floats, a harmonized database of more than 40 million entries is created. A variational inverse method implemented in the software DIVA (Data-Interpolating Variational Analysis) allows the production of time series of monthly analyses of temperature and salinity over the North Atlantic (NA). These time series are used to derive two GS indices: the GS North Wall (GSNW) index for position and the GS Delta (GSD) index as a proxy of its transport. We find a significant correlation (0.37) between the GSNW and the NAO at a lag of 1 year (NAO preceding GS) since 1940 and significant correlations (0.50 and 0.43) between the GSD and the NAO at lags of 0 and 2 years between 1960–2014. We suggest this 2-year lag is due to Rossby waves, generated by NAO variability, that propagate westwards from the center of the NA. This is the first reconstruction of GS indices over a 75-year period based on an objective method using the largest in situ dataset so far. This enhanced tracking and quantification of the GS confirms and extends the temporal scope of this property: NAO+ phases lead to a stronger and more northward GS, and conversely for NAO− phases. The teleconnections between the NAO and the variability of the GS were extensively studied these last years, often exhibiting time delays between both phenomena. These time lags, usually ranging between 0–2 years, are sometimes explained by the hypothesis of baroclinic Rossby waves generated by the NAO in the central NA and travelling westward before interacting with the GS. In this study, we use a numerical hindcast at an eddy-resolving resolution (1/12°) from the DRAKKAR project to examine the occurrence and properties of such Rossby waves between 1970–2015, thus including a large pre-TOPEX/Poseidon period. Through the use of a two-dimensional Radon Transform (2D-RT) on Hovmöller diagrams of the Sea Surface Height (SSH), a methodology easily portable to other oceanic model outputs, we show evidence of baroclinic Rossby waves travelling at 39°N at a speed of 4.17 cm/s. This study extends the period over which Rossby waves have been found that far north to a much longer period, which reinforces the findings of previous works. These results are consistent with the time lags observed between the NAO and the GS transport while the GS latitudinal shifts might obey additional processes. The Barents Sea, located between the Norwegian Sea and the Arctic Ocean, is one of the main pathways of the Atlantic Meridional Overturning Circulation. Changes in the water mass transformations in the Barents Sea potentially affect the thermohaline circulation through the alteration of the dense water formation process. In order to investigate such changes, we present here a seasonal atlas of the Barents Sea including both temperature and salinity for the period 1965–2016. The atlas is built as a compilation of datasets from the World Ocean Database, the Polar Branch of Russian Federal Research Institute of Fisheries and Oceanography, and the Norwegian Polar Institute using the DIVA tool. DIVA allows for a minimization of the expected error variance with respect to the true field. The atlas is used to provide a volumetric analysis of water mass characteristics and an estimation of the ocean heat and freshwater contents. The results show a recent "Atlantification" of the Barents Sea, that is a general increase of both temperature and salinity, while its density remains stable. The atlas is made freely accessible as handy NetCDF files to encourage further research in the Barents Sea physics.
Research center :
GeoHydrodynamics and Environment Research
Disciplines :
Earth sciences & physical geography
Author, co-author :
Watelet, Sylvain ;  Institut Royal Météorologique - IRM > Observations > RADLI
Language :
English
Title :
Reconstruction of the Gulf Stream variability since 1940 using a variational inverse method and study of its interaction with the North Atlantic Oscillation
Defense date :
January 2021
Institution :
ULiège - Université de Liège
Degree :
Docteur en Sciences
Promotor :
Beckers, Jean-Marie  ;  Université de Liège - ULiège > Freshwater and OCeanic science Unit of reSearch (FOCUS)
President :
Fettweis, Xavier  ;  Université de Liège - ULiège > Sphères
Secretary :
Munhoven, Guy ;  Université de Liège - ULiège > Sphères
Jury member :
Barth, Alexander  ;  Université de Liège - ULiège > Freshwater and OCeanic science Unit of reSearch (FOCUS)
Goosse, Hugues
Nicolay, Samuel  ;  Université de Liège - ULiège > Mathematics
Sagen, Helge
European Projects :
FP7 - 283607 - SEADATANET II - SeaDataNet II: Pan-European infrastructure for ocean and marine data management
Name of the research project :
SeaDataNet II
Funders :
CE - Commission Européenne [BE]
Available on ORBi :
since 04 January 2021

Statistics


Number of views
204 (17 by ULiège)
Number of downloads
171 (8 by ULiège)

Bibliography


Similar publications



Contact ORBi