Realistic modelling of the exceptional inflows into the central Baltic Sea in 2003 using terrain-following coordinates

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.