Barotropic and baroclinic oscillations in strongly stratified ocean basins: Numerical study of the Black Sea

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.