Modeling the seasonal, interannual, and long-term variations of salinity and temperature in the Baltic proper

Salinity and temperature variations in the Baltic proper and the Kattegat have been analyzedwith a numerical ocean model and a large amount of observational data. In the model, theBaltic Sea is divided into 13 sub-basins with high vertical resolution, horizontally coupled bybarotropic and baroclinic flows and vertically coupled to a sea-ice model which includesdynamics as well as thermodynamics. The model was integrated for a 15-year period (1980—1995) by using observed meteorological forcing data, river-runoff data and sea-level datafrom the Kattegat. The calculated 15-year median profiles of salinity and temperature in thedifferent sub-basins are in good agreement with observations. However, the calculated middepthsalinities in the Arkona Basin and Bornholm Basin were somewhat overestimated, andthe calculated deep-water temperatures in the Arkona Basin and the Bornholm Basin aresomewhat lower than the observed values. Frontal mixing and movements in the Kattegat andthe entrance area of the Arkona Basin were important to consider in the model. Water masseswere simulated well, and prescribing constant deep-water properties in the Kattegat proved tobe a reasonable lateral boundary condition. Further, comparisons were made between observedand calculated seasonal and interannual variations of the hydrographic properties in the EasternGotland Basin, as well as the interannual variations of the annual maximum ice extent. Weconclude that the model can simulate these variations realistically. The major Baltic inflow of1993 was also simulated by the model, but the inflowing water was 1—2° degrees too cold. Finally, the response times to changes in forcing of the Baltic proper and the Kattegat wereinvestigated by performing the so-called lock-exchange experiment. Typical stratificationspin-up times were of the order of 10 years for the Kattegat, and 100 years for the Baltic proper.