SEASONAL VARIABILITY OF WIND AND THERMOHALINE-DRIVEN CIRCULATION IN THE BLACK-SEA - MODELING STUDIES

The seasonal variability of the Black Sea circulation is studied using an eddy-resolving primitive equation model. A series of numerical exp eriments is carried out to determine the relative importance of wind s tress, air-sea thermohaline fluxes, and river-induced lateral buoyancy forcing in driving the circulation on the monthly and seasonal timesc ales. A synthesis is made of the results with those obtained under yea rly climatological conditions by Oguz et al. [1995] to assess whether the major circulation features are a response to the yearly forcings o r are dominated by the seasonal cycle. The model experiments indicate that under all forcing mechanisms, the overall basin circulation is ch aracterized by a very strong seasonal cycle dominating the yearly sign al described by Oguz et al. [1995]. The purely wind-driven circulation reveals most of the observed circulation features including a well-de fined meandering boundary current system and subbasin scale cyclonic g yres forming the interior flow structure of the basin. Topography obvi ously remains a crucial factor in controlling the pattern of the persi stent rim current system all year long. The dynamical instabilities of the rim current produce strong meandering and mesoscale eddies which often modulate the basin and subbasin scale structures of the circulat ion. The surface thermohaline fluxes generate simpler circulation patt erns with a comparable strength but mostly in the opposite direction t o the wind-driven circulation. Two important by-products emerge from t he present work. First is the necessity of reanalyzing the heat flux c limatology. The existing surface thermohaline fluxes, even though not affecting critically the general characteristics of the surface circul ation patterns, may induce rather unrealistic horizontal temperature d istributions and water mass properties in the surface layer. Second, t he role of the northwestern shelf in the cold intermediate water (CIW) mass formation process is shown to be secondary during moderate-to-hi gh winter discharge conditions from the northwestern rivers. In these conditions the freshwater outflow reduces the density of the cold wate r formed on the shelf by about 1 kg/m(3) as compared with that of the basin interior, which is the major reservoir for the formation of the winter CIW.