WIND AND THERMOHALINE CIRCULATION OF THE BLACK-SEA DRIVEN BY YEARLY MEAN CLIMATOLOGICAL FORCING

Using an eddy-resolving ocean circulation model endowed with active th ermodynamics and a turbulence closure parameterization, a hierarchy of numerical experiments is carried out to investigate the relative cont ributions of the wind forcing, the surface thermohaline fluxes, the ri ver runoff, and the Bosphorus inflow/outflow in driving the yearly mea n circulation in the Black Sea. The model accommodates a topographic a nd boundary-fitted curvilinear coordinate system and resolves steep to pographical changes around the periphery of the basin using O(10 km) g rid spacing and 18 stretched vertical levels. Model experiments show t hat topography, wind forcing, and buoyancy forcing are all first-order contributors to the primary circulation of the Black Sea. If any of t hese features are neglected, significant elements of the model circula tion do not reproduce observations. Subbasin scale gyres are caused by both wind and thermohaline forcing. Annual mean wind stress is suffic ient to produce the major interior cyclonic gyres. Stronger winds prod uce more defined interior flow than the weaker winds of the Hellermann and Rosenstein (1983) fields. Heat flux is an important contributor t o subbasin scale cyclonic circulation. However, the annual mean heat f lux with spatial structure given by the climatology produces unrealist ic features in the interior circulation, The latter ones disappear whe n including the seasonal heat flux variability. This results strongly suggests that the seasonal cycle of the wind stress is much less cruci al than the heat flux seasonal cycle in producing a realistic basin ci rculation. The Rim Current is locked to the steep topographic shelf sl ope, regardless of the forcing mechanism. Without including the strong topography the Rim Current is absent for all forcings. Mesoscale vari ability arises from the dynamic evolution of the Rim Current. This var iability is enhanced by the Danube inflow and the Bosphorus inflow/out flow, demonstrating the importance of these buoyancy sources in enhanc ing the mesoscale. The deep-layer circulation is controlled by the bar otropic pressure gradient and is insensitive to the magnitude, seasona lity, or strength of the surface forcing. A transition zone separates the surface and deep-layer circulation patterns, Its circulation is ma inly driven by the slope of the pycnocline developed as a response to the surface forcing.