Model study of the seasonal variability of the Black sea circulation

Seasonal variability of the Black sea circulation is studied using a general circulation model. An attention is paid to the simulation of an upper layer of the sea, where the Cold Intermediate Layer (CIL) exists at the depths of 30-75 m throughout the year. Basic model is a primitive equation ocean general circulation model developed in the Institute of Numerical Mathematics, RAS with Richardson number dependent parameterization of vertical turbulent mixing and nonlinear horizontal viscosity. Horizontal grid spacing is 15' longitude and 12' latitude. There are 21 vertical levels. Forcing of the model includes climatological monthly temperature, salinity and wind stress at the sea surface. Climatological monthly values of velocity are set on the open lateral boundaries that approximate river Danube inflow and the Bosphorus Straits inflow/outflow. A quasi periodical solution for the upper layer of the sea is analyzed. An extended series of experiments with different values of parameters of the Munk-Anderson formula for vertical mixing coefficients gave no reasonable result when background value of coefficient (a(hb)) was constant. The introduction of depth dependent ahb changing from 1.0 cm(2)/s at the sea surface to 0.01 cm(2)/s af:the depths deeper than 50 m, parameterizing high frequency component of the forcing, resulted in realistic simulation of CIL. Horizontal pattern of currents shows strong seasonal variability. Cyclonic gyre in the south-west deep part of the sea is a permanent feature of circulation. An estimate of the heat fluxes through the sea surface obtained from a model solution gives a belt (about 40 km width) of low heat gain along the coasts of the sea in spring and summer. The value of heat flux in the sea is 100-200 W/m(2) in the deep sea regions and 50-100 W/m(2) close to the coast. The model experiments indicate strong space and seasonal variability of heat fluxes. Pattern of wind stress that forces upwelling and downwelling obviously controls local regions of anomalous heat losses and gains.