Changing coastal oceanography of the Black Sea II: Mediterranean effluent

The behavior and physical mechanisms during the Mediterranean flow, entering the Black Sea from the Bosphorus, are described using existing field observations and hydrodynamic models for the vertical and along axis circulation in the strait. Soviet studies in the 1960-1970's showed that a well-defined sill north of the Bosphorus does not prevent the overflow of dense Mediterranean water onto the shelf even during unfavorable conditions (high surface elevation in the Black Sea and strong northerly winds). The stream makes a sharp westward turn after exiting from the strait and then starts its track downward to the shelf slope. Intensive dispersion of the Mediterranean water occurs at a distance of 25-50 km from the strait. Recent laboratory and mathematical models have been reviewed to describe the behavior of the density and current interfaces in the strait and the role of temporary blockage of the dense flow upon the Mediterranean effluent. Two hypothetic physical mechanisms for sill overflow are discussed. One is associated with bifurcation of the coastal flows, approaching the strait orifice, and formation of a diversion zone in the upper layer which facilitates the flow over the sill. Another hypothesis suggests that the sill exercises frictionless rotational hydraulic control in the short, 4 km continuation of the strait channel. Persistent northeasterly or southerly winds can substantially affect the intensity of the Mediterranean effluent by modifying the average flow fields and density structures in the strait. TS-analysis of far-field property distribution shows that the Mediterranean water contributes to the laminated structure of the Black Sea water column. Under average conditions, the Mediterranean waters follow the mainstream of the Black Sea current system along the Turkish coast, reaching the deepest strata in the southeastern corner of the sea. Ongoing Soviet diversions of fresh water from the northern slope of the Black Sea and projected damming of the major estuaries and the Azov Sea will intensify the Mediterranean effluent and, at the same time, will increase the vertical mixing due to more effective convective overturn. These phenomena will lead to partial or complete disappearance of the H ₂S layer and general warming of the water column.