The upper layer waters of the Black Sea are characterized by a predominantly cyclonic, strongly time-dependent and spatially-structured basinwide circulation. Many details of the circulation system have been explored using recent hydrographic data (Oguz et al., 1994; 1998; Oguz and Besiktepe, 1999; Gawarkiewicz et al., 1999; Krivosheya et al., 2000), AVHRR data (Oguz et al., 1992; Sur et al., 1994, 1996; Sur and Ilyin, 1997; Ginsburg et al., 2000, 2002a; Afanasyev et al., 2002; Zatsepin et al., 2003), altimeter data (Korotaev et al., 2001 and 2003; Sokolova et al., 2001), and CZCS and SeaWIFS data (Ozsoy and Unluata, 1997; Oguz et al., 2002a; Ginsburg et al., 2002b). These analyses reveal a complex, eddy-dominated circulation with different types of structural organizations within the interior cyclonic cell, the Rim Current flowing along the abruptly varying continental slope and margin topography around the basin, and a series of anticyclonic eddies in the onshore side of the Rim Current. The interior circulation comprises several sub-basin scale gyres, each of them involving a series of cyclonic eddies. They evolve continuously by interactions among each other, as well as with meanders, and filaments of the Rim Current. The Rim Current structure is accompanied by coastal trapped waves with an embedded train of eddies and meanders propogating cyclonically around the basin (Sur et al., 1994; Sur et al; 1996; Oguz and Besiktepe, 1999; Krivosheya et al., 2000; Ginsburg et al., 2002a,b). Over the annual time scale, westward propogating Rossby waves further contribute complexity to the basinwide circulation system (Stanev and Rachev, 1999). According to the Acoustic Doppler Current Profiler measurements (Oguz and Besiktepe, 1999), the Rim Current jet has a speed of 50-100 cm/s within the upper layer, and about 10-20 cm/s within the 150-300 m depth range. The mesoscale features evolving along the periphery of the basin as part of the Rim Current dynamic structure apparently link coastal biogeochemical processes to those beyond the continental margin, and thus provide a mechanism for two-way transports between nearshore and offshore regions. Taking the relatively narrow width of the basin into account, such mesoscale processes can easily give rise to meridional transports from one coast to another.
Apart from complex eddy-dominated features, larger scale characteristics of the upper layer circulation system possess a distinct seasonal cycle, as suggested by objectively analyzed, optimally interpolated and dynamically assimilated sea level anomaly data provided by the Topex-Poseidon and ERS-1/2 altimeters period from 1 January 1993 to 31 December 1998 (Korotaev et al., 2003). As shown by the model-derived circulation patterns for the middle of February, July and October, the interior cyclonic cell in winter months involves a two-gyre system surrounded by a rather strong and narrow peripheral jet without any appreciable lateral variations. This system transforms into a multi-centered composite cyclonic cell surrounded by a broader and weaker Rim Current zone in summer. The interior basin flow field further weakens and finally disintegrates into smaller scale cyclonic features in autumn. A composite peripheral current system is hardly noticeable in this season (Afanasyev et al., 2002). The turbulent flow field is, however, rapidly converted into a more intense and organized structure after November-December.
The most notable quasi-persistent and/or recurrent features of the circulation system, as schematically presented in Figure below, include (i) the meandering Rim Current system cyclonically encircling the basin, (ii) two cyclonic sub-basin scale gyres comprising four or more gyres within the interior, (iii) the Bosphorus, Sakarya, Sinop, Kizilirmak, Batumi, Sukhumi, Caucasus, Kerch, Crimea, Sevastopol, Danube, Constantsa, and Kaliakra anticyclonic eddies on the coastal side of the Rim Current zone, (iv) bifurcation of the Rim Current near the southern tip of the Crimea; one brach flowing southwestward along the topographic slope zone, and the other branch deflecting first northwestward into the shelf and then contributing to the southerly inner shelf current system, (v) convergence of these two branches of the original Rim Current system near the southwestern coast, (vi) presence of a large anticyclonic eddy within the northern part of the northwestern shelf.
The basic mechanism which controls the flow structure in the surface layer of the northwestwern shelf is spreading of the Danube outflow. Wind stress and Rim Current structure along the offshore side of the shelf are additional modifiers of this system. The freshwater discharge influences not only the circulation and mixing properties, but also the ecosystem of the entire shelf region along the western coast. The Danube plume generally forms an anticyclonic bulge confined within the upper 25 m layer. The leading edge of this plume protrudes southward (i.e downstream) as a thin baroclinic boundary current along the western coastline. The coastal jet is separated from the interior waters by a well defined front with salinity differences of more than 3.0 over an approximately 50 km zone along the coast. It is often unstable, exhibits meanders and spawns filaments, which extend across the wide topographic slope zone. The shelf and interior waters undergo cross-shelf exchanges as reported consistently in hydrographic surveys, satellite imagery, and altimeter data. An anticyclonic circulation system accompanying with small scale structures over the northwestern shelf have also been reproduced by modeling studies (e.g. Oguz et al., 1995; Staneva et al., 2001; Beckers, et al., 2002).