On the summer mesoscale variability of the Black Sea

The evolution of the Black Sea temperature, salinity and circulation, from large scale to mesoscale, is studied using a data-driven primitive equation simulation. The data are drawn from (i) a basin-wide hydrographic survey, CoMSBlack'92, obtained in the Summer of 1992; (ii) wind stress derived from wind analyses of the Sevastopol MSIA/URHI Office; (iii) climatological heat fluxes; and (iv) climatological river outflows. The primitive equation model is from the Harvard Ocean Prediction System. The simulation is used to examine the evolution of the circulation at mesoscale resolution, its dominant variabilities and dependencies in the summer period. The large-scale upper layer circulation over the deep portion of the basin is generally cyclonic with a system of anticyclonic eddies evolving in its periphery. The edge of the cyclonic circulation is dominated by an inertial jet: the Rim Current. As the Rim Current transverses the edge of the deep basin, the meandering and secondary circulation associated to the jet varies according to internal dynamics and interactions with the bottom topography and shelf water circulation. The relatively broad northwestern shelf is found to be mostly wind driven with a buoyancy-driven coastal current and interacting with the quasi-stationary Crimea and Kaliakra anticyclones. The seasonal thermocline is strengthened during this period and a zonal large-scale temperature gradient with warmer/colder sea-surface temperatures in the east/west is driven by the observed weak/strong winds. Some of the major circulation elements are partially verified using qualitative comparisons with the Summer of 1992 data and historical data; both in situ, and infrared and color remotely sensed data. The Rim Current meander shape and propagation parameters, eddy size and distribution, and the generation of rapid surface bound jets are found to be in good agreement with observations. The simulation shows two previously unobserved events: an anticyclonic eddy is shed near Sinop; and the anticyclones moving north along the Caucasian coast are formed and shed from the Batumi eddy. Imprint, of these events are found in the historical record