MESOSCALE CIRCULATION AND THERMOHALINE STRUCTURE OF THE BLACK-SEA OBSERVED DURING HYDROBLACK-91

Abstract

Results from nearly 300 hydrographic stations occupied during HydroBlack '91, a basin-wide survey performed during September 1991, reveal a complex, eddy-dominated three-dimensional circulation in the Black Sea. Except in the areas of anticyclonic eddies distributed around the periphery of the basin, the inshore portion of the Rim Current along the relatively smooth continental margin topography forms a narrow coastal jet characterized by small amplitude (approximately 20-30 km) undulations with a wavelength of approximately 125 km. The offshore margin of the Rim Current, however, exhibits large meanders, offshore filaments and dipole eddy structures. The Rim Current is accompanied by a strong frontal zone having cross-frontal temperature and salinity differences of approximately 1.0-degrees-C and approximately 0.5, respectively, near the surface. The circulation within the interior of the sea is composed of an interconnecting series of cyclonic eddies and gyres, varying in size and shape with depth. The mesoscale variability introduces a different scale of complexity to the general circulation at each pressure level, although larger and quasi-persistent features of the general circulation are fairly coherent down to 500 dbar. Important structural changes, however, take place in both large scale and mesoscale components of the circulation at 500 dbar and deeper, consistent with the findings of the September 1990 survey (OGUZ et al., Deep-Sea Research I, 40, 1597-1612). The circulation is strongest in the upper layer (approximately 150 dbar), where the calculated geostrophic currents have speeds of 0.2-0.3 ms-1 along the axis of the Rim Current. The strength, width, and frontal intensity of the Rim Current, however, reduce considerably below the permanent pycnocline.