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On the ADCP-derived Rim Current structure, CIW formation and the role of mesoscale eddies on the CIW transport in the Black Sea: Results from April 1993 observations

Oguz, T
Besiktepe, S
Ivanov, LI
Diacanu, V
The shipboard CTD and ADCP current measurements, performed during 2-15 April 1993, yield new information on the current and hydrographic structures of the continental shelf and slope of the western Black Sea. The region is shown to be populated by pronounced current meandering and eddy activity. The narrow Rim Current frontal zone of the width similar to 50 km has a uniform upper layer speed in excess of 50 cm/s, with maximum measured currents of similar to 100 cm/s over the steepest topographic slope. Following a relatively sharp decrease across the pycnocline, uniform currents of similar to 20 cm/s art observed up to the depth of similar to 300 dbar, being the approximate limit of the ADCP measurements. The cross-stream velocity structure exhibit a narrow core region (similar to 10-20 km) confined over the topographic slope, flanked by a narrow zone of anticyclonic shear and a broader region of cyclonic shear on its coastal and offshore sides, respectively. The anticyclonic shear is stronger, comparable with the Coriolis parameter, and extends to greater depths. Except in the localized regions of the onshelf meanders of the Rim Current, the northwestern shelf circulation is decoupled from the influence of the basin-wide circulation, and characterized by much weaker currents of similar to 10 cm/s. The southward coastal flow associated with Danube and Dinepr Rivers is considerably weak at this time of the year and is confined to a narrow zone of similar to 20-30 km along the coast. The CTD data suggest temperature-induced overturning prior to the measurements, and subsequent formation of the Cold Intermediate Water (CIW) within the Northwestern Shelf (NWS) and center of the cyclonic gyre occupying the interior of the western basin. The newly formed shelf CIW is transported in part by the coastal current system, and in part, it flows downslope along the shelf in response to mesoscale structure of the local circulation system and intrude subsequently into the residual cold intermediate layer preserved in the frontal zone from the early winter season. The data indicate that the CIW mass, injected into the Rim Current zone quasi-horizontally from the shelf and periphery of the cyclonic gyre of the interior region, is carried by the Rim Current system around the basin.