Distinctive features of water exchange across the Słupsk Sill (a full-scale experiment)

The flows of brackish waters in the upper layer and saline watersin the lower layer meet above the Słupsk Sill, which makes thisone of the most significant features of the Baltic Sea, controllingas it does the ventilation of the deep basins in its centralregion. Earlier high-resolution measurements using towed scanningprobes conducted here for more than ten years had revealed thecomplexity and variability of the water dynamics in this area. Mapping surveys repeated in quick succession are needed to studythe water exchange in such an area. A survey of this kind wasattempted in October 2003 during the 57th cruise of the r/v"Professor Shtokman". Three surveys were carried out inthe areas of the Słupsk Sill, the eastern Bornholm Basin, andthe western Słupsk Furrow by means of a scanning probe towedalong closely-spaced transects. The water structure around thesill was different each time, despite the rather short time gapsbetween the surveys. As follows from the data analysis, duringthe first survey, the saline Bornholm waters flowed over thesill as an axially symmetrical jet and entrained the adjacentfreshened cold waters of the intermediate layer. In ten days,this joint flow displaced to the southern flank of the sill andpropagated in the Słupsk Furrow along its southern border, withthe dense core of saline waters gradually moving over the bottomto the northern border. Concurrently, the contrary flow of themain volume of cold freshened waters, originating from northernareas and leaving the Baltic Sea, was pushed away from the southernwall of the furrow and blocked at a significant distance fromthe sill. In three days, the blocked waters forced their waythrough towards its northern flank. Just below these waters,waters of elevated salinity were found above the eastern slopeof the sill at the depth of its ridge, while waters of a similarsalinity occurred below the depth of the ridge above the westernslope of the sill. There were no indications of intensive overflowin the central and southern areas of the sill. Accordingly, thereturn flow of Bornholm waters across the sill became possible