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

On the possibility of convective overturning in the Słupsk Furrow overflow of the Baltic Sea

Alosely spaced CTD transects across the Slupsk Furrow displayed a "downward-bending" of salinity contours below the salinity interface on the southern flank due to a transverse circulation in the saline water overflow. Numerical simulation of a gravity current in an idealized channel with geometry, dimensions and initial density stratification all much the same as in the Slupsk Furrow was applied to verify whether the downward-bending could be transformed into an inverted density stratification. Some arguments in favour of the possibility of convective overturning due to the differential transverse advection beneath the gravity current, brought on by the numerical simulations, are discussed

Ventilation of the Baltic Sea deep water: A brief review of present knowledge from observations and models

The ventilation of the Baltic Sea deep wateris driven by either gale-forced barotropic or baroclinic salt water inflows.During the past two decades, the frequency of large barotropic inflows(mainly in winter) has decreased and the frequency of medium-intensity baroclinic inflows(observed in summer) has increased. As a result of entrainment of ambient oxygen-rich water,summer inflows are also important for the deep water ventilation.Recent process studies of salt water plumes suggest that the entrainmentrates are generally smaller than those predicted by earlier entrainment models.In addition to the entrance area, the Słupsk Sill andthe Słupsk Furrow are important locations for the transformation of water masses. Passing the Słupsk Furrow, both gravity-driven dense bottom flows and sub-surface cyclonic eddies,which are eroded laterally by thermohaline intrusions,ventilate the deep water of the eastern Gotland Basin.A recent study of the energy transfer from barotropic to baroclinicwave motion using a two-dimensional shallow water model suggests thatabout 30% of the energy needed below the halocline for deep water mixingis explained by the breaking of internal waves.In the deep water decade-long stagnation periods with decreasingoxygen and increasing hydrogen sulphide concentrations might be caused by anomalously largefreshwater inflows and anomalously high mean zonal wind speeds. In differentstudies the typical response time scale of average salinity was estimated tobe between approximately 20 and 30 years.The review summarizes recent research resultsand ends with a list of open questions and recommendations