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

Zhurbas V., Elken J., Paka V., Piechura J., Chubarenko I., Väli G., Golenko N., Shchuka S.

Oceanologia

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2011

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No. 53 (3)

771-791

EN

Closely spaced CTD transects across the Słupsk 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 Słupsk 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.

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

Meier H. E. M., Feistel R., Piechura J., Arneborg L., Burchard H., Fiekas V., Golenko N., Kuzmina N., Mohrholz V., Nohr C., Paka V. T., Sellschopp J., Stips A., Zhurbas V.

Oceanologia

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2006

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No. 48 (S)

133-164

EN

The ventilation of the Baltic Sea deep water is 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 entrainment rates are generally smaller than those predicted by earlier entrainment models. In addition to the entrance area, the Słupsk Sill and the 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 baroclinic wave motion using a two-dimensional shallow water model suggests that about 30% of the energy needed below the halocline for deep water mixing is explained by the breaking of internal waves. In the deep water decade-long stagnation periods with decreasing oxygen and increasing hydrogen sulphide concentrations might be caused by anomalously large freshwater inflows and anomalously high mean zonal wind speeds. In different studies the typical response time scale of average salinity was estimated to be between approximately 20 and 30 years. The review summarizes recent research results and ends with a list of open questions and recommendations.

3

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

Paka V., Golenko N., Korzh A.

Oceanologia

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2006

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No. 48 (S)

37-54

EN

The flows of brackish waters in the upper layer and saline waters in the lower layer meet above the Słupsk Sill, which makes this one of the most significant features of the Baltic Sea, controlling as it does the ventilation of the deep basins in its central region. Earlier high-resolution measurements using towed scanning probes conducted here for more than ten years had revealed the complexity and variability of the water dynamics in this area. Mapping surveys repeated in quick succession are needed to study the water exchange in such an area. A survey of this kind was attempted in October 2003 during the 57th cruise of the r/v "Professor Shtokman". Three surveys were carried out in the areas of the Słupsk Sill, the eastern Bornholm Basin, and the western Słupsk Furrow by means of a scanning probe towed along closely-spaced transects. The water structure around the sill was different each time, despite the rather short time gaps between the surveys. As follows from the data analysis, during the first survey, the saline Bornholm waters flowed over the sill as an axially symmetrical jet and entrained the adjacent freshened cold waters of the intermediate layer. In ten days, this joint flow displaced to the southern flank of the sill and propagated in the Słupsk Furrow along its southern border, with the dense core of saline waters gradually moving over the bottom to the northern border. Concurrently, the contrary flow of the main volume of cold freshened waters, originating from northern areas and leaving the Baltic Sea, was pushed away from the southern wall of the furrow and blocked at a significant distance from the sill. In three days, the blocked waters forced their way through towards its northern flank. Just below these waters, waters of elevated salinity were found above the eastern slope of the sill at the depth of its ridge, while waters of a similar salinity occurred below the depth of the ridge above the western slope of the sill. There were no indications of intensive overflow in the central and southern areas of the sill. Accordingly, the return flow of Bornholm waters across the sill became possible.

4

Some characteristics of fine structure and microstructure of Baltic Sea waters

Golenko N.

Biuletyn Instytutu Morskiego w Gdańsku

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2001

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Vol. 28, No. 2

23--33

EN

It is generally agreed that fine structure and microstructure are usually beyond the resolution of numerical models. Nevertheless, corresponding processes have a considerable influence on water dynamics and exchanges in the Baltic Sea. Measurements of fine structure were carried out with the scanning CTD probe. Microstructure measurements were performed by towing a system with fast-response velocity, conductivity, and temperature sensors. Bottom turbulence was investigated with the Acoustic Doppler Velocimeter. Domains of high mixing and stining activity were revealed by means of high-resolution profiling on temperature and salinity transects. These domains were found near inhomogeneities of bottom relief such as the Słupsk Sill, the bottom elevation on eastern boundary of the Słupsk Furrow and in areas of ray concentration of quasi-inertial internal waves. Data on transects with manifestations of convection provide an evidence of the non-uniform distribution of turbulence in the upper layer. A bottom turbulence patch with duration of about 2 hours was registered over the slope of the Gotland Deep. The rate of turbulent energy dissipation is about ]105cm2/s3. In the area of Hamrarne and Słupsk Sill, horizontal scales of turbulent patches are between O(10m) and 0(100 m). All obtained data testify the high level of turbulent intermittency, so probability of detecting turbulent patches is high.