Numerical simulation experiments with a high-resolution circulation model were carried out to study nutrient transport from different depths to the surface 10-m layer during an upwelling event along the northern coast of the Gulf of Finland in July 1999. The initial nutrient distribution is based on field measurements performed in the north-western part of the Gulf. Wind forcing covering the period of the upwelling along the northern coast was turned through 180° to simulate an upwelling along the southern coast. The simulation results showed that the main phosphorus transport to the upper 10-m layer occurred from depths shallower than 30 m for the upwelling events along both the northern and the southern coasts. Nitrogen transport to the upper 10-m layer was the largest from depths of 40-55 m for the upwelling along the northern and 40-65 m for the upwelling along the southern coast. Simulated cumulative volume transports to the upper 10-m layer from different depths showed that the contribution from deeper layers was larger in the case of the upwelling along the southern coast. The reduction of wind stress had a bigger influence on water transport from the deeper layers.
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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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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.
A sigma-coordinate ocean model, by Blumberg and Mellor (POM). is applied to study the formation processes of mesoscale cyclones observed in the Baltic Sea, following the inflow events. The initial conditions simulate a situation when the Arkona and Bomholm Basins (or Arkona Basin solely) are already filled with the inflow water of the North Sea origin. while the rest of the sea still contains the old water of pre-inflow stratification. The model runs with constant and time dependent northerly/easterly wind, changing the buoyancy forcing, grid geometry and bottom topography, display the following. Entering the East Gotland Basin from the Slupsk Furrow. the bottom intrusion of saline inflow water splits in two: one goes northeast towards the Gotland Deep and the second moves southeast towards the Gulf of Gdansk. An intensive mesoscale cyclonic eddy carrying the inflow water is generated just east of the Slupsk Furrow with the inflow pulse. A number of smaller cyclones, with boluses of the inflow water, form in the intermediate layer along the saline intrusion pathway to the Gotland Deep. A similar cyclonic eddy is generated in the Bomholm Basin with the inflow pulse from the Arkona Basin. Following Spall and Price [19}, the cyclones are expected to form by the geostrophic adjustment of high potential vorticity inflow water column to a low potential vorticity environment. Some evidence for the PV outflow/inflow hypothesis is obtained in a numerical experiment, with a virtual dam restricting water exchange between the Bomholm and Gotland Basins to the limits of the Słupsk Furrow.
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