Deep water exchange in the Baltic Proper

Deep-water exchange and mixing properties in the Baltic Proper were analysed on the basis of temperature and salinity data measured during the period 1970–90. The data were analysed applying basic model concepts as the conservation principles, the two-layer approach and the geostrophic flow assumption. The renewal of the deep water in the Baltic Proper consists of inflow from upstream basins. The inflowing dense water is diluted by surface water and on a 20-year average increased by a factor of 4, when entering from the Kattegat into the Landsort Deep. Three main mixing zones were localized. Firstly, the Belt Sea and the Sound, where the deep-water inflow increases by 79%; secondly, the Arkona Basin, where vertical mixing causes the increase of deep current volume flow by an average of 53%; and thirdly, in the Stolpe Channel, where the turbulent entrainment adds an average of 28% to the deep current. Applying the geostrophic flow model on salinity data, time series of deep current flow rates were calculated. The model was calibrated by 20-year mean flows calculated from conservation principles. The effective sill depths were introduced as calibration coefficients. It was found that the geostrophic flow model described deep-layer flows in the Bornholm Channel well, but the flow rate was underestimated in the Stolpe Channel and overestimated in the Fårö Channel. In the Stolpe Channel and in the Fårö Channel, the deep-layer flow showed seasonal variations with rapid increase during the autumn and winter seasons, respectively.