9 Salinity decreases from the Belt Sea to the Gulf of Finland and the Bothnian Bay. Stratification is reduced with distance from the Baltic Sea entrance. Almost no salinity di?erence between bottom and surface water is apparent in the most northern areas of the Baltic Sea and the Gulf of Finland. Mean salinity in surface and deep water layers for di?erent basins may be seen in Table 1. Prevailing winds in the area of the Baltic Sea are from the west and southwest. Characteristic values of the wind speed 10 m above the sea surface are 8, 6, 5 and 7 m/s for winter, spring, summer and autumn respectively [15]. The annual mean wind speed is approximately 6 m/s. The global frequency of winds in the Baltic is shown in Table 2. These data correspond to Vilsandi Island located in the Baltic proper, which are very similar to those of the Swedish island Gotland. The Baltic may be significantly covered by ice during the winter months. However, there is currently no scientific consensus on the influence this has on circulation in the Baltic Sea. The periods during which the ice cover is present in di?erent areas of the Baltic Sea for mild, normal and severe winters are presented in Table 3. TABLE 1. SALINITY IN SURFACE AND DEEP WATERS FOR DIFFERENT ZONES IN THE BALTIC Zone Ssurface Sdeep Arkona 8.5 17 Bornholm 8 16 Central Baltic 7.5 11.5 Gulf of Finland 6.5 10 Bothnian Sea 6 7 Bothnian Bay 3.5 4 TABLE 2. GLOBAL FREQUENCY OF WINDS IN THE BALTIC SEA Wind E SE S SW W NW N NE Frequency (%) 8 10 14 20 14 10 16 8 TABLE 3. DURATION (DAYS) OF THE ICE COVER [16] Zone Mild winter Normal winter Severe winter Bothnian Bay 90 120 150 Bothnian Sea 0 90 120 Gulf of Finland 90 120 150 Baltic Proper 0 0 90 TABLE 4. MEAN FRESHWATER SUPPLIES TO SEVERAL SUB-BASINS [17] AND SPM CONCENTRATIONS [18] Basin Supply (m3/s) SPM (g/m3) Bothnian Bay 3104 5.0 ± 1.5 Bothnian + Aland Sea 2860 4.5 ± 1.5 Gulf of Finland 3556 5.5 ± 1.5 Gotland + Northern + Southern Baltic 4630 3.0 ± 1.5