Interannual salinity variations in the Baltic Sea during the period 1954–1990

The Baltic Sea is a semi-closed sea with a large net freshwater supply and a strong permanent halocline at 60–70 m. Above the halocline the salinity is low and rather homogeneous, and below the halocline there is a pronounced vertical stratification. Long time series of the salinity in different depth intervals are shown for the three major sub-basins, namely the Baltic proper, the Bothnian Sea and the Bothnian Bay. These are compared with time series of the freshwater supply (runoff from land), the estimated salt-water inflow and the estimated mixing rate. During the last decade, the salinity in the Baltic proper has decreased by approximately 1 psu in and below the halocline. Above the halocline the decrease is about 0.4 psu. Over the same period, there was an increased freshwater supply. How this increase may have diluted the mixed layer and impeded the input of saltwater from the Kattegat is discussed. It is argued that vertical mixing also has increased, thereby altering the vertical distribution of salt within the Baltic Sea.