On temporal wind variations forcing salt water inflows into the Baltic Sea

Salt water inflows into the Baltic Sea are important processes for maintaining the general stratification and the ventilation of the bottom water in deep basins of the central Baltic. These events occur randomly during the winter season at intervals from one to several years. This pattern changed in the mid-seventies when only weak or no major inflows were observed. During that period a steady loss of salt together with a steady increase in hydrogen sulphide concentrations was observed in the central Baltic deep water.

It is generally assumed that strong westerly winds force a salt water inflow. Long time series of daily wind records at the meteorological station Arkona and sea level observations at Landsort between 1951 and 1990 have been analyzed in order to find characteristic sequences being associated with inflow events. A necessary condition for a salt water inflow is for the wind to blow from west for several tens of days. The weighted mean of the yearly cycle of the wind components for years without and with salt water inflows revealed that this condition happens usually in November and December. However, in years with inflows, a long-lasting easterly wind occurs in October and early November just before the strengthening of westerly winds. A similar sequence is observed in the yearly cycle of the mean sea level of the Baltic Sea, i.e., in years with inflows a lowering of the mean sea level precedes the increase of the sea level in November to December. Hence, major salt water inflows are very likely forced by a sequence of easterly winds in late autumn lasting for 20—30 days followed by strong to very strong westerly winds of similar duration.