Main characteristics of the long-term sea level variability in the Baltic sea

The horizontal variation of the sea level variability for periods between a few days and several years has been investigated using daily means of sea level records obtained along the coasts of Sweden in the period 1977—1987. Motions on these periods are forced, either “externally” by the varying sea level outside the mouth and the freshwater supply, or “internally” by varying air pressure, wind and density in the Baltic Sea. Free, natural oscillations (seiches) of the Baltic Sea are generally considered to have periods from two days and shorter, and may thus contribute some variance to the daily means of the sea level and by that to the studied motions. The externally and internally forced contributions to the sea level records in the Baltic Sea are separated using a model for the externally forced contribution. The externally forced sea level variations explain most of the variance for periods longer than one month, and between 50 and 80% of the total sea level variance in the Baltic Sea with maximum in the central parts (the Stockholm area). It is also found that for periods shorter than about one month the internally forced oscillations are kinematically similar to those occurring in the first natural seiche mode in a closed Baltic Sea, with maximal variability in the extreme north and south, and minimum in the Stockholm area. For longer periods, however, the internally forced oscillations are kinematically similar to those occurring in an open bay with increasing amplitudes from the mouth and inwards. The shift in the kinematics of the internally forced oscillations is explained by the limited transport capacity of the straits in the mouth for “high frequency” motion.