Modelling sea level variability in different climates of the Baltic Sea

Within the Baltic Sea Region INTERREG IIIB project `Sea level change affecting the spatial development in the Baltic Sea region' (SEAREG, 2002-2005) sea levels in past and future climate were investigated based upon 6-hourly regional model results. The main factors affecting the longterm mean sea level in the Baltic Sea are the land-uplift, the eustatic sea level rise, and the water balance of the Baltic Sea. The land uplift (or the glacio-hydro-isostatic effect) is the Earth's response to the past changes in ice and water loads. Relative to the mean sea level a maximum uplift of 9.0 mm yr in the Bay of Bothnia is found from long records of observations (Ekman, 1996). The mean sea level is not stationary in time but rising relative to the geoid. This eustatic sea level rise is estimated to be 1-2 mm yr during the 20 century (Church et al., 2001). In global atmosphere-ocean general circulation models (AOGCMs), at least a third of the 20 century anthropogenic eustatic sea level rise is caused by thermal expansion, which has a geographically non-uniform signal in sea level change (Church et al., 2001). Other factors are the recent melting of glaciers and ice caps in Greenland and Antarctica and the long-term development of ice sheets. The largest uncertainty of the model results is in the terrestrial storage terms. The global average sea level is projected to rise from 1990 to 2100 in the range between 0.09 and 0.88 m (Church et al., 2001). Thereby it is assumed that the West Antarctic ice sheet, containing ice to rise the global average sea level by 6 m, is stable. The third factor affecting the long-term mean sea level is the water balance of the Baltic Sea, which is closely related to the sea level pressure (SLP) patterns over the North Atlantic. The dominant pattern over Northern Europe in winter is the North Atlantic Oscillation (NAO). The sea level variability on time scales longer than 1 yr correlates significantly with the NAO index. In greenhouse gas scenarios of some global models an increase of the NAO index was found giving rise to an increased winter mean wind speed. Consequently, an increase of the winter mean sea level in the Baltic Sea should be expected.