Regionalization of climate change for the North Sea and Baltic Sea

Using a coupled ice/ocean model, an impact study on the influence of increased westerly winds in the North Sea and Baltic Sea due to strengthening of the polar front was carried out, as part of the German research project KLINO (Modelling inter-annual variability of hydrographic conditions in the North Sea and the Baltic Sea). By means of a simple sensitivity experiment it could be shown that the impact of a positive westerly wind anomaly (i.e. high North Atlantic Oscillation Index, NAO) results in an intensification of the cyclonic circulation in both the Baltic Sea and the North Sea, However, the impact on salt exchange and content is opposite for the 2 shelf seas: exchange between the North Sea and the North Atlantic increases with intensified westerly wind forcing and thus the North Sea salt content increases; for the Baltic Sea, the intensification of westerly winds inhibits exchange with the Kattegat. In combination with the increased doming of the lower layer (i.e. saltier) water in the center of the cyclonic eddies, and thus intensified vertical mixing, an overall decrease of salt content in the Baltic Sea and an erosion of the permanent halocline result. Thus, as it is expected that future climate, under the influence of increased atmospheric CO2 concentration, will lead to an intensification of the NAO, it was concluded that expected anthropogenic climate change will result in a weakening of haline stratification and a decrease in the salt content of the Baltic Sea.