Multidecadal climate variability dominated past trends in the water balance of the Baltic Sea watershed

The Baltic Sea watershed includes the territories of 14 countries in Northern, Central, and Eastern Europe. Long-term observations have shown that the climate in this area is characterised by a pronounced multidecadal variability, with a period of about 30 years, but its origin is thus far unknown. We propose that the observed ~30-year fluctuations in Baltic Sea salinity are caused by the Atlantic Multidecadal Variability and the North Atlantic Oscillation, which together modulate precipitation over the watershed and hence the river discharge into the Baltic Sea. The return of a large portion of the outflowing brackish Baltic Sea water with the inflowing salt water, due to mixing at the entrance area results in a positive feedback mechanism that amplifies the multidecadal variations in salinity. The strength of this self-amplification is considerable since atmospheric forcing has nearly the same periodicity as the response time of the freshwater content to external freshwater inputs.