Decadal-to-Centennial Variability of Salinity in the Baltic Sea

A transient multicentury simulation mimicking natural variability has been performed for the Baltic Sea. The simulation is used for investigations of long-term trends of salinity in the Baltic Sea with special focus on periods of salinity reduction. Periods with decreasing salinity over 10 yr are found to appear approximately once per century. Considering extended periods of salinity reduction, as observed from 1976 to 1992, such events are found to be quite exceptional. Based on the climate simulation, a return period of 200 yr is estimated. River discharge, net precipitation (precipitation minus evaporation), and zonal wind are identified as the most important drivers for salinity variations in the Baltic Sea. For multidecadal periods, almost two-thirds of the salinity variability can be explained by annual means of river discharge, precipitation, both wind components, temperature, and the North Atlantic Oscillation, when multilinear regression techniques are used. However, the evaluation of wavelet coherences among the time series highlights that this relationship is not constant in time. At least three periods exist, each spanning roughly 50 yr, where the coherence between salinity and runoff as the common main driver is rather weak. This indicates that the importance of river discharge might be limited for certain periods, and drivers such as zonal wind may become more important. Finally, the variability of the Baltic Sea salinity shows increased power on time scales of 100 yr and longer. Such periodicity has never been shown for Baltic Sea salinity, and the driving mechanism remains unclear.