Dependence of salinity and oxygen concentrations in the Baltic Sea on large-scale atmospheric circulation

The relationship between the low-frequency (annually averaged) salinity and oxygen concentrations in the last 30 yr in the Baltic Sea and the large-scale atmospheric circulation was statis- tically analyzed. It was found that at these time scales the evolution of salinity and oxygen is quite homogeneous in depth, i.e. deviations from the long-term mean tend to be of the same sign in the upper and deeper layers simultaneously. Moreover, the evolution of the salinity and oxygen concentrations are negatively correlated within each layer: lower than normal salinities tend to be observed simulta- neously with higher than normal oxygen concentrations, and vice versa. The statistical analysis seems to indicate that the forcing by the atmospheric circulation may be responsible for this anticorrelation between salinity and oxygen. A stronger meridional sea-level-pressure (SLP) gradient over the North Atlantic, and therefore stronger westerly winds, causes positive rainfall anomalies in the Baltic Sea catchment area and increase run-off giving rise to decreased salinities at all depths. The mechanisms by which a stronger zonal atmospheric circulation enhances the oxygen concentrations may be related either to a weakened stratification through the reduced salinity (at long time scales), or by stronger or more frequent inflows of North Sea waters (at short time scales). The influence of the atmospheric cir- culation can explain at these time scales of the order of 60 and 40% of the variability of these hydro- graphic variables, respectively. The question of how salinity and oxygen in the Baltic Sea may respond to changes of the atmospheric circulation in a global climate change scenario is also addressed. For this purpose the intensities of the relevant atmospheric circulation patterns in a transient experiment with a coupled atmosphere-ocean general circulation model forced with increasing anthropogenic green- house gas concentrations between the years 1860 and 2099 were estimated. In this integration the change of the atmospheric circulation most relevant for salinity and oxygen in the Baltic Sea seems to be an enhancement of the meridional SLP gradient in the North Atlantic. Thus, under the assumption that the statistical relationship between SLP and the hydrographic variables also holds in the future, this trend in the atmospheric circulation would lead to decreased salinity and improved oxygen condi- tions in the Baltic Sea.