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-sc ale atmospheric circulation was statistically analyzed. Ii: was found that at these time scales the evolution of salinity and oxygen is guile homogene ous in depth, i.e. deviations from the long-term mean tend to be of the sam e sign in the upper and deeper layers simultaneously. Moreover, the evoluti on of the salinity and oxygen concentrations are negatively correlated with in each layer: lower than normal salinities tend to be observed simultaneou sly with higher than normal oxygen concentrations, and vice versa. The stat istical analysis seems to indicate that the forcing by the atmospheric circ ulation may be responsible for this anticorrelation between salinity and ox ygen. A stronger meridional sea-level-pressure (SLP) gradient over the Nort h Atlantic, and therefore stronger westerly winds, causes positive rainfall anomalies in the Baltic Sea catchment area and increase run-off giving ris e to decreased salinities at all depths. The mechanisms by which a stronger zonal atmospheric circulation enhances the oxygen concentrations may be re lated either to a weakened stratification through the reduced salinity (at long time scales), or by stronger or more frequent inflows of North Sea wat ers (at short time scales). The influence of the atmospheric circulation ca n explain at these time scales of the order of 60 and 40 % of the variabili ty of these hydrographic variables, respectively. The question of how salin ity 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 i n a transient experiment with a coupled atmosphere-ocean general circulatio n model forced with increasing anthropogenic green house gas concentrations between the years 1860 and 2099 were estimated. In this integration the ch ange of the atmospheric circulation most relevant for salinity and oxygen i n the Baltic Sea seems to be an enhancement of the meridional SLP gradient in the North Atlantic. Thus, under the assumption that the statistical rela tionship between SLP and the hydrographic variables also holds in the futur e, this trend in the atmospheric circulation would lead to decreased salini ty and improved oxygen conditions in the Baltic Sea.