Sensitivity of the Baltic Sea Overturning Circulation to Long-Term Atmospheric and Hydrological Changes

The sensitivity of the overturning circulation in the Baltic Sea is analyzed with respect to long-term changes in atmospheric and hydrological conditions by using two state-of-the-art ocean circulation models: RCO (Rossby Centre Ocean model) with a reference simulation and various sensitivity experiments as well as MOM (Modular Ocean Model). Historical reconstructions since 1850 lasting for >150 years are considered in order to identify coherences between the overturning stream function and surface wind, river runoff, major Baltic inflows, salinity, and water temperature. Long-term time series are evaluated statistically for several subbasins of the Baltic Sea for the two different models and for the sensitivity experiments concerning the interannual, multidecadal, and centennial variability. We found that the simulated overturning circulation has a response time scale with respect to wind or runoff of around 30 years. The overturning circulation will decrease basin-wide under anomalous wind conditions which hamper the deep water flow within the Baltic Sea, under a warmer climate or when river runoff increases. However, a global sea level rise would reinforce the overturning circulation. Overall, multidecadal variations of the overturning circulation are anticorrelated to and caused by the wind parallel to the cross-section of the overturning circulation. They are not caused by river runoff and the coherence with major Baltic inflows is small. Hence, the overturning circulation does not act as a good proxy for major Baltic inflows.