Contribution of Regional Climate Drivers to Future Winter Sea-Level Changes in the Baltic Sea Estimated by Statistical Methods and Simulations of Climate Models

A statistical downscaling approach is applied to the output of different global climate model simulations driven by SRES A2 future scenarios of greenhouse gas concentrations to estimate the contribution of changes in the atmospheric circulation and in precipitation to regional future winter sea-level changes. The method is based on observed statistical relationships between Sea level as predictand and large-scale climate fields as predictors. The approach is applied to the Baltic Sea as it is one of the largest brackish seas in the world and a clear example for a complex coupled ocean-atmosphere land system with a complex coastline and bathymetry. Recent studies indicated that Baltic Sea level variations on decadal and longer time scales are strongly influenced by atmospheric forcings, but the influence of different large-scale forcing factors on sea level vary geographically. While the decadal sea level variations in the northern and eastern Baltic gauges are strongly influenced by the atmospheric circulation, the decadal variations in the southern Baltic Sea can be (statistically) better explained by area-averaged precipitation. The results indicate that future trends in sea-level rise caused by these forcing are larger than the past variability. Using sea level pressure as predictor for the central and eastern Baltic Sea level stations, three climate models lead to 21st century future trends in the range of the order of 1 to 2 mm/year. Using precipitation as predictor for the stations in the Southern Baltic Coast all five models lead to significant trends with a range of the order of 0.4 mm/year. These numbers are smaller, but of the order of magnitude as the predicted future global sea level rise. Nevertheless, these estimations comprise only a partial contribution of selected large-scale regional predictors and an estimation of the total regional sea-level rise has to consider other regional factors such as the isostatic contribution to relative sea-level changes or substantial changes in the sea-ice cover and global sea level rise.