Temperature variability of the Baltic Sea since 1850 and attribution to atmospheric forcing variables

The Baltic Sea is highly impacted by global warming and other anthropogenic changes and is one of the fastest-warming marginal seas in the world. To detect trends in water temperature and to attribute them to atmospheric parameters, the results of two different ocean circulation models driven by reconstructed atmospheric forcing fields for the period 1850-2008 were analyzed. The model simulations were analyzed at temporal and spatial scales from seasonal to centennial and from intrabasin to basin, respectively. The strongest 150-year trends were found in the annual mean bottom temperature of the Bornholm Deep (0.15K/decade) and in summer mean sea surface temperature (SST) in Bothnian Bay (0.09-0.12K/decade). A comparison of the time periods 1856-2005 and 1978-2007 revealed that the SST trends strengthened tenfold. An attribution analysis showed that most of the SST variability could be explained by the surface air temperature (i.e., sensible heat flux) and the latent heat flux. Wind parallel to the coast and cloudiness additionally explained SST variability in the coastal zone affected by the variations in upwelling and in offshore areas affected by the variations in solar radiation, respectively. In contrast, the high variability in stratification caused by freshwater and saltwater inflows does not impact the long-term variability in the SST averaged over the Baltic Sea. The strongest SST trends since the 1980s can be explained by the superposition of global warming and a shift from the cold to the warm phase of the Atlantic Multidecadal Oscillation.