Research Seminar - Tian Tian

11.00 -12.00

Sea surface fields in small ocean basins from the global circulation models (GCMs) are often greatly biased due to the coarse resolution. We configured a high-resolution coupled model system, comprising a regional climate model (RCM) and a regional ocean model, for the North Sea and Baltic Sea regions. Here, we assess the bias in the air-sea interface in the Baltic Sea, induced by the GCM boundary forcing in the uncoupled simulation, and improvements by the coupled one. The first experiment is forced by ERA-Interim (ERAI) reanalysis for the period 1990-2010. ERAI overestimates the Baltic ice extent by 72% in winter. The coupled run resulted in a 33% lower value than the uncoupled one, showing the best agreement in daily ice extent to observations. The second experiment is forced by EC-Earth CMIP5 climate simulations, with a focus on the historical period 1986-2005 for assessment. The EC-Earth forcing showed a significant warm bias in winter surface air temperatures and hence very low sea ice cover. This bias resulted in poor amplitude of seasonal cycle in surface air temperature and underestimation of sea ice cover in the uncoupled run. A remarkable improvement was found in the coupled run. In both experiments, the coupled simulation tends to mitigate the effects of bias induced by the forcing. In the presentation, we will discuss about the importance of choosing interactive (flux) coupling in a coastal region with complex coastlines.