ALBERT

Description: This work deals with various aspects of the atmospheric response to mid-latitude SST fronts, in particular in the Gulf Stream region. It consists of three parts: In the first part the effect of enhanced model resolution on the correct representation of the processes in the frontal region is discussed, based on analysis of a set of experiments using the atmospheric general circulation model ECHAM5. Precipitation shows sensitivity to resolution, but most other quantities that we analysed do not. Therefore, this sensitivity is likely caused by the parameterisation of atmospheric convection. In winter, also the representation of the North Atlantic storm track seems to benefit from higher model resolution. The second part focuses on the question of whether or not SST variability in the Gulf Stream region in general has an influence on the atmospheric processes related to the SST front. A transient ECHAM5 ensemble run, shows that convective precipitation has a high sensitivity to atmospheric boundary conditions in the region of the SST front and that local SSTs and convective precipitation are highly correlated there. In summer the so-called pressure adjustment mechanism, which has been identified in the climatological mean state, is enhanced when warm SST anomalies occur. In winter the enhanced precipitation is likely related to atmospheric fronts. Their frequency is not enhanced significantly, but the amount of precipitation per front is effected. The last part of this work deals with ocean-atmosphere interactions in a warming climate, based on a long-term integration with the Earth System Model MPI-ESM. The objective was to assess to what extend SST changes in the North Atlantic contribute to the global warming signal in the atmosphere, locally as well as on larger spatial scales. For the region of the historical Gulf Stream SST-front - a region were the historical run shows a local maximum in precipitation - the model projection shows a decrease in winter-time precipitation that is connected to a weakening of the SST gradients there. The large-scale atmospheric response in contrast was found to be mainly a direct response to the changed radiative forcings and the large scale SST increase and shows only little evidence of an impact from changes in the SST patterns in the Gulf Stream region.