Combined effects of global warming and an AMOC shutdown on West African and European climate

We investigate the effects of an AMOC shutdown, for example, due to an influx of fresh water from Arctic ice sheet melting, in combination with global warming (IPCC's SRESA2 emissions scenario) on West African and European climate. Shutdown of the AMOC by the end of this century is generally seen as possible but not likely, but Arctic ice is melting more quickly than predicted by global models, and the consequences for climate may be severe and the changes abrupt. The NCAR/NOAA WRF model (v. 3.1.1.) on a large domain with 90-km grid spacing is used. The present-day control simulation uses the National Center for Environmental Prediction (NCEP) reanalysis 2 to specify initial surface and lateral boundary conditions. For the future simulation we use anomalies generated by coupled atmosphere-ocean GCMs applied to the NCEP2 reanalysis values to specify regional model boundary conditions, and SSTAs from GCM water hosing experiments that force a shutdown of the AMOC. In both the boreal spring and summer months, cooling in the eastern Atlantic due to the AMOC shutdown causes an eastward extension of the North Atlantic subtropical high over Europe, and rainfall rates decrease markedly throughout most of Europe. In May and June, rainfall rates decrease by 50-80% over Sahelian Africa as a secondary response to the eastern Atlantic cool SSTs, as dry air from Europe is advected southward. In contrast, the responses over Europe and West Africa are decoupled during the summer months, rainfall over Europe continues to decrease by up to 90%, and the response over West Africa is largely attributable to the response of the African monsoon to global warming. Along the coast, rainfall rates actually increase by up to 10%, while in the Sahel rainfall rates decrease by 40%, and small regions in the Sahara also experience an increase in rainfall.