Climatic Feedbacks during the 2003 European Heat Wave

Abstract

Abstract During the summer of 2003, a record heat wave over Europe occurred, to which the deaths of over 20 000 people in 18 countries were attributed, 10 000 of those in France alone. Temperatures across Europe were above normal for most of the summer, but reached their peak during the first two weeks of August, when most of the deaths occurred. Ensemble simulations done with a recent version of the Center for Ocean–Land–Atmosphere Studies (COLA) atmospheric general circulation model (AGCM) have been analyzed over the European region. The simulations were forced by weekly mean observed sea surface temperature (SST). Relative to the 1982–2001 period, the COLA AGCM simulated anomalous warmth over the European region during June–August 2003, in response to the observed SST; however, the simulated magnitude was smaller than that observed. A series of simulations in which the observed SST was used only south of 25°N or only north of 25°N suggest that it was the influence of the local SST rather than remote SST that was an important influence on the heat wave. By early June, the soil over much of the European region was anomalously dry, consistent with the below-normal precipitation observed over the region in the preceding months. The model was restarted in early June with observation-based soil wetness anomalies imposed on the soil wetness from the control simulations. Although the model soil wetness was then allowed to evolve as usual, the simulations with the imposed initial soil wetness anomaly enhanced the simulated surface temperature anomaly during June–August by 1°–2°C. This weaker-than-observed temperature anomaly may be related to the persistence of imposed soil wetness anomalies, which was weaker than that observed. The experiments suggest that both the warm local SST and the dry local soil were important in intensifying the 2003 European heat wave.