Response to the summer of 2003 Mediterranean SST anomalies over Europe and Africa

The sensitivity of the atmospheric circulation to the warm Mediterranean sea surface temperature (SST) anomalies observed during the summer of 2003 (July and August) is studied using the European Centre for Medium-Range Weather Forecasts (ECMWF) model. A control integration imposes climatological Mediterranean SSTs as a lower boundary condition. The first sensitivity experiment uniformly increases these Mediterranean SSTs by 2 K, the approximate mean observed in the 2003 summer season. A second experiment then investigates the additional impact of the SST distribution by imposing the observed SST summer anomaly. The response of the atmospheric circulation in the European area shows some resemblance to the observed anomaly. The weakness of this response suggests, however, that the warm Mediterranean played a minor role, if any, in maintaining the anomalous atmospheric circulation as observed in the summer of 2003. Increasing SST in the Mediterranean locally leads to an increase in precipitation, particularly in the western Mediterranean. Furthermore, significantly increased Sahelian rainfall is simulated, deriving from enhanced evaporation in the Mediterranean Sea. In the ECMWF model the anomalously high moisture is advected by the climatological Harmattan and Etesian winds, where enhanced moisture flux convergence leads to more precipitation. The associated diabatic heating leads to a reduction of the African easterly jet strength. A similar Sahelian response has been previously documented using a different atmospheric model, increasing confidence in the robustness of the result. Finally, the results are discussed in the context of the seasonal predictability of European and African climate.