Heat waves in the mediterranean: A local feature or a larger-scale effect?

We analyse the anomalously warm summer months in the Mediterranean region using the 850 hPa temperature, T850, extracted from the ERA-40 reanalysis, in order to find how these anomalies are related to the anomaly of the jet stream over the Euro-Atlantic area. In this region, the westerly jet has two main branches: the Scandinavian jet and the Mediterranean jet, which, in summer, are at a meridional minimum distance. In addition, we analyse the heat waves in the Central Mediterranean in the last half century using the temperature observations collected at fifty surface stations distributed throughout the Italian peninsula and its two major islands, and we relate these events to the position of the Mediterranean jet. We find that, when these two jets are almost aligned, there is a streak of the Mediterranean jet over the Alps and, to the south of them, an anticyclonic vorticity aloft, which forces a strong subsidence and an adiabatic warming of the troposphere over the Mediterranean. This configuration is also a characteristic of anomalously warm spells over the basin. While, when the Mediterranean jet resides further south and along the northern rim of Africa, its meridional distance from the Scandinavian jet is relatively large, the vorticity over the Mediterranean is cyclonic and this region is relatively cool. The tropospheric temperature difference between these two configurations (Mediterranean jet over the Alps and jet over North Africa) is of the order of 3 degrees C. Since the correlation between the observed temperatures over Italy and the ERA-40 T850 in the Central Mediterranean is more than 90%, and more than 60% in the T850 of the entire Mediterranean basin, we conclude that the heat waves over Italy are representative of exceptionally warm episodes over the Central Mediterranean, and that a warm spell over Italy is a country-scale symptom of warm spells in the Mediterranean basin. Copyright (c) 2006 Royal Meteorological Society.