INTERNATIONAL JOURNAL OF CLIMATOLOGY Int. J. Climatol. 26: 1477–1487 (2006) Published online in Wiley InterScience (www.interscience.wiley.com) DOI: 10.1002/joc.1389 HEAT WAVES IN THE MEDITERRANEAN: A LOCAL FEATURE OR A LARGER-SCALE EFFECT? MARINA BALDI,* GIOVANNI DALU, GIAMPIERO MARACCHI, MASSIMILIANO PASQUI and FRANCESCO CESARONE Institute of Biometeorology, IBIMET – CNR, Via dei Taurini, 19-00185 Rome, Italy Received 29 May 2005 Revised 11 March 2006 Accepted 11 April 2006 ABSTRACT 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 ° 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  2006 Royal Meteorological Society. KEY WORDS: climate variation; heat waves; jetstreak; jetstream; Mediterranean; Italy 1. INTRODUCTION The Mediterranean basin is located in a transitional geographical zone bounded by the Euro-Asian region and by the North African desert, and it is directly under the effect of the Atlantic Ocean (Meteorological Office, 1962). In the longitudinal direction, the basin extends between 10 ° W and 35 ° E, showing a distinct pressure seesaw pattern between east and west not only at the surface (Maheras et al., 1997; Palutikof, 2003), but also at 500 hPa (Piervitali et al., 1999). However, during the warm season, the amplitude of this oscillation is at its minimum, and the pressure and temperature fields are almost levelled throughout the entire Mediterranean basin. During this season, the high Alpine mountain range reduces the airflow from the Atlantic into the Central Mediterranean basin. This is when the Western and Central Mediterranean basins are mainly influenced by a western extension of the Azores high (Xoplaki, 2003a), while the Eastern most part of the basin is under the influence of the Asian monsoon (Ziv et al., 2004), since a Rossby wave generated by this monsoon enhances the geopotential of the ridge over the East Mediterranean (Rodwell and Hoskins, 1996). * Correspondence to: Marina Baldi, CNR – IBIMET, Via dei Taurini, 19, 00185 – Roma, Italy; e-mail: m.baldi@ibimet.cnr.it Copyright  2006 Royal Meteorological Society