Synoptic Situations Causing Drought in the Mediterranean Basin

Abstract

Anomalously dry months in the Mediterranean basin during the period 1900–1990 were identified from meteorological observations at seven stations located along the West-East axis of the Mediterranean basin (Lisbon, Madrid, Barcelona, Florence, Malta, Athens and Jerusalem). Different patterns of dry months are qualitatively identified on the basis of the spatial distribution of a Drought Index: 1) An almost uniform distribution of negative precipitation anomalies over the whole Mediterranean; 2) Strong negative anomalies to the West, but weaker or normal ones to the East, 3) Negatives anomalies to the West and East, but weaker or normal ones, or even positive anomalies in the central Mediterranean. The standardised sea level pressure at 56 grid points in the geographical domain from 35°N to 65°N and 30°W to 40°E are subjected to a T-mode Principal Component Analysis for each of the anomalously dry months. Monthly charts of mean standardised anomaly and mean sea level pressure are constructed for each principal component and each season in order to interpret the spatial distribution of the negative precipitation anomalies in terms of the mean circulation over the geographical domain. Finally, the frequencies of anticyclonic and cyclonic circulation types over Greece are compared with the mean precipitation and the mean number of days of precipitation in Greece (period 1950–1994) in order to interpret the severe drought in Greece during the period 1988–1990.