The outstanding 2004/05 drought in the Iberian Peninsula: associated atmospheric circulation

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The 2004/05 hydrological year (October 2004 to September 2005) was characterized by intense dry conditions affecting most of western Europe (35 degrees-55 degrees N and 10 degrees W-10 degrees E). In Iberia the drought affected every month of this period, with the southern half of Iberia receiving roughly 40% of the usual precipitation by June 2005. Moreover, this episode stands as the driest event in the last 140 yr, producing major socioeconomic impacts particularly due to the large decrease in hydroelectricity and agricultural production in both Iberian countries (Portugal and Spain). To assess the atmospheric submonthly circulation associated with this drought an Eulerian [weather types (WTs)] and a Lagrangean (objective storm tracks) analysis were combined. There was a dramatic drop in "wet" WT frequency during winter, with less than 50% of the normal value, and a corresponding increase of "dry" WTs. The storm-track analysis reveals an impressive northward displacement of cyclone trajectories in the North Atlantic sector in winter months, resulting in an almost complete absence of cyclones crossing Iberia and western Europe. At the monthly scale, the intense drought in Iberia was due to a combination of different physical mechanisms. First, the scarce precipitation observed between November 2004 and January 2005 was associated with positive North Atlantic Oscillation (NAO) indexes for these months. In February, the East Atlantic (EA) pattern seems to be the main driver. In March neither the negative NAO (-1.8) nor the positive EA (1.1) are capable of explaining the large negative precipitation anomalies. However, it is shown that during March 2005, an intense and anomalous blocking was displaced southward of its usual location, inhibiting the occurrence of precipitation over Iberia and leading to a negative NAO index anomalously associated with low precipitation records.
© 2007 American Meteorological Society. This work was partially supported by the Spanish Ministry of Science through Grant HP 2004-0039. We also thank the Instituto Nacional de Meteorología (INM) for providing the precipitation data for the Spanish stations. R. M. Trigo was supported by Projects VAST (Variability of Atlantic Storms and their impact on land climate), Contract POCTI/CTA/ 46573/2002, and CLIMAAT (Climate and Meteorology of the Atlantic Archipelagos), Interreg IIIb A3/2.3, both cofinanced by the EU under program FEDER. Three anonymous reviewers also helped to improve this manuscript.
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