A gradual decrease in rainfall in most Mediterranean areas during the 20th century is an undeniable fact verified by statistically significant negative trends in precipitation series (Fig.1). As a consequence, severe drought episodes have become more frequent and persistent late in the century (e.g. Garcia-Herrera et al., 2007; Sousa et al., 2010). The IPCC report in 2007 supports most recent studies based on global and regional climate models, predicting also a trend towards less precipitation during the 21st century. The combined effects of precipitation decrease and the increment in the surface temperature will create important changes in the region’s water cycle, raising concerns of water shortage impacts in ecosystems, on agricultural and human use (e.g. Trigo et al., 2010), with serious social and economic effects. In short, the risk of drought episodes in the Mediterranean basin appears to be rising in recent decades and is bound to keep the upward tendency for the foreseeable future.

Drought indices are an important tool when analyzing moisture availability. One of the most widely employed indices of regional moisture availability is the Palmer Drought Severity Index (PDSI), which can be calculated using a relatively simple water budget system (Palmer, 1965). The PDSI was developed for the United States but it has been used extensively for other areas, often resulting in exaggerated classification of extreme conditions. This fact which led other authors to design other indices, such as the self-calibrated PDSI (scPDSI) introduced by Wells et al. (2004), or the Standardized Precipitation-Evapotranspiration Index (SPEI) introduced by Vicente-Serrano et al. (2010). These indices have proved to be quite efficient in terms of drought estimation, independently of the target area.

The climatological and climate change group has dedicated a large effort to characterize the large scale atmospheric circulation associated with major drought events such as: 1) the 2005 drought in Iberia (Garcia-Herrera et al., 2007), 2) the Middle East drought in 2007-2009 (Trigo et al., 2010) or the 2010 drought in China (Barriopedro et al., 2012). Additionally the group has been involved in the use of different drought indices to characterize long-term drought occurrence in the entire Mediterranean basin (Sousa et al, 2011) and remote sensed drought indices (Gouveia et al., 2009; 2012)

Figure 1. Relative trends (%) of mean monthly annual precipitation in the 1901-2000 period – only areas with mean monthly precipitation above 10mm and with statistical significance of 10% achieved are represented – and circled areas are statistically significant at 5% (modified Mann-Kendall test). (From Sousa et al., 2011)