Trends of extreme precipitation and associated synoptic patterns over the southern Iberian Peninsula

This paper investigates the extreme precipitation events in the southern Iberian Peninsula (Andalusia) using 86 stations with daily precipitation records for the period 1955-2006. Seasonal (excluding summer) and monthly trends were established for different extreme-precipitation indices. Both the stability and the significance of the trends were examined. The findings indicate a significant spatial and seasonal variability. The most noteworthy results were found in winter, with a predominance of decreasing trends for intensity indices in western and central (CW) Andalusia, whereas positive trends appeared in the south-eastern (SE) zone. These marked differences in the sign of trends in short distances evidence the importance of using a dense network of stations for regional analysis of extremes. Overall, spring analysis reveals a negative trend in the intensity indices, especially in March. In autumn, the significance of the trends proved lower in comparison to winter and spring, and only September presented significant increasing trends in intensity indices. The synoptic patterns associated with heavy precipitation events were compared with two large-circulation indices affecting the precipitation over Andalusia (the North Atlantic Oscillation, NAO, and the Western Mediterranean Oscillation, WeMO). This analysis was made for the two sub-areas of Andalusia separately. The synoptic patterns at the surface and 500-hPa level were determined by the use of a principal component analysis (PCA) in T-mode coupled with clustering techniques. The seasonal distribution and the location of disturbances of the atmospheric patterns found were clearly differentiated in the two areas. The tendencies found for the indices agree with the changes in the frequency of the main synoptic patterns for each area. Furthermore, most of these synoptic patterns were related to highly negative WeMO values. Conversely, only some of the patterns associated with heavy rainfall in the western and central area presented significant negative values of the NAO index. (C) 2011 Elsevier B.V. All rights reserved.