Changes in the probability of extreme daily precipitation observed from 1951 to 2002 in the Iberian Peninsula

In this study trends in the probability of extreme rainfall in the Iberian Peninsula at a daily scale from 1951 to 2002 have been analysed. A simple statistical model of daily precipitation based on the gamma distribution is applied to data from 22 stations in the Iberian Peninsula from 1951 to 2002. The Mann-Kendall test was applied to look for trends of the distribution parameters. It is shown that the scale parameter remains stable, while the shape parameter is most variable, spatially and temporally, with significant negative trends in many stations. As a consequence, the trend of the probability of daily rainfall lesser than the 5th percentile is positive, mainly to the north and to the south of the Iberian Peninsula; meanwhile the probability of daily rainfall higher than the 95th percentile is negative. This behaviour, with slight spatial differences, is observed in the four seasons of the year. Results corresponding to the probability of heavy rainfalls are confirmed using the stretched exponential distribution. The possible influence of teleconnection patterns (North Atlantic Oscillation, East Atlantic, Mediterranean Oscillation pattern) is analysed calculating the correlation coefficients between teleconnection indices and the gamma distribution parameters. The winter NAO index shows significant relationships with the scale parameter, but not with the shape parameter. The East Atlantic pattern shows significant correlation coefficients with the shape parameter of 5 stations in winter, and 5 stations in autumn, but these results are not conclusive. These results suggest that the decrease of this parameter, and the behaviour of extreme rainfall probabilities, must be explained by other mechanisms. Copyright (C) 2009 Royal Meteorological Society