Trends in daily rainfall in the Iberian Peninsula from 1951 to 2002

Rodrigo F.S, Trigo R.M.
International Journal of Climatology. 27: 513-529, DOI: 10.1002/joc.1409

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An analysis of 22 sites of daily precipitation records over the period 1951-2002 for the Iberian Peninsula is presented. Annual and seasonal total precipitation (P), number of wet days (N), precipitation intensity (I), the 95th percentile (P95), and percentage of rain falling on days with rainfall above the 95th percentile (%) are investigated. The annual and seasonal trends for these variables and for all 22 rain gauges are analysed, using the Mann-Kendall statistic, and a linear regression model. Moreover, a t-test is applied to the difference between the means of two subperiods, respectively 1951-1976 and 1977-2002. Principal results indicate a decreasing trend in P, I, and P95 for several northern and southern stations in winter; P, I, and P95 in some southern stations in spring, I and P95 in some southern stations in summer, and I in some northern and southern stations in autumn. The general behaviour is a decrease in the daily intensity of rainfall, while the number of wet days does not reveal pronounced changes. This pattern is valid for both annual and seasonal values of the indices. The decreasing trend found for I in winter and annual series for some localities may be related to the predominance of the positive phase of the North Atlantic Oscillation (NAO), but it is necessary to find other mechanisms for those stations and seasons not linked directly to NAO.