Abstract. In this work, cluster and principal component analysis are used to divide Spain in a limited number of climatically homogeneous zones, based on seasonal rainfall for 32 Spanish localities for the period 1912-2000. Using the hierarchical technique of clustering Ward's method, three clusters have been obtained in winter and spring, and four clusters have been obtained in summer and autumn. Results are similar to those obtained by applying principal component analysis. Centroid series of each cluster and principal component series of each EOF have been compared to analyze the temporal patterns. The comparison of both methods indicates that cluster analysis is suitable to establish spatiotemporal patterns of seasonal rainfall distribution in Spain.

In many regions of the world, planning agricultural and water management activities is usually done based on probabilities for seasonal or monthly rainfall, for specified intervals of values. These intervals of rainfall amounts are commonly grouped into three categories: drought, normal rainfall, and abundant rainfall. Changes in the probabilities of occurrence of rainfall amounts within these climatic rainfall categories will influence the decisions that farmers and water managers will take. This research explores the changes produced by the North Atlantic oscillation (NAO) on the probability that local monthly rainfall takes in the southern Iberian Peninsula. The evolution of the NAO was divided into three phases: negative NAO, 'normal' NAO, and positive NAO, and local rainfall series were divided into three groups, corresponding to each NAO phase. The resulting empirical distribution functions were analysed and modelled by Gamma distributions. The results allow one to estimate the change in the probabilities of wet and dry months when a change in NAO phase is produced. The main result of this work is that changes in the probability of occurrence of climate categories of rainfall are more complex than only an increase of rainfall amount during the negative NAO phase and a decrease during the positive NAO phase. In fact, a certain asymmetry is detected in January, with more extremes linked to the negative NAO phase.

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In many regions of the world, agricultural and water management is usually based on probabilities of seasonal or monthly rainfall, commonly grouped into 3 categories: drought, normal rainfall, and abundant rainfall. Changes in the probabilities of the occurrence of rainfall amounts within these climatic rainfall categories will influence decision-making by farmers and water managers. This study explores the changes induced by the North Atlantic Oscillation (NAO) on the probability of winter rainfall in Spain. The evolution of the NAO was divided into 3 phases: negative NAO (NAO-), neutral NAO (NAOn), and positive NAO (NAO+), and rainfall series were divided into 3 groups corresponding to each NAO phase. Resulting empirical distribution functions allow an estimate of the changes in the probabilities of wet and dry winters due to changes in the NAO phase. Changes in the probability of the occurrence of the rainfall categories are more complex than mere increases in rainfall amount during NAO-and decreases during NAO+ phase. The spatial distribution of impacts is asymmetric, with higher probability of extremes linked to NAO-in the western area of the Iberian Peninsula. KEY WORDS: North Atlantic Oscillation · Rainfall · SpainResale or republication not permitted without written consent of the publisher

Agricultural and water management is usually based on probabilities of seasonal or monthly rainfall, the quantity of which is commonly grouped into 3 categories: drought, normal, and abundant. Changes in the probability of rainfall amounts will influence decision-making by farmers and water managers. This study explores the changes induced by the El Niño-Southern Oscillation (ENSO) on the probability of seasonal rainfall in Spain. We analyzed the seasonal ENSO index, divided into 3 phases: positive (La Niña), neutral, and negative (El Niño). Seasonal rainfall in Spain for 1912-2000 was divided into 3 groups corresponding to each ENSO phase. Resulting empirical distribution functions allow an estimate of the changes in the probability of seasons being wet or dry that are due to changes in the ENSO phase. The analysis was made considering contemporaneous seasons, and rainfall series lagged 1 and 2 seasons. ENSO affects rainfall in Spain as follows: (1) In autumn, El Niño leads to a null probability of drought, while La Niña leads to a low probability of wet conditions in the whole country, except in the north; in the Mediterranean area in the following spring, an autumn El Niño leads to drought and an autumn La Niña to a null probability of drought.(2) In winter there is no ENSO influence, perhaps as a consequence of the predominance of the North Atlantic Oscillation (NAO). (3) In spring, La Niña leads to a low probability of drought in the north. (4) In summer, La Niña leads to drought in SW Spain, as well as in the north in the following winter; in addition, in the following autumn there are low probabilities of drought after El Niño, and of wet conditions after La Niña. KEY WORDS: ENSO · Seasonal rainfall · Extreme events · SpainResale or republication not permitted without written consent of the publisher Clim Res 30: 1-12, 2005 and Europe during warm (cold) ENSO events presents a generally negative (positive) SLP over the central North Atlantic and a positive (negative) one north of 60°N. This pattern is consistent with a weakening (strengthening) of the normal north-south SLP gradient across the North Atlantic and thus a negative (positive) phase of the North Atlantic Oscillation (NAO) during the warm (cold) ENSO events (Moron & Gouirand 2003, Pozo-Vázquez et al. 2005.For the warm ENSO phase, low surface temperatures and precipitation are observed in Scandinavia, while wet and warm conditions tend to prevail in SW Europe (Knippertz et al. 2003). In the spring season following a cold event, temperatures are lower than normal over western Europe and NW Africa. ENSO strongly influences precipitation in spring, with reduced (enhanced) rainfall in NW Africa and the eastern parts of the Iberian Peninsula after a warm (cold) event, and opposite anomalies in central Europe (Knippertz et al. 2003). The basic mechanism for the interaction between ENSO and precipitation in Europe is poorly understood. It may be related to ENSO-induced SST anomalies in the tropical North Atlantic, an effect that is stronges...