Impacts of the North Atlantic Oscillation on the probability of dry and wet winters in Spain

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.