Influence of the El Nino-Southern Oscillation on the probability of dry and wet seasons in Spain

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 Nnio-Southern Oscillation (ENSO) on the probability of seasonal rainfall in Spain. We analyzed the seasonal ENSO index, divided into 3 phases: positive (La Nina), neutral, and negative (El Nino). 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 Nino leads to a null probability of drought, while La Nina 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 Nino leads to drought and an autumn La Nina 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 Nina leads to a low probability of drought in the north. (4) In summer, La Nina 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 Nino, and of wet conditions after La Nina.