The Impact of the North Atlantic Oscillation on river flow of three major Iberian rivers

Abstract

We have assessed the impact of the North Atlantic Oscillation (NAO) on the winter precipitation at various spatial scales, namely; Europe, Iberian Peninsula and Portugal. Results show that the large inter-annual variability of winter precipitation in Portugal is largely modulated by the NAO mode. The precipitation composite corresponding to high NAO (> 0.5) index presents a considerable lower median value than the corresponding low NAO (< –0.5) index class. Furthermore, the entire precipitation distribution associated to the low NAO index composite encompasses a wider range of values than the corresponding high NAO index composite. Finally, we have assessed the impact of the NAO on winter river flow regimes for the three main international Iberian rivers, the Douro (north), the Tagus (centre) and the Guadiana (south). Results show that the large inter-annual variability of flow of these three rivers is largely modulated by the NAO. Such modulation, associated with the recent positive trend of the NAO index, might implicate a significant decrease of the available flow. This reduction can represent an important hazard for the Portuguese and Spanish economy due to its impact in agricultural yield and hydroelectric power production.