The Impacts of the NAO on Hydrological Resources of the Western Mediterranean

The present study analyses the impact of the North Atlantic Oscillation (NAO) on the precipitation and river flow regime of the western Mediterranean region. The spatial pattern of the NAO impact over western Europe and the Mediterranean is evaluated on a monthly basis showing that the NAO impact is well apparent and significant between December and March, and not so well defined between October and November. I have focused the attention on the impact of the NAO pattern on the river flow regimes for the three largest transboundary Iberian river basins, namely the Douro (north), the Tejo (centre) and the Guadiana (south). Results show that the large inter-annual variability of these three rivers flow is largely modulated by the NAO phenomena. Moreover, the magnitude of the relationship between NAO and river discharges increases substantially for the last period considered (1973-1998), with values being statistically significant at the 1%. Major changes in the precipitation regime of Iberia have occurred in the last 5 decades, with an outstanding decrease in precipitation being noticed in March since the 1960s, which is significantly associated with increasing probabilities of positive values of the NAO index. It is also shown that this decline of precipitation is inducing a significant decrease of river flow for the three Iberian river basins considered. Finally, it is well known that the precipitation and river flow regimes in Iberia present large values of inter-annual variability, being characterised by large disparities between wet and dry years. These characteristics will probably exacerbate in the coming decades as, according to modelling results, the entire Mediterranean basin is bound to become hotter and drier under an increased frequency of positive NAO winters. Therefore, if we consider the rising use of water in both Iberian countries (Portugal and Spain) for agricultural, touristic, hydroelectricity production and urban purposes, one can foresee an increasing stressed situation for the water resources management in the region directly driven by the NAO conditions.