Quantifying the Impact of the North Atlantic Oscillation on Western Iberia

Abstract

The main objective of this work is to evaluate the influence, bothphysical and socio-economical, of the most important large-scale phenomenon of the Northern Hemisphere, the North Atlantic Oscillation (hereafter NAO), on the climate of the western Iberian region. Using high and low NAO index composites, statistically significant anomaly fields of climate variables are then interpreted based on physical mechanisms associated with anomalous large-scale circulation.

The Iberian Peninsula precipitation and river flow regimes are characterized by large values of inter-annual variability, with large disparities between wet and dry years. This is a major problem for water resource management, in general, and for the production of hydroelectricity, in particular. We have assessed the impact of the NAO on Iberian winter precipitation and river flow regimes for the three main international Iberian river basins, namely the Duero (north), the Tagus (centre) and the Guadiana (south). Results show that the large inter-annual variability of these three river flows is mostly modulated by the NAO phenomenon. Throughout most of the 20th century, the January-to-March river flow is better correlated with the 1-month-lagged (December-to-February, DJF) NAO index than is the simultaneous (DJF) river flow. Correlation values for the period 1973–1998 are highly significant, –0.76 for Duero, –0.77 for Tagus and –0.79 for Guadiana, being consistently of higher magnitude than those obtained in previous decades.

The majority of landslide episodes in the area north of Lisbon are associated with rainfall events of short (less than 3 days) or long duration (more than 20 days). Results for the low NAO class are crucial because these months are more likely associated with long-lasting rainfall episodes responsible for large landslide events. This is confirmed by the application of a 3-month moving average to both the NAO index and the precipitation time series. This procedure allows the identification of virtually all months with landslide activity as being characterized by negative average values of the NAO index and high values of average precipitation (above 100 mm/month).