The NAO signal in daily rainfall series over the Iberian Peninsula

The North Atlantic Oscillation (NAO) is the principal mode of climatic variability over the North Atlantic, playing an important role in the climate of the Iberian Peninsula. Generally, investigations into the effect of the NAO over this area have been performed using data at a monthly scale. Here, we conducted a more detailed study using series of the daily rainfall over Iberia, defining several indices related to the frequency and intensity of the rainfall in this area. The objective was to search for possible relationships between these daily rainfall indices, which hold information that cannot be reproduced using monthly rainfall series, and the NAO index. The winter NAO index (wNAO) was found to have clear negative correlations with high values of both the frequency and the intensity of winter rainfall over certain regions of the peninsula, and with the winter rainfall maxima, and a positive correlation with winter dry spell maxima. The regions showing the strongest correlations were the west, southwest, and interior of the peninsula. While the wNAO was found to affect essentially the category of intense rainfall in this area, both in frequency and in accumulated values, other rainfall categories (especially light and moderate rainfall) were less affected by changes in the wNAO. In sum, the association between the wNAO and the winter total rainfall indices, in number of cases and in accumulated values, is primarily due to the effect that wNAO has on intense winter rainfall events.