Spatial patterns of statistical relationship between the North Atlantic Oscillation and the Western Mediterranean Oscillation, and precipitation in Peninsular Spain

The Iberian Peninsula, located in a transitional zone between the temperate oceanic climate of the mid-latitudes and the Mediterranean climate, exhibits significant temporal precipitation variability. The contrasting topography further enhances considerable spatial variability. Using the MOPREDAS monthly precipitation database for Peninsular Spain, with a fine resolution (10 × 10 km), the spatial patterns of correlation between the NAOi and the WeMOi and precipitation for the period 1915-2015, as well as three subperiods, are analyzed. The results reveal almost opposite spatial patterns between the two low-frequency variability indexes, especially in December, the month with the highest correlation. The NAOi shows a clearly negative correlation with precipitation in the southwestern quadrant of peninsular Spain, sometimes extending diagonally towards the northeast. In contrast, the most typical pattern of the WeMOi is a dipole, with negative correlations in the eastern-southeastern peninsula and positive correlations along the eastern Cantabrian coast. Additionally, we examine the similarities between the aforementioned correlation maps and other rainfall-related variables, such as the interannual coefficient of variation, drought duration, the consecutive disparity index, the concentration index, and precipitation seasonality. The location of the maximum concentration index value corresponds to the maximum negative correlation between the WeMOi and precipitation, as well as to an autumn rainfall maximum.