Low-frequency variability of seasonal rainfall over the Iberian peninsula and ENSO

Abstract

This study is concerned with the influence of El Nino-Southern Oscillation (ENSO) on seasonal precipitation over the Iberian Peninsula (IP), and the stability of this association over time. Principal Component Analysis (PCA) is applied to rainfall data in order to identify regions of coherent interannual variability. Two regions are identified in the IP, namely its south-eastern part, and the remaining region. Lag correlations have been applied to rainfall and the Southern Oscillation Index (SOI), the latter leading the former. For the entire period, 1900-1996, ENSO appears to be related to above-average autumn precipitation in both regions, and below-average spring rainfall over the south-eastern IP. For winter, the ENSO signal in the region is weak, and not statistically significant. Strongest correlations occur for leads of 3-6 months. However, when the ENSO-rainfall association is analysed over a 20-year moving window from 1900 to 1996, it becomes clear that it has changed substantially over time. For the last decades of the period, ENSO has been associated with below-average spring and winter rainfall over the south-eastern IP, and above-average autumn rainfall in the Peninsula as a whole. Over the south-eastern IP, the association for autumn has weakened considerably recently. The strength of the ENSO-rainfall association appears to be related to the frequency and/or intensity of ENSO events during the 1900-1996 period.