The western Mediterranean oscillation and rainfall in the Iberian Peninsula

Seasonal precipitation variability in the cast of the Iberian Peninsula is weakly linked to the North Atlantic Oscillation (NAO) during autumn and winter. For the purpose of improving the study of its performance, low-frequency variability patterns specific to the Mediterranean basin have been searched for. In this way, the Western Mediterranean Oscillation (WeMO) has been defined by means of the dipole composed, in its positive phase, by the anticyclone over the Azores and the depression over Liguria, and its index (WeMOi), as a result of the difference of the standardised values in surface atmospheric pressure in San Fernando (Spain) and Padua (Italy). This new index allows the detection of the variability relevant to the cyclogenesis next to the western Mediterranean basin, which determines in a predominant way the types of rainfall in the Gulf of Valencia. In this area, the WeMO is significantly better than the NAO to explain the monthly pluviometric anomalies during these seasons. Also, a daily resolution of the WeMOi can provide a useful tool to forecast torrential rainfall events in the north-western zones of the Mediterranean (eastern part of the Iberian Peninsula and the south of France), and such significantly daily rainfall frequencies for different thresholds. Copyright (c) 2006 Royal Meteorological Society.