Mediterranean wave climate variability and its links with NAO and Indian Monsoon

This study examines the variability of the monthly average significant wave height (SWH) field in the Mediterranean Sea, in the period 1958-2001. The analysed data are provided by simulations carried out using the WAM model (WAMDI group, 1988) forced by the wind fields of the ERA-40 (ECMWF Re-Analysis). Comparison with buoy observations, satellite data, and simulations forced by higher resolution wind fields shows that, though results underestimate the actual SWH, they provide a reliable representation of its real space and time variability. Principal component analysis (PCA) shows that the annual cycle is characterised by two main empirical orthogonal functions (EOF) patterns. Most inter-monthly variability is associated with the first EOF, whose positive/negative phase is due to the action of Mistral/Etesian wind regimes. The second EOF is related to the action of southerly winds (Libeccio and Sirocco). The annual cycle presents two main seasons, winter and summer characterised, the first, by the prevalence of eastwards and southeastwards propagating waves all over the basin, and the second, by high southwards propagating waves in the Aegean Sea and Levantin Basin. Spring and fall are transitional seasons, characterised by northwards and northeastwards propagating waves, associated to an intense meridional atmospheric circulation, and by attenuation and amplification, respectively, of the action of Mistral. These wave field variability patterns are associated with consistent sea level pressure (SLP) and surface wind field structures. The intensity of the SWH field shows large inter-annual and inter-decadal variability and a statistically significant decreasing trend of mean winter values. The winter average SWH is anti-correlated with the winter NAO (North Atlantic Oscillation) index, which shows a correspondingly increasing trend. During summer, a minor component of the wave field inter-annual variability (associated to the second EOF) presents a statistically significant correlation with the Indian Monsoon reflecting its influence on the meridional Mediterranean circulation. However, the SLP patterns associated with the SWH inter-annual variability reveal structures different from NAO and Monsoon circulation. In fact, wave field variability is conditioned by regional storminess in combination with the effect of fetch. The latter is likely to be the most important. Therefore, the inter-annual variability of the mean SWH is associated to SLP patterns, which present their most intense features above or close to Mediterranean region, where they are most effective for wave generation.