Regional and local scale atmospheric forcing upon sea level along the coast of SW Europe

Sea level height variability along the coast of southwestern Europe is analyzed in relation with local and regional scale atmospheric forcing mechanisms. A circulation type (CP) catalogue, derived from a combination of principal component analysis and cluster analysis was used to characterize the regional scale atmospheric circulation, whereas the local scale was highlighted through the analysis of hourly wind records during extreme events. A large portion of common anomalous sea level variability at 6-hourly time frame can be explained by the effect of one CP, depicting a deep extra-tropical disturbance, close to the Iberian Peninsula, which represents the dominance of the inverse barometer mechanism. Other CPs can induce occasionally similar effects, but showing a spatially restricted scope, in relationship with the location of the main centres of action. Embedded into this synoptic framework, a local forcing, related to both meteorological processes (wind set up) and coastal oceanographic features (depth of the continental shelf) usually control the timing and less frequently, the magnitude of the surges. The decreasing trend observed in the magnitude of the sea level residuals derived from a hindcast model as well as some spatial differences (more intense trend in the upper percentiles along the northern Spain and Mediterranean coasts) can also be explained by long-term changes in the frequency, and specially, in the internal variability of the CPs, which might be linked to the recent positive phase and eastward displacement of the southern node of the North Atlantic Oscillation. An overall increase in sea level pressure, detected in all CPs, has been accompanied by significant increases in types characterized by anticyclones over the Iberian Peninsula and decreases of northerly flows, related to a reduction of the number of cyclones passing through the Gulf of Biscay and the Western Mediterranean Basin.