Characteristics of large positive and negative surges in the Mediterranean Sea and their attenuation in future climate scenarios

This study analyzes large positive and negative storm surges along the Mediterranean coast in a 7-member climate model ensemble covering the period 1951-2050 under the A1B emission scenario. A hydro-dynamical shallow water model (HYPSE, Hydrostatic Padua Sea Elevation model) is driven by 6-hourly meteorological fields produced by the state-of-the-art global and regional climate models that have been used in the CIRCE fp6 project (Climate Change and Impact Research: the Mediterranean Environment). Model validation is based on the comparison of a model hindcast (1958-2001) and climate simulations with observed sea level (SL) values at 21 tide gauges. The accuracy of the models in reproducing large positive and negative storm surges appears to depend primarily on the quality of the atmospheric forcing (which is mainly related to their space resolution). Also the resolution of the hydro-dynamical model is somehow relevant along some stretches of the coastline. Climate signal is computed as the difference between the surge statistics in the 2021-2050 and 1971-2000 periods. The choice of the global climate simulation, which is used for the boundary conditions of the regional climate models, is shown to be the largest source of uncertainty for the assessment of the climate change signal. Other, less relevant, sources of uncertainty are the choice of the regional climate model and that of the hydro-dynamical model resolution. In spite of these uncertainties, the model ensemble mean shows a modest (about -. 5%), but clear and widespread decrease of the amplitude of both positive and negative large storm surges along the coast of the Mediterranean Sea. However, increase of mean SL and land subsidence (which are not considered in this study) might increase significantly the hazard posed by coastal floods in spite of the likely attenuation of storminess that is caused by climate change in the Mediterranean Se