The contrasting effect of increasing mean sea level and decreasing storminess on the maximum water level during storms along the coast of the Mediterranean Sea in the mid 21st century

The maximum level that water reaches during a storm along the coast has important consequences on coastal defences and coastal erosion. It depends on future sea level, storm surges, ocean wind generated waves, vertical land motion. The future sea level in turn depends on water mass addition and steric contributions (with a thermosteric and halosteric component). This study proposes a practical methodology for assessing the effects of these different factors (which need to be estimated at sub-regional scale) and applies it to a 7-member model ensemble of regional climate model simulations (developed and carried out in the CIRCE fp6 project) covering the period 1951-2050 under the A1B emission scenario. Sea level pressure and wind fields are used for forcing a hydro-dynamical shallow water model (HYPSE), wind fields are used for forcing a wave model (WAM), obtaining estimates of storm surges and ocean waves, respectively. Thermosteric and halosteric effects are diagnosed from the projections of sea temperature and salinity. Steric expansion and storminess are shown to be contrasting factors: in the next decades wave and storm surge maxima will decrease while thermosteric expansion will increase mean sea level. These two effects will to a large extent compensate each other, so that their superposition will increase/decrease the maximum water level along two comparable fractions of the coastline (about 15-20%) by the mid 21st century. However, mass addition across the Gibraltar Strait to the Mediterranean Sea will likely become the dominant factor and determine an increase of the maximum water level along most of the coastline.