Increasing mean sea level and decreasing storminess: a multi-model and multi-scenario estimate of contrasting factors that will affect the Mediterranean coastline in the 21st century

This study estimates the factors (storm surges, ocean wind generated waves, sea level rise) responsible for the maximum level that water reaches during a storm along the coast of the Mediterranean Sea. The results of regional climate models are used for driving wave and storm surge models and for estimating thermosteric and halosteric effects on sea level. Seamless wave and sea level simulations covering the period 1950-2100 have been carried out for the whole Mediterranean Sea with forcings that have been produced by different regional climate models under multiple emission scenarios (A1B, RCP4.5, RCP8.5). This large set of results allow to describe the likely future changes of regional marine storminess and their uncertainty depending on emission scenario, climate model and inter-decadal variability. In the Mediterranean Sea, steric expansion and storminess are shown to be contrasting factors: during the 21st century, 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 mass addition from the global ocean that will enter through the Gibraltar Strait in the Mediterranean Sea will likely become the dominant factor and determine an increase of the maximum water level along most of the coastline. The superposition of hazard level changes and morphology of the coast allows to identify parts of the Mediterranean coastline that are potentially at risk in the future.