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The Mediterranean climate is expected to become warmer and drier during the twenty-first century. Mediterranean Sea response to climate change could be modulated by the choice of the socio-economic scenario as well as the choice of the boundary conditions mainly the Atlantic hydrography, the river runoff and the atmospheric fluxes. To assess and quantify the sensitivity of the Mediterranean Sea to the twenty-first century climate change, a set of numerical experiments was carried out with the regional ocean model NEMOMED8 set up for the Mediterranean Sea. The model is forced by air-sea fluxes derived from the regional climate model ARPEGE-Climate at a 50-km horizontal resolution. Historical simulations representing the climate of the period 1961-2000 were run to obtain a reference state. From this baseline, various sensitivity experiments were performed for the period 2001-2099, following different socio-economic scenarios based on the Special Report on Emissions Scenarios. For the A2 scenario, the main three boundary forcings (river runoff, near-Atlantic water hydrography and air-sea fluxes) were changed one by one to better identify the role of each forcing in the way the ocean responds to climate change. In two additional simulations (A1B, B1), the scenario is changed, allowing to quantify the socio-economic uncertainty. Our 6-member scenario simulations display a warming and saltening of the Mediterranean. For the 2070-2099 period compared to 1961-1990, the sea surface temperature anomalies range from +1.73 to +2.97 °C and the SSS anomalies spread from +0.48 to +0.89. In most of the cases, we found that the future Mediterranean thermohaline circulation (MTHC) tends to reach a situation similar to the eastern Mediterranean Transient. However, this response is varying depending on the chosen boundary conditions and socio-economic scenarios. Our numerical experiments suggest that the choice of the near-Atlantic surface water evolution, which is very uncertain in General Circulation Models, has the largest impact on the evolution of the Mediterranean water masses, followed by the choice of the socio-economic scenario. The choice of river runoff and atmospheric forcing both have a smaller impact. The state of the MTHC during the historical period is found to have a large influence on the transfer of surface anomalies toward depth. Besides, subsurface currents are substantially modified in the Ionian Sea and the Balearic region. Finally, the response of thermosteric sea level ranges from +34 to +49 cm (2070-2099 vs. 1961-1990), mainly depending on the Atlantic forcing. [ABSTRACT FROM AUTHOR]

Despite global circulation models have been steadily evolving in terms of complexity and resolution refinement, some coastal and shelf phenomena are still poorly replicated, especially for complex-geometry regions. Small-scale ocean features and processes must be thus explicitly computed by means of regional and coastal models with finer horizontal grid spacing but for a particular delimited area. In this context, there is a significant wealth of ocean forecasting systems routinely operated on overlapping regions, offering the opportunity to compare them, judge the strengths and weaknesses of each system and eventually evaluate the added-value of high-resolution coastal models respect to coarser parent model solutions. In this work, a multi-parameter model inter-comparison was conducted, ranging from global to local scales in a two-phase strategy. Firstly, a comparison between GLOBAL and the nested IBI regional system, both operated in the frame of the Copernicus Marine Environment Monitoring Service (CMEMS), was performed with a focus on the impact of impulsive-type river freshwater discharge on local surface circulation in NW Spain. The increased horizontal resolution of IBI allowed a more accurate representation of horizontal salinity gradients, the horizontal extent of the river plume and the strength and position of the freshwater front, according to the results derived from the validation against in situ observations of sea surface temperature, salinity and currents provided by a moored buoy. Secondly, a 1-year multi-model inter-comparison exercise was performed in the Strait of Gibraltar between CMEMS GLOBAL, CMEMS IBI and a high-resolution coastal system (developed by Puertos del Estado -PdE- and dynamically embedded in IBI) named PdE-SAMPA in order to elucidate both their ability to both characterize the Atlantic Jet (AJ) inflow and reproduce singular coastal ocean processes, such as the quasi-permanent (up to 48 h long) full reversal of the AJ surface flow under intense and prolonged easterlies. To this end, a High-Frequency radar system was used as benchmark since it regularly provides quality-controlled hourly maps of surface currents. Results showed the steady improvement in AJ representation in terms of speed and direction when zooming from global to coastal model configurations, highlighting both the benefits of the dynamical downscaling approaches and the relevance of a variety of factors at local scales such as the refined horizontal resolution, the tailored bathymetry along with the high spatio-temporal resolution of the atmospheric forcing. [ABSTRACT FROM AUTHOR]