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Characterizing and understanding the basic functioning of the Mediterranean Sea in terms of heat and salt redistribution within the basin is a crucial issue to predict its evolution. Here we quantify and analyze the heat and salt transfers using a simple box model consisting of four layers in the vertical for each of the two (western and eastern) basins. Namely, we box-average 14 regional simulations of the Med-CORDEX ensemble plus a regional and a global reanalysis, computing for each of them the heat and salt exchanges between layers. First, we analyze in detail the mechanisms behind heat and salt redistribution at different time scales from the outputs of a single simulation (NEMOMED8). We show that in the western basin the transfer between layer 1 (0–150 m) and layer 2 (150–600 m) is upwards for most models both for heat and salt, while in the eastern basin both transfers are downwards. A feature common to both basins is that the transports are smaller in summer than in winter due to the enhanced stratification, which dampen the mixing between layers. From the comparison of the 16 simulations we observe that the spread between models is much larger than the ensemble average for the salt transfer and for the heat transfer between layer 1 and layer 2. At lower layers (below 600 m) there is a set of models showing a good agreement between them, while others are not correlated with any other. The mechanisms behind the ensemble spread are not straightforward. First, to have a coarse resolution prevents the model to correctly represent the heat and salt redistribution in the basin. Second, those models with a very different initial stratification also show a very different redistribution, especially at intermediate and deep layers. Finally, the assimilation of data seems to perturb the heat and salt redistribution. Besides this, the differences among regional models that share similar spatial resolution and initial conditions are induced by more subtle mechanisms which depend on the variable and process analyzed. In order to reduce the uncertainties in the Mediterranean regional climate projections further modelling studies and better observational datasets are needed to constrain the main sources of discrepancies among models. In the absence of those, an ensemble modelling approach as the one followed in the Med-CORDEX initiative seems to be the best solution to evaluate model uncertainties into the future climate projections. © 2016 Springer-Verlag Berlin Heidelber

Summary:We present an overview of the changes expected during the 21st century in key marine parameters (sea surface temperature, sea surface salinity, sea level and waves) in the sector of the NE Atlantic Ocean close to the Spanish shores. Under the A1B scenario, open-sea surface temperatures would increase by 1°C to 1.5°C by 2050 as a consequence of global ocean warming. Near the continental margin, however, the global temperature rise would be counteracted by an enhancement of the seasonal upwelling. Sea surface salinity is likely to decrease in the future, mainly due to the advection of high-latitude fresher waters from ice melting. Mean sea level rise has been quantified as 15-20 cm by 2050, but two contributions not accounted for by our models must be added: the mass redistribution derived from changes in the large-scale circulation (which in the NE Atlantic may be as large as 15 cm in 2050 or 35 cm by 2100) and the increase in the ocean mass content due to the melting of continental ice (for which estimates are still uncertain). The meteorological tide shows very small changes, and therefore extreme sea levels would be higher in the 21st century, but mostly due to the increase in mean sea level, not to an increase in the storminess. The wave projections point towards slightly smaller significant wave heights, but the changes projected are of the same order as the natural variability.Keywords: climate change; surface temperature; surface salinity; sea level, waves. Escenarios climáticos marinos regionalizados en el sector NE del Océano Atlántico cercano a las costas españolasResumen: En este trabajo se presenta una visión de conjunto de los cambios esperados en el siglo XXI en los principales parámetros marinos (temperatura y salinidad superficiales, nivel del mar y oleaje) en el sector NE del Océano Atlántico más cercano a las costas españolas. Bajo el escenario A1B, se prevé que la temperatura superficial en mar abierto suba del orden de 1-1.5°C para el año 2050, como consecuencia del calentamiento global del océano. Cerca del margen continental, sin embargo, el aumento de la temperatura superficial podría ser contrarrestado por un aumento del afloramiento estacional. La salinidad superficial es probable que disminuya en el futuro, debido principalmente a la advección desde latitudes más altas de aguas provenientes de la fusión de hielos polares. El aumento del nivel del mar obtenido de los modelos se ha cuantificado en 15 a 20 cm para el año 2050, pero esa estima no incluye dos contribuciones adicionales que deben ser añadidas: la redistribución de masa derivada de los cambios en la circulación a gran escala (que en el Atlántico NE se ha estimado en unos 15 cm para 2050 i en 35 cm para 2100) y el aumento de masa debido a la fusión de hielos continentales (para el cual las estimas son todavía inciertas). La marea meteorológica muestra cambios muy pequeños, y por tanto el aumento de los niveles extremos del mar en el siglo XXI serán debidos principalmente al aumento del nivel medio, no a un aumento en la ...