Future projections of the surface heat and water budgets of the Mediterranean Sea in an ensemble of coupled atmosphere–ocean regional climate models

Abstract

Within the CIRCE project "Climate change and Impact Research: the Mediterranean Environment", an ensemble of high resolution coupled atmosphere-ocean regional climate models (AORCMs) are used to simulate the Mediterranean climate for the period 1950-2050. For the first time, realistic net surface air-sea fluxes are obtained. The sea surface temperature (SST) variability is consistent with the atmospheric forcing above it and oceanic constraints. The surface fluxes respond to external forcing under a warming climate and show an equivalent trend in all models. This study focuses on the present day and on the evolution of the heat and water budget over the Mediterranean Sea under the SRES-A1B scenario. On the contrary to previous studies, the net total heat budget is negative over the present period in all AORCMs and satisfies the heat closure budget controlled by a net positive heat gain at the strait of Gibraltar in the present climate. Under climate change scenario, some models predict a warming of the Mediterranean Sea from the ocean surface (positive net heat flux) in addition to the positive flux at the strait of Gibraltar for the 2021-2050 period. The shortwave and latent flux are increasing and the longwave and sensible fluxes are decreasing compared to the 1961-1990 period due to a reduction of the cloud cover and an increase in greenhouse gases (GHGs) and SSTs over the 2021-2050 period. The AORCMs provide a good estimates of the water budget with a drying of the region during the twenty-first century. For the ensemble mean, he decrease in precipitation and runoff is about 10 and 15% respectively and the increase in evaporation is much weaker, about 2% compared to the 1961-1990 period which confirm results obtained in recent studies. Despite a clear consistency in the trends and results between the models, this study also underlines important differences in the model set-ups, methodology and choices of some physical parameters inducing some difference in the various air-sea fluxes. An evaluation of the uncertainty sources and possible improvement for future generation of AORCMs highlights the importance of the parameterisation of the ocean albedo, rivers and cloud cover

Coupled atmosphere ocean climate model simulations in the Mediterranean region: effect of a high-resolution marine model on cyclones and precipitation

“…The eddy-permitting model, with its higher horizontal resolution, more vertical levels and more accurate surface process parameterizations than CMCC-CM, allows a better representation of the Mediterranean dynamics, with a positive feedback on SST simulation. The cold bias of CMCC-CM is clearly reduced by CMCC-MED everywhere, especially in the central and northern part of the basin, but it is still around 2 • C. On the other hand, Dubois et al (2012) found that all CIRCE models simulate SSTs (and air temperatures) that are cooler than the observations. Such a shortcoming encompasses several reasons, ranging from radiative forcings to the marine albedo, but the problem is still under debate and a clear understanding of its reasons is still missing.…”

“…It is worth noticing that the improved information due to the very high resolution of MFS is partly deteriorated in the coupling procedure by averaging the SST over the coupling frequency time and by interpolating the field to the coarse atmospheric model. On the other hand, Dubois et al (2012), examining the performances of CIRCE models in terms of surface heat and water budgets over Mediterranean Sea, showed that the presence of an underlying marine model, which realistically simulates the small-scale spatial structures over the Mediterranean, is still beneficial in terms of the air-sea interactions.…”

“…Going in the same direction, Artale et al (2010), using the PROTEUS regional coupled model over Mediterranean area, produced more realistic simulations of SST (sea surface temperature), winds and, as a consequence, of air-sea fluxes. More recently, within the framework of the CIRCE EU-FP6 project, and Dubois et al (2012) investigated the effect of high-resolution Mediterranean components within fully coupled atmosphere-ocean models on present climate and climate projection, using the IPCC scenario A1B (IPCC TAR, 2001), which shows that only high-resolution models permit a realistic simulation of the surface air-sea fluxes.…”

Mediterranean Sea response to climate change in an ensemble of twenty first century scenarios

“…The uncertainty related to the choice of the GCM or ARCM or ORCM has been partially tackled in CIRCE (Gualdi et al 2013;Dubois et al 2012) and a larger multi-model exercice is currently being organised under the umbrella of the Hymex, Med-CORDEX and Med-CLIVAR programs.…”

“…A review of the studies dealing with Mediterranean climate projections is presented in Planton et al (2012). The regional coupled models intercomparison study by Dubois et al (2012) presented Mediterranean Sea projections under A1B scenario for 1950-2050 and found a decrease in the heat loss and an increase in water loss over the ocean, which may strongly impact the Mediterranean water masses and the associated MTHC. A recent study by Lazzari et al (2013) investigated the impact of A1B climate change scenario on the Mediterranean trophic regimes, but their time slice approach presented some limitations compared to the transient approach of the CIRCE project simulations (Gualdi et al 2013), with smaller SST increase at the end of the twenty-first century compared to other studies.…”