Climate modeling over the Mediterranean Sea: impact of resolution and ocean coupling

Feedback between the Mediterranean Sea and the atmosphere on various temporal and spatial scales plays a major role in the regional climate system. We studied the impact of horizontal atmospheric grid resolution (grid-spacing of similar to 9 vs. similar to 50 km) and dynamic ocean coupling (the ocean model NEMOMED12) in simulations with the regional climate model COSMO-CLM. The evaluation focused on sea surface heat fluxes, 10-m wind speed, and sea surface temperature (SST) parameters on both seasonal and annual timescales. The finer grid improved the wind speed (particularly near coastal areas) and subsequently the turbulent heat flux simulations. Both parameters were better simulated with the interactive ocean model NEMOMED12 than with prescribed daily ocean SSTs (using near-observation ERA-Interim reanalysis based SSTs), but coupling introduced a warm SST bias in winter. Radiation fluxes were slightly better represented in coarse-grid simulations. Still, only the higher-resolution coupled simulations could reproduce the observed net outgoing total heat flux over the Mediterranean Sea. Investigation of the impact of sub-diurnal SST variations showed a strong effect on sub-daily heat fluxes and wind speed but minor effects at longer time scales. Therefore, a coupled atmosphere-ocean climate model should be preferred for studying the Mediterranean Sea climate system. Higher-resolution models should be preferred, but they are not yet able to perform better than their coarse-resolution predecessors in all aspects.