The effect of ocean-atmosphere coupling on a summer Mistral event

In this paper the effect of air-sea coupling on a Mistral event in the north-western Mediterranean is discussed, using a coupled numerical model and satellite observations. The focus is on how the sea surface cooling induced by the strong winds affects the surface fluxes, as well as interior properties of the oceanic and atmospheric boundary layers. The model used is the Coupled Ocean-Atmosphere Mesoscale Prediction System (COAMPS® ), developed at the Naval Research Laboratory. This system includes an atmospheric sigma coordinate, non-hydrostatic model, coupled to a hydrostatic sigma-z level ocean model (Naval Coastal Ocean Model), using the Earth System Modeling Framework (ESMF). Both models are at high resolution: the inner nest of the COAMPS domain is on a 4 km grid with 40 vertical levels, and that for NCOM on a 2 km grid with 50 levels. Two non-assimilating runs, coupled and uncoupled, are run for a three-day period of a Mistral event, 26-28 June 2007, to examine the impact of coupling on the flux fields and Sea Surface Temperature (SST). The cooling of SST up to 3°C over 72 hours in the strong wind jet regions in the fully coupled run significantly reduces the surface stress, and the surface turbulent heat fluxes, relative to an uncoupled simulation where the SST is kept fixed at the initial value of the coupled run. In other words, the coupling provides a negative feedback on the surface forcing under strong winds, and this influences the ocean mixed layer and the atmospheric boundary layer. A heat budget is performed to examine the role of surface heat loss, entrainment and Ekman pumping in the SST response.