The response of the Ligurian and Tyrrhenian Seas to a summer Mistral event: A coupled atmosphere–ocean approach

Abstract

In this paper the effect of a summer Mistral event on the Ligurian and Tyrrhenian Seas in the north-western Mediterranean is discussed, using a coupled numerical model and satellite and in situ observations. The focus is on the spatial and temporal distribution of the ocean mixed layer response to the strong winds, and on how this is affected by atmosphere–ocean coupling. The model used is the Coupled Ocean–Atmosphere Mesoscale Prediction System (COAMPS®1COAMPS® is a registered trademark of Naval Research Laboratory.1), 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). The model is run at high (km scale) resolution to capture the fine structure of wind jets and surface cooling.Two non-assimilating numerical experiments, coupled and uncoupled, are run for a 3-day period of a Mistral event, to examine more closely the impact of coupling on the surface flux and sea surface temperature (SST) fields. The cooling of SST up to 3°C over 72h in the coupled run significantly reduced the surface momentum and heat fluxes, relative to the uncoupled simulation, where the SST was kept fixed at the initial value. Mixed layer depths increase by as much as 30m during the event. A heat budget analysis for the ocean is carried out to further explain and investigate the SST evolution. Shear-induced mixing in inertial waves is found to be important to the surface cooling. Effects of coupling on the atmospheric boundary layer are found to be significant, but overall the effect of coupling on the synoptic low pressure system is small.