Air-sea interaction in the Ligurian Sea: Numerical simulations and in-situ data in the summer of 2007

Abstract

In situ experimental data and numerical model results are presented from the Ligurian Sea in the northwestern Mediterranean. Here surface winds are light in summer except during occasional Mistral events which are often associated with cyclogenesis in the lee of the Alps. The Ligurian Sea Air-Sea Interaction Experiment (LASIE07) took place in June 2007, with a focus on the coincident measurement of oceanic and atmospheric boundary layer properties. To help interpret the observational results we use 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 2km grid with 50 levels. The coupled model system is evaluated for a month-long simulation which includes data assimilation in the atmosphere but not the ocean. Correlatation coefficients between model and observed values for near-surface wind speeds, and turbulent heat fluxes are above 0.5 (significant at 99%) at a deep water mooring. A comparison of the coupled run with an uncoupled atmospheric run using analysis SST at the surface boundary, both of which use atmospheric data assimilation, does not reveal significant or systematic differences. Therefore a non-assimilating run for the period of a strong wind event (26–29 June), is performed to examine more closely the impact of coupling on the flux fields and SST. Here, the cooling of SST up to 3°C over 72 hours in a fully coupled run affects the surface stress, which is reduced by ~20%, and the surface latent heat flux which is reduced by 50%, 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.