Intercomparison of oceanic and atmospheric forced and coupled mesoscale simulations - Part I: Surface fluxes

A mesoscale non-hydrostatic atmospheric model has been coupled with a mesos cale oceanic model. The case study is a four-day simulation of a strong sto rm event observed during the SEMAPHORE experiment over a 500 x 500 km(2) do main. This domain encompasses a thermohaline front associated with the Azor es current. In order to analyze the effect of mesoscale coupling, three sim ulations are compared: the first one with the atmospheric model forced by r ealistic sea surface temperature analyses; the second one with the ocean mo del forced by atmospheric fields, derived from weather forecast re-analyses ; the third one with the models being coupled. For these three simulations the surface fluxes were computed with the same bulk parametrization. All th ree simulations succeed well in representing the main oceanic or atmospheri c features observed during the storm. Comparison of surface fields with in situ observations reveals that the winds of the fine mesh atmospheric model are more realistic than those of the weather forecast re-analyses. The low -level winds simulated with the atmospheric model in the forced and coupled simulations are appreciably stronger than the re-analyzed winds. They also generate stronger fluxes. The coupled simulation has the strongest surface heat fluxes: the difference in the net heat budget with the oceanic forced simulation reaches on average 50 Wm(-2) over the simulation period. Sea su rface-temperature cooling is too weak in both simulations, but is improved in the coupled run and matches better the cooling observed with drifters. T he spatial distributions of sea surface-temperature cooling and surface flu xes are strongly inhomogeneous over the simulation domain. The amplitude of the flux variation is maximum in the coupled run. Moreover the weak correl ation between the cooling and heat flux patterns indicates that the surface fluxes are not responsible for the whole cooling and suggests that the res ponse of the ocean mixed layer to the atmosphere is highly non-local and en hanced in the coupled simulation.