EUMELI OLIGOTROPHIC SITE - RESPONSE OF AN UPPER OCEAN MODEL TO CLIMATOLOGICAL AND ECMWF ATMOSPHERIC FORCING

Within the frame of the EUMELI program-component of FRANCE-JGOFS-in th e Northeast tropical Atlantic ocean, we investigate the potential of a one-dimensional eddy-kinetic-energy model (Gaspar et al., 1990, GGL) to characterize the vertical dynamics of the oceanic mixed layer (ML) at the EUMELI oligotrophic site (21 degrees N, 31 degrees W) north of the Cape Verde Frontal Zone, The atmospheric forcings used are derived from two different sources: the operational Atmospheric General Circu lation Model of ECMWF (over two 12-month periods: August 1985-July 198 6 and the full year 1990) and climatologies (Esbensen and Kushnir, EK, 1981; Hsiung, H, 1986; Oort, 1983). At the site, depending on the dat a base, the annual mean of the total energy flux at the ocean-atmosphe re interface differs in sign and intensity and its monthly evolution p resents significant variation both in amplitude and timing of the maxi mum. The monthly wind stress evolution due to the regular north-east t rade winds prevailing in this region is quite consistent as derived by the different data sources. In our area, a net evaporation rate occur s throughout the year. The simulated ML depth, based on GGL's ML depth definition, is always shallower than climatological observations of M L depth, whatever the surface atmospherical forcing used, the exceptio n being the simulation performed with the atypical ECMWF85-86 forcing. The simulated SST's using H forcing compare rather well (within 1 deg rees C) with the observed SST's of the climatologies of Lamb and EK. S ampling experiments on the surface boundary conditions showed that sim ulated evolutions of the ML depth and SST differ quite significantly d ue to differences in data bases rather than differences in forcing fre quencies. An error analysis on the ocean surface energy fluxes and the prescription of evaporation and precipitation rates under various for ms demonstrate the crucial need for heat, momentum and freshwater flux es estimates as accurate as possible. From the distributions of the tu rbulent kinetic energy (TKE) budget on different time scales, it is fo und that most often shear production and viscous dissipation dominate in the ML. Gravitational production or destruction, turbulent diffusio n and storage of TKE are of second-order. The use of a daily instead o f a 3-hour forcing creates an underestimation of 19% of the total annu al energy produced by the shear. When taking into account freshwater f luxes, gravitational production becomes of first order during fall and winter and intervenes in the balance between shear production and vis cous dissipation.