A 1 year mesoscale simulation of the northeast Atlantic : Mixed layer heat and mass budgets during the POMME experiment Article - 2005

A simplified oceanic model is used to perform a realistic yearlong simulation to study fine-scale mixed layer processes in an area of the northeast Atlantic (21 degrees-15 degrees W, 38 degrees-45 degrees N) during the Programme Ocean Multidisciplinaire Meso Echelle (POMME) experiment (September 2000-October 2001). This simplified three-dimensional oceanic model is obtained by degenerating the primitive equations system by prescribing continuously analysis-derived geostrophic currents U-g (called ``guide’’) into the momentum equation by the substitution of the horizontal pressure gradient. The realism of the simulated mesoscale structures was validated by a comparison with in situ mesoscale and submesoscale data and results from the geostrophic adjustment to the guide. With this adjustment being applied at all scales, this model was able to simulate energetic small horizontal scales taking place around stirring eddies and frontal regions. Moreover, filamentary mixed layer depth structures were simulated and correlated to intense vertical velocities confined in the mixed layer. The circulation structure during POMME consisted of three quasi-permanent mesoscale eddies and a front. The mixed layer heat and mass budgets were thus strongly driven by the horizontal advection and the vertical transport associated with this mesoscale circulation. Although no effective detrainment or entrainment occurred at the domain scale, three regions were found to be the site of intense detrainment ( around anticyclonic and cyclonic mesoscale eddies and along an axis of strong currents), in which the lateral induction and the vertical velocity at the mixed layer base are the major components driving the detrainment, respectively.