Modeling the deep convection in the northwestern Mediterranean Sea using an eddy-permitting and an eddy-resolving model:: Case study of winter 1986-1987

In the northwestern Mediterranean Sea, winter 1986-1987 was particularly cold, inducing a strong open ocean convection event. In order to investigate the impact of numerical models spatial resolution on the convection representation and the effects of deep convection on the northwestern Mediterranean circulation, we perform two numerical three-dimensional simulations (eddy-permitting versus eddy-resolving). Models are forced at the surface by the ERA40 atmospheric fluxes, with a simple heat flux correction to better mimic the observed value. We examine the characteristics of the deep convection (mixed layer, water masses characteristics, convection zone, and mesoscale structures) and perform temporal analysis of this event in terms of kinetic energy, buoyancy equilibrium, and deep water (DW) evolution. The convection characteristics are similarly represented on a global scale by both models and are in good agreement with observations, except for the size of the convection region. However, the eddy-resolving model better reproduces the mesoscale structures, whose role in the DW formation, mixing, and transport is shown to be essential. The boundary circulation and the overturning are enhanced during the convection event. Sixty-six percent of the DW spreading is due to the bleeding effect into the Catalan sea during the convection event, whereas 33% is due to the mesoscale structures southwestward advection after the event. Sixty percent of the restratification with respect of the water column initial structure occurs before July 1987 and is due to light water advection. Afterward, restratification is due to the mixing and is not complete before next year convection.