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Modeling the deep convection in the Northwestern Mediterranean Sea using an eddy-permitting and an eddy-resolving model: case study of winter 1986-87

Herrmann Marine

In the Northwestern Mediterranean sea, winter 1986-87 has been particularly cold and windy, inducing a strong open-ocean convection event. In order to investigate the impact of numerical models spatial resolution on the convection representation, two numerical tridimensional simulations of the year 1986-87 have been performed: one with the eddy-permitting Mediterranean version of the model OPA and one with the eddy-resolving model SYMPHONIE. Both models are forced at the surface by the atmospheric fluxes coming from the ERA40 reanalysis. A simple heat flux correction is added to better mimic the value observed during this event. We examine in both simulations the mixed layer depth evolution, the characteristics of the newly formed WMDW (Western Mediterranean Deep Water), the geographic characteristics of the convection zone and the associated mesoscale structures. We also carry out temporal analysis of this event in terms of kinetic energy, buoyancy equilibrium and deep water formation. The convection is represented similarly on a global scale by both models (convection timing, deep water characteristics, maximum mixed-layer depth, deep water transport). However, due to a better representation of the mesoscale structures, the eddy-resolving model is able to reproduce more correctly the horizontal extension of the convection volume, the restratification process and the WMDW surface formation and mixing. During the convection event, 57%, resp 8%, of the newly formed WMDW is mixed with less dense water for the eddy-resolving, resp. eddy-permitting, model and the WMDW formation rate is 1.7 Sv resp. 2.0 Sv.