Equilibres multiples de la circulation thermohaline

Abstract : Forced by the surface density field gradients, large scale convective cells develop in the ocean. They induce an important meridional redistribution of heat and chemicals. The temperature and freshwater surface forcings have antagonistic effects on the density field and on the resulting thermohaline circulation. Due to the competition between the two forcings, the circulation can operate in different modes. Here, the multiple equilibria of the thermohaline circulation are studied through the comparison between numerical results of a bidimensional model and predictions of an asymptotic expansion derived from the model in the limit of a thin layer. The bidimensional model (latitude-depth) is based on the Boussinesq equations in a rectangular basin with flat bottom. Mixed boundary conditions (fixed temperature field and salinity flux) are imposed at the surface. The model is controlled by five adimensional parameters and the two forcing meridional profiles. Through a numerical parametric study, the location of multiple equilibria has been located in the forcing amplitude plane. We considered many forcing profiles, chosen for their ability to induce interesting dynamical behaviour of the thermohaline circulation (multiple equilibria, thermohaline fronts). Performing an asymptotic expansion in the limit of a thin fluid layer, the bidimensional model is reduced to only one monodimensional (latitude) evolution equation. The asymptotic equation supports multiple solutions, corresponding to the minima of a potential. One of these solutions is globally defined in the basin. The others are only defined on certain ranges of latitudes. A geometric interpretation of the asymptotic equation is proposed. The topology of its solutions is deduced, as well as the parts of the domain where the asymptotic theory is not valid. Comparing asymptotic solutions and numerical results, the asymptotic analysis is found to be a very simple and powerful tool to predict the simulated equilibria.
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Laurence Fleury. Equilibres multiples de la circulation thermohaline. Océanographie. Université Paul Sabatier - Toulouse III, 1996. Français. ⟨tel-02014914⟩



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