Rôle du rail équatorial dans la circulation méridienne océanique : interactions des échelles spatiales conduisant au mélange

The large scale meridional overturning circulation in the ocean is maintained by a combination of several mechanisms. This dissertation focuses on turbulent equatorial mixing, one of the key mechanisms to balance heat forcing. The motivation for the research is observations of thin layers of well-mixed density and tracers distributed intermittently over the vertical that coincide with particular features of a larger scale pattern of equatorial jets. A large number of numerical experiments at moderately high resolution was performed in order to successfully replicate the observed equatorial dynamics, which involves both quasi-barotropic extra-equatorial jets and small vertical scale equatorial deep jets. The combination of the two systems of jets builds low-potential-vorticity niches, favouring the development of inertial instability and mixing. A very high resolution simulation was performed to study the formation of fine-scale layer structures with properties similar to those observed in the Atlantic. The creation of layers appears to be irreversible and the spatial distribution of the layers matches that of the marginal condition for inertial stability. The new appreciation gained for the importance of equatorial mixing leads one to revisit the question of the global distribution of diapycnal diffusivity coefficients, the estimation of which is often based on the breaking of internal waves, whose activity decreases with latitude. The resolution of the simulations performed for this study is not fine enough to produce a direct cascade of energy toward mixing scales but invites further investigation in that area.