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Dynamique agéostrophique dans l'océan intérieur

Abstract : The ocean is the largest solar energy collector on Earth. The amount of heat it can store is modulated by its complex circulation, which spans a broad range of spatial scales, from centimeters to thousands of kilometers. This dissertation investigates two types of physical processes: mesoscale eddies (100-300 km size) and submesoscale fronts (£ 50 km size). To date, ageostrophic submesoscale motions are thought to be mainly trapped within the ocean surface mixed layer, and to be weak in the ocean interior. This is because, in the classical paradigm, motions below the mixed layer are broadly assumed to be in quasigeostrophic balance, preventing the formation of strong buoyancy gradients at depth. This dissertation introduces a paradigm shift; based on a combination of high-resolution in situ CTD data collected by instrumented elephant seals, satellite observations of sea surface height, and high-resolution model outputs in the Antarctic Circumpolar Current, we show that ageostrophic motions (i) are generated by the backgound mesoscale eddy field via frontogenesis processes, and (ii) are not solely confined to the ocean surface mixed layer but, rather, can extend in the ocean interior down to depths of 1 000 m. Deepreaching ageostrophic fronts are shown to drive an anomalous upward heat transport from the ocean interior back to the surface that is larger than other contributions to vertical heat transport and of comparable magnitude to air-sea fluxes. This effect can potentially alter oceanic heat uptake and will be strongest in eddy-rich regions such as the Antarctic Circumpolar Current, the Kuroshio Extension, and the Gulf Stream, all of which are key players in the climate system. As such, ageostrophic fronts at submesoscale provide an important, yet unexplored, pathway for the transport of heat, chemical and biological tracers, between the ocean interior and the surface, with potential major implications for the biogeochemical and climate systems.
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Lia Siegelman. Dynamique agéostrophique dans l'océan intérieur. Sciences de la Terre. Université de Bretagne occidentale - Brest, 2019. Français. ⟨NNT : 2019BRES0094⟩. ⟨tel-02998433⟩

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