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Impact des marées sur la circulation générale océanique dans une perspective climatique

Abstract : Tides have long been considered as a high frequency phenomenon which was supposed to be limited to continental shelves. Thereby, tides were not likely to interact with the oceanic general circulation and thus on the climate's low frequency dynamic. Since altimetric data and global tidal hydrodynamic solutions have reached a centimetric resolution, a connection between tides and climate is now conceivable. The goal of this study is thus to explore what are the effects of the tides on the oceanic general circulation. These effects take place through two essential physical processes: (i) the highly non-linear dynamic of the tides and, (ii) the dissipation of their energy in deep ocean as vertical mixing or heat.
In order to examine each of these aspects, the chosen method consists in parameterizing the tides in a three-dimensional ocean general circulation model (OGCM) dedicated to climate: NEMO. For this, the off-line outputs of a two-dimensional hydrodynamical tidal model (MOG2D-G) are used. At first, a map of the global residual mean circulation (CRM) generated by the tidal non-linear dynamic is computed for the first time and described. This CRM, obtained with MOGD2-G is then introduced as an external forcing in the OGCM NEMO. In a second time, the tidal energy dissipation is examined. At first, the fraction of tidal energy dissipated as heat is quantified. This enables to determine if, like geothermal flux, it could play a significant role in the abyssal circulation, and it is concluded that it is not the case. Then, the fraction of tidal energy dissipated locally as vertical mixing via internal waves is considered. This tidal mixing (TM) results from the energy transfer of the barotropic mode to the baroclinic ones i.e the tidal conversion rate. This transfer is diagnosed using MOG2D-G and integrated in NEMO through a vertical turbulent mixing parameterization.
It is concluded that: (i) the sole TM can have a significant impact on the oceanic general circulation and thus on climate, (ii) that introducing local TM in OGCM is a key point for a proper modeling of abyssal water masses transport and, (iii) that from now on it is crucial to consider the fraction of TM which is generated away from the generation site
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Submitted on : Friday, September 14, 2007 - 12:31:05 PM
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Laurent Bessières. Impact des marées sur la circulation générale océanique dans une perspective climatique. Océan, Atmosphère. Université Paul Sabatier - Toulouse III, 2007. Français. ⟨tel-00172154⟩

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