Interactions entre les Cyclones Tropicaux et l'Océan : de l'échelle synoptique à l'échelle climatique

Abstract : Tropical Cyclones' (TCs) intensity heavily depends on the processes occurring at the air-sea interface. TCs draw their energy from the warmth of the ocean surface but they induce a sea surface cooling (Cold Wake CW) that negatively feeds back on their intensity. This dissertation first describes the processes explaining the surface cooling along with the characteristics of the TC and of the ocean that control its magnitude. Three main processes participate in the CW : vertical mixing, surface heat fluxes (mainly evaporation) and advection. By mixing together warm water from the surface and cold water from below, the first process cools the surface while warming the sub-surface. Mixing is responsible for the major part of the cooling right under the TC and is thus the key process of the CW feedback. By contrast, surface heat fluxes poorly contribute to the cooling right under the TC but they cool the ocean surface over very large regions. Horizontal advection participates in cooling by surface divergence of warm water and modifies the shape of the CW. In this thesis, I define an index integrating the wind power during the TC passage (Wind Power index; WPi) and show that the CW magnitude increases with WPi. However, for a given wind power, the state of the ocean sub- surface modulates the magnitude of the CW by a factor of 10 ! To account for this control of the CW by the ocean sub-surface, I propose an index measuring the potential energy of the oceanic stratification (Cooling Inhibition index ; CI). These results have been made possible thanks to an experimental setup allowing the simu- lation of oceanic response to TCs for a moderate numerical cost. It allows a statistical analysis of the CW characteristics induced by more than 3,000 TCs over the last 30 years. By directly simulating the ocean response to TCs within a global ocean model, it also allows to link the processes occurring at the event scale to their effect on the ocean at climatic scale. Warm water injected in the sub-surface by mixing under TCs has been suggested to modify the global ocean heat transport (OHT). However, warm anomalies injected in the sub-surface during the cyclonic season (local summer) hardly reach depth below the seasonal thermocline. Mixed layer deepening in winter reabsorb these warm anomalies, hence considerably reducing the potential effect of TCs on the OHT. On the other hand, enhanced surface heat fluxes by TCs act to significantly cool cyclonic basins during summer. Adding this to the winter surface warming evoked above, this process leads to a 10% reduction of surface temperature seasonal cycle within TC basins. Finally, this thesis open perspectives for a better understanding of how TCs' activity may be affected by the natural and anthropogenic variability of ocean characteristics.
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Submitted on : Monday, July 30, 2012 - 10:31:02 AM
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Emmanuel Vincent. Interactions entre les Cyclones Tropicaux et l'Océan : de l'échelle synoptique à l'échelle climatique. Océan, Atmosphère. Université Pierre et Marie Curie - Paris VI, 2011. Français. ⟨tel-00721714⟩

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