Les eaux modales de l'Océan Austral

Abstract : Subantarctic mode water (SAMW) are formed in the Southern Ocean in the deep winter mixed layers north of the Subantarctic front. They influence the climate at interannual and decadal scales and play a fundamental role in the ventilation of Southern Hemisphere thermocline. We study the details of SAMW formation using the recent deployment of ARGO profiling floats and GDP surface drifters, which provide an excellent space-time coverage of the Southern Ocean upper ocean processes. Since the beginning of the ARGO international program, the number of vertical hydrographic profiles in the Southern Ocean have increased considerably so that nowadays we have a comparable number of profiles to decades of hydrographic ship data. Based on this dataset, we found that the dominant forcing for SAMW formation in winter in the Southern Indian Ocean was due to air-sea and Ekman fluxes. We found a rapid transition to thicker surface mixed layers in the central South Indian Ocean, at about 70°, associated with a reversal of the horizontal eddy heat diffusion in the surface layer and the meridional expansion of the ACC as it rounds the Kerguelen Plateau. These effects are ultimately related to the bathymetry of the region, leading to the seat of formation in the region southwest of Australia.

SAMW formation is tightly linked to the Southern Ocean dynamics and position of the main polar fronts. A second study concerned the ACC circulation and frontal variability. In this study we mixed in-situ and altimeter data to monitor the position of the two main fronts of the ACC during the period 1993-2005. Then, we related their movements to the two main atmospheric climate modes of the Southern Hemisphere, the Southern Annular Mode (SAM) and the El-Nino Southern Oscillation (ENSO). We found that although the fronts are steered by the bathymetry, which sets their mean pathway at first order, in flat-bottom areas the fronts are subject to large meandering due to mesoscale activity and atmospheric forcing.

In parallel, we developed a new estimate of the circumpolar distribution of eddy diffusion in the Southern Ocean. Diffusion has almost never been studied in the Southern Ocean with in-situ data. We analysed up to 10 years of surface drifter trajectories and applied a statistical analysis previously developed in other oceans. We mapped a climatological eddy diffusion coefficient and derived an altimetric parametrization of the coefficient for easy use by the modeling community and for future studies on the interannual evolution of eddy diffusion. This study shows that the Southern Ocean is highly diffusive north of the ACC core, with several spots of very strong diffusion: the Agulhas Retroflection, the Campbell Plateau region, and the Brazil Current region.

These results lead us to a circumpolar analysis of the SAMW formation, and to a representaion of the link between Southern Ocean dynamics and SAMW formation. The constant increase in hydrological profiles in the Southern Ocean via the ARGO program allowed us to have a better spatial representation of the regions of SAMW formation. We found that eddy heat diffusion play a substantial role in the local heat balance. South of the western boundary currents and north of the SAF, the eddy heat diffusion is positive and counterbalances the cooling of the mixed layer by winter Ekman and air-sea fluxes. Specifically, it reduces the mixed layer destabilisation north of the SAF in the Western Indian Ocean downstream of the Agulhas Retroflection and in the Western Pacific downstream of Campbell Plateau.
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Submitted on : Tuesday, December 4, 2007 - 3:53:40 PM
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Jean-Baptiste Sallée. Les eaux modales de l'Océan Austral. Océan, Atmosphère. Université Paul Sabatier - Toulouse III, 2007. Français. ⟨tel-00193791⟩

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