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Subantarctic mode water (SAMW) and Antarctic Intermediate water (AAIW) formed in the Southern Ocean, play a fundamental role in the ventilation of Southern Hemisphere thermocline. Along their circulation pathway these fresh, cold, and high in oxygen water masses are modified and exported either in the tropics, or in the Southern Ocean carried by the Antarctic circumpolar current (ACC). The understanding of these water masses is one of the 4 main priorities of the CLIVAR ``Southern Ocean'' because they are recognized as an important index of climatic variability in the Southern Hemisphere, and are very important in entrapping and transporting carbon dioxide and nutrients. SAMW are formed in the Southern Ocean in the deep winter mixed layer north of the Subantarctic front and influence the climate at interannual and decadal scales.

Despite their substantial role for the climate, characteristics of this water mass are poorly known. For example, their exact pathway, their formation processes, and their rate of subduction are not known with accuracy. As such, SAMW are not well represented in climate or ocean general circulation models, although they are of primary importance. In contrast, models can help us to improve our understanding of these physical processes. The study of SAMW and AAIW is a typical field where observational and modeling community collaboration is required.

I began my research by focusing on the in-situ data. The Southern Ocean is historically poorly sampled, so fewer observational studies of mode water have been undertaken in comparison to modelling studies. 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. I took advantage of this recent dataset during my Ph.D. I began with a study of the SAMW formation in the South East Indian Ocean. I focused on this region because it is a key location of SAMW formation in the Southern Ocean. The objective of this study was to begin an investigation into the mechanisms controlling SAMW formation, through a heat budget calculation. We found a rapid transition to thicker layers in the central South Indian Ocean, at about $70^{\circ}$S, 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.

The results of this study gave rise to two main questions: 1) are these results applicable in other locations in the Southern Ocean? 2) Since these formation mechanisms are strongly linked with the ocean circulation, how does climate variability affect this circulation and consequently the SAMW formation?

To answer the first question we began a study 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. Following work developed in other oceans, we analysed up to 10 years of surface drifter trajectories. We mapped an eddy diffusion coefficient and derived an altimetric parametrization of the coefficient for easy use in the modeling community and for future studies on the interannual evolution of the 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. We are now interested in quantifying the role of these high eddy diffusion regions on the position of SAMW formation. We showed in our first study that the high diffusion in the Eastern Indian Ocean contributes to both preventing the formation of mode waters South of Africa and favouring the destabilization of the surface mixed layer around Kerguelen. And we recently found that similar processes occur when the ACC passes around the Campbell Plateau.

To address the second question, we studied the ACC dynamics and related it to the main modes of climate variability in the Southern Hemisphere. 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. Three typical scenarios occur in response to atmospheric forcing: poleward movement of the frontal structure in the Indian basin during positive SAM events, an equatorward movement in the Central Pacific, and an intensification without substantial meridional movement in the Indo-Pacific basin. The study also shows the geographical regions which are dominated by a SAM or ENSO respond at low and high frequency band.

In the wake of all these studies on the formation processes of SAMW the central question remains concerning the rate of subduction of this water mass when formed. The subduction rate determines the effective volume of water formed that will be introduced into the ocean interior and consequently that will link atmospheric characteristics with the deeper ocean. Plenty of data are available today to perform such a study. In-situ data such as ARGO floats, hydrographic ship data combined with surface altimeter data would be very useful to this end.

Although the number of in-situ data have increased dramatically over the last 5 years, their distribution is still irregular in space and time. As such, collaborations with the modeling community would also be very efficient and productive for such a study. As mentioned earlier, in such field the collaboration with modelers improve both understanding of the physical processes and the realism of the current generation of climate models.