Outflow of Weddell sea waters into the Scotia Sea through the western sector of the South Scotia Ridge

[eng] This work compiles the results of the analysis of hydrographical data collected in January 2008 over the western sector of the South Scotia Ridge (SSR). The cruise was carried out on board R/V Hesperides in the framework of the Synoptic Antarctic Shelf-Slope Interaction (SASSI) study, one of the core projects endorsed by the International Polar Year. SASSI focused on shelf-slope processes taking place all along the Antarctic continental slope, paying particular attention to the Antarctic Slope Front (ASF) and its associated westward Antarctic Slope Current (ASC). The Spanish contribution to SASSI (framed by the E-SASSI project) focused on the SSR region between the South Shetland Islands and the South Orkney Islands, bounded to the north by the Scotia Sea and to the south by the Weddell Sea. The main objectives of E-SASSI were (1) to quantify the outflow of Weddell Sea waters into the Scotia Sea and to determine how these waters contribute to the modification of the Southern Boundary (SB) of the Antarctic Circumpolar Current (ACC); (2) to determine the role of the Antarctic Slope Front in these processes; and (3) to track the path of the Antarctic Slope Current before diluting into the Scotia Sea. This thesis aims to answer these questions. The sector of the SSR located between the South Shetland Islands and the South Orkney Islands is a region of especial interest. First because the gaps indenting the ridge constitute the first gate for the outflow of relatively shallow, recently ventilated waters from the northwestern Weddell Sea into the Scotia Sea. Second, because of the complexity of the bathymetry: a deep trough (the Hesperides Trough) separates the northern and southern flanks of the ridge and the location and depth of the different gaps indenting the ridge constrain the pathway of the Antarctic Slope Current. A key feature of the E-SASSI cruise with respect to previous studies conducted in the region is the unprecedented high spatial resolution of the hydrographic survey, particularly over the continental slopes. Also the coverage of all the gaps of the northern flank of the ridge was a novelty of E-SASSI. Both features have allowed a better quantification of the water mass transports in the region. The E-SASSI physical data consist mainly of Conductivity-Temperature-Depth (CTD) and ship-mounted Acoustic Doppler Currentmeter Profiler (ADCP) measurements. The presence of narrow jets, the rough topography, the strong tidal currents observed in theregion, and the fact that velocity measurements were available only for the upper 500 meters of the water column, they all handicapped the determination of the barotropic component of the flow. Inverse modeling based on the conservation of volume, heat, and salt over an enclosed region was used to refine the barotropic component of the velocity pattern initially estimated from the adjustment of the baroclinic component of velocity profiles to the ADCP measurements. The regional circulation, including the pathway of the Antarctic Slope Current, was inferred from the joint analysis of CTD profiles and the velocity field inferred from the inverse model. Results from a cross-slope section located in the Weddell Sea side show the well-defined structure of the Antarctic Slope Front before reaching the SSR. At the firsts gaps indenting the southern flank of the SSR the ASC has been observed to break into two branches: an inshore branch following the upper levels of the slope (700m) and an offshore branch extending over the 1600m isobath. At the northern flank the sampling covered all the gaps of the ridge and several cross-slope sections into the Scotia Sea. The inshore branch of the ASC was detected crossing a relatively shallow gap that prevents the outflow of the offshore, deeper branch and acts as a barrier for Weddell Sea Deep Water (WSDW). In spite of the higher velocities of the outflow, this shallow gap is less important in terms of Warm Deep Water (WDW) transport than the deeper Hesperides Passage hosting the outflow of the deeper branch of the ASC. This passage accounts for most of the outflow of Weddell Sea waters into the Scotia Sea and is the only gate of WSDW through the western sector of the SSR. The transports inferred from the inverse model give a net outflow of 7 ± 5 Sv, 2 Sv corresponding to WSDW and most of the other 5 Sv being WDW. In addition to the determination of the circulation pattern we have also analyzed inflow/outflow θS diagrams. They show an overall homogenization of the outflowing waters with respect to the more variable incoming Weddell Sea waters. In the last part of this thesis we show that isopycnal mixing between inshore and offshore water masses taking place within the Hesperides Trough is the main process for the modification of subsurface and intermediate layers. We also describe the role of the ASF in the formation of the most modified WDW observed before reaching the SSR and study the contribution of this water to the modification of the Southern Boundary of the ACC, in the southwestern sector of the Scotia Sea

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