Dynamics of the Southern Ocean Circulation Off West Antarctica

Abstract

This thesis presents an investigation of the dynamics of the ocean circulation off West Antarctica, a region characterised by intense ice shelf loss. The Ross Gyre, located in the western side of West Antarctica, is studied through remote sensing techniques that retrieve information of the sea level from sea ice leads. The seasonality of the gyre is driven by the winds through two distinct modes. The first mode, associated with circumpolar winds, reduces sea level at the end of summer through northward Ekman transport. Regional sea level rises in winter due to ice cover, which reduces the wind stress on the ocean. The second mode, associated with the Amundsen Sea Low (ASL), controls the transport and area of the Ross Gyre and the coastal throughflow with a semi-­‐ annual period, with peaks in May and November. A deeper ASL allows the gyre’s expansion and intensification, and enhances the westward throughflow. On the eastern side of West Antarctica, warm Circumpolar Deep Water (CDW) inflows to the Amundsen Sea are studied with a regional model and in situ data. The key inflow-­‐controlling mechanisms are identified by decomposing CDW temperature variability into two components associated with (1) changes in the depth of isopycnals (heave; HVE), and (2) changes in the temperature of isopycnals (water mass property changes; WMP). In the western Amundsen Sea, the deeper thermocline and shallower shelf break hinder CDW access onto the shelf, and CDW inflow is regulated by the uplift of isopycnals at the shelf break – which is itself controlled by wind-­‐driven variations in the speed of an undercurrent flowing eastward along the continental slope. In contrast, the shallower thermocline and deeper shelf break in the eastern Amundsen Sea permit CDW to persistently access the continental shelf, and CDW temperature in the area responds to wind-­‐driven modulation of the water mass’ on-­‐ shelf volume by changes in the rate of inflow across the shelf break and in Ekman pumping-­‐ induced vertical displacement of isopycnals within the shelf. A multi-­‐year mooring located in the Dotson Trough (western Amundsen Sea) supports our model-­‐based conclusions. This mooring shows a transition to cooler conditions between 2010 and 2015, associated with WMP induced by weakening winds at the shelf break. The ASL was identified as the main driver of the local winds. A deeper ASL favours strengthening of the easterlies, weakening the undercurrent and reducing access of CDW to the continental shelf. In turn, a shallow ASL favours strengthening of the westerlies, intensifying the undercurrent and increasing access of CDW to the continental shelf. Thus, the Amundsen and Ross Seas are connected by a coastal current accelerated during periods of deeper ASL, which favours the export to the Ross Sea of meltwater from the Amundsen Sea coast. A future change in the ASL strength could impact both regions, with consequences for the regional heat budget and melting of the ice shelves downstream.

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ORCID for Alberto Naveira Garabato: orcid.org/0000-0001-6071-605X

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