The Southern Boundary of the Antarctic Circumpolar Current

Antarctic ice sheet mass loss is a major contributor to global sea level rise. Circumpolar Deep Water, transported by the Antarctic Circumpolar Current (ACC), contributes heat to amplify ice shelf basal melting, when it accesses the Antarctic continental shelf. The Southern Boundary of the ACC provides the closest connection of warm ACC waters to Antarctica. Strong density gradients across the Southern Boundary constitute a frontal jet and modulate the southward heat transport. Mechanisms modulating the poleward transport of heat are poorly understood. This thesis explores two such mechanisms. Firstly, the frontal structure of the Southern Boundary is investigated with observed hydrographic transects. A global ocean reanalysis is used to identify the interannual and seasonal variability of the Southern Boundary and deep-water temperatures on the Bellingshausen Sea continental shelf. Annual means of potential temperature reveal interannual variability and enable separation into warm and cold regimes with deep-water temperature differences of up to 1°C. The results suggest that the deep-water temperature in the southern Bellingshausen Sea is due to a combination of shelf break heat transport and surface heat fluxes. Surface heat flux variability is related to the variability of the Amundsen Sea Low and its influence on sea ice extent and local formation of cold, dense water in winter. Secondly, the variability of the Southern Boundary at the Greenwich Meridian is investigated using ocean gliders and satellite altimetry. It is demonstrated that a cyclonic eddy near the Southern Boundary increases its density gradients and amplifies its frontal jet. Mixing length scales are suppressed across the frontal jet while the eddy is crossing and increase to about 25 km after the eddy has crossed the Greenwich Meridian. This increase supports an increased exchange of water parcels across the Southern Boundary, implying that the ‘barrier’ properties of the Southern Boundary are reduced

Wind-induced variability of warm water on the Southern Bellingshausen Sea continental shelf

The Bellingshausen Sea hosts heat transport onto the continental shelf, potentially enhancing ice shelf basal melt. Here we use the GLORYS12V1 1993-2018 reanalysis to identify physical processes that set seasonal and interannual variability of water mass properties in the Eltanin and Latady bays on the southern Bellingshausen Sea continental shelf. Annual means of potential temperature from 300 m to the seabed reveal interannual variability and allow separation into warm and cold regimes. The Amundsen Sea Low is more intense and extends further east during the warm regime than the cold regime. In the warm regime, a wind-induced reduction of sea ice concentration near the coast increases surface heat loss, convection, and formation of cold dense water in winter, associated with a decrease in heat content of the southern Bellingshausen Sea over time and a net northward heat transport. In contrast, in the cold regime, increased sea ice concentration reduces surface heat loss and thus formation of cold, dense water. Combined with an increase in heat content over time and a net southward heat transport, this results in a warming of the southern Bellingshausen Sea. This suggests that variability in the deep water temperature in the southern Bellingshausen Sea is primarily due to local surface heat fluxes above the shelf. The variability of surface heat fluxes is related to the variability of the Amundsen Sea Low and its influence on sea ice extent and local formation of cold, dense water in winter

Stirring across the Antarctic Circumpolar Current's southern boundary at the prime meridian, Weddell Sea

At the southern boundary of the Antarctic Circumpolar Current (ACC), relatively warm ACC waters encounter the colder waters surrounding Antarctica. Strong density gradients across the southern boundary indicate the presence of a frontal jet and are thought to modulate the southward heat transport across the front. In this study, the southern boundary in the Weddell Sea sector at the prime meridian is surveyed for the first time in high resolution over 2 months during an austral summer with underwater gliders occupying a transect across the front on five occasions. The five transects show that the frontal structure (i.e. hydrography, velocities and lateral density gradients) varies temporally. The results demonstrate significant, transient (a few weeks) variability of the southern boundary and its frontal jet in location, strength and width. A mesoscale cold-core eddy is identified to disrupt the southern boundary’s frontal structure and strengthen lateral density gradients across the front. The front's barrier properties are assessed using mixing length scales and potential vorticity to establish the cross-frontal exchange of properties between the ACC and the Weddell Gyre. The results show that stronger lateral density gradients caused by the mesoscale eddy strengthen the barrier-like properties of the front through reduced mixing length scales and pronounced gradients of potential vorticity. In contrast, the barrier-like properties of the southern boundary are reduced when no mesoscale eddy is influencing the density gradients across the front. Using satellite altimetry, we further demonstrate that the barrier properties over the past decade have strengthened as a result of increased meridional gradients of absolute dynamic topography and increased frontal jet speeds in comparison to previous decades. Our results emphasise that locally and rapidly changing barrier properties of the southern boundary are important to quantify the cross-frontal exchange, which is particularly relevant in regions where the southern boundary is located near the Antarctic shelf break (e.g. in the West Antarctic sector)