Characterising the influence of deep-ocean eddies and fronts on the shelf seas of a sub-Antarctic Archipelago in the Southern Ocean: The Prince Edward Islands

Marcel Van den Berg, Oceans and Coasts research, Department of Forestry,Fisheries and the Environment, Cape Town, South Africa, Tarron Lamont, Oceans & Coasts Research, Department of Forestry, Fisheries and the Environment; University of Cape Town; Bayworld Centre for Research and Education, Department of Forestry, Fisheries and the Environment, Cape Town 8012, South Africa and Isabelle Jane Ansorge, University of Cape Town, Department of Oceanography, Cape Town, South Africa
Within the Southern Ocean, ecosystems such as the Prince Edward Islands (PEIs) play a pivotal role in sustaining rich environments with large populations of top predators, and thus there is a crucial need to enhance our understanding of drivers of oceanographic variability and impacts on biological communities in such regions. Daily (April 2014-April 2019) averaged bottom temperatures and water-column current speeds from two moorings at the PEIs were investigated to determine the influence of passing deep-sea mesoscale eddies and fronts on hydrographic conditions on the inter-island shelf. Warming/cooling events of the order of 0.5 to 2 °C, concomitant with changes in current speed and direction were associated with advection of waters into the shelf region from passing anticyclonic/cyclonic eddies. Some of these eddies were observed to influence shelf circulation for more than 30 days at a time. The impact of frontal movement was quantified by increased current speeds throughout the water column when the southern branch of the sub-Antarctic Front (S-SAF), or the northern branch of the Antarctic Polar Front was in close to the islands. When the S-SAF was north of the PEIs, bottom temperatures were lower due to stronger influx of Antarctic surface and intermediate waters. In contrast, when the S-SAF was south of the PEIs, bottom temperatures were elevated due to the occurrence of larger proportions of warmer, more saline surface and intermediate sub-Antarctic and even Subtropical waters. Predominance of westerly flow in the southern portion of inter-island region suggested the perpetual existence of a Taylor column which was at times enhanced by the juxtaposition of some eddies. As a result of its course spatial resolution, satellite altimetry fails to capture this westerly flow, demonstrating the critical need to sustain and enhance in situ observations in the region.