Origin, pathways, and characteristics of deep-ocean mesoscale eddies influencing the shelf seas of a sub-Antarctic Archipelago in the Southern Ocean: The Prince Edward Islands

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 Marcel Van den Berg, Oceans and Coasts research, Department of Forestry,Fisheries and the Environment, Cape Town, South Africa
Within the Southern Ocean, the Andrew Bain Fracture Zone (ABFZ) of the Southwest Indian Ridge (SWIR) is known to be a hotspot for the generation of mesoscale eddies. These eddies have been shown to profoundly impact on heat and salt budgets across frontal zones, transporting physical characteristics and biota of both sub-Antarctic and Antarctic origin across the Antarctic Polar Frontal Zone toward the Prince Edward Island (PEI) archipelago. The interaction of oceanic features such as mesoscale eddies and fronts with the PEIs contributes to the maintenance of a rich, biologically diverse ecosystem which supports a large number of top predators, and thus it is necessary to improve our knowledge of when and how these features interact with the island ecosystem. We applied an eddy detection scheme to satellite altimetry to further explore the influence of mesoscale eddies on the PEI archipelago by quantifying the number of eddies reaching the islands from 1993 to 2018. In addition, we also investigated the origin, pathways, and characteristics of these eddies. A total of 395 cyclonic and 377 anticyclonic eddies, with diameters ranging mostly between 60 and 100 km, were observed in the PEI region (44.6-48.6°S; 35-41°E) over the 25-year period. The vast majority of these eddies were short-lived (< 90 days) and formed within this region, with only 19 % of anticyclonic and 25 % of cyclonic eddies entering the PEI region from further afield. While previous studies have described an extensive train of eddies extending from the ABFZ of the SWIR (48.6-52.6°S; 27-33°E) to the PEIs, our study shows that only 1 anticyclonic and 14 cyclonic eddies born within this hotspot area entered the PEI region over the 25-year period, and only 3 of these cyclonic eddies propagated close enough to directly interact with the PEI archipelago.