Impact of deep-ocean eddies and fronts on the shelf seas of a sub-Antarctic Archipelago: The Prince Edward Islands

Ecosystems within the Southern Ocean, such as the Prince Edward Islands (PEIs), fulfil a pivotal role in sustaining rich environments with large populations of top predators. Many of these have already undergone dramatic shifts in response to changes in climate, highlighting the crucial need to enhance our understanding of drivers of oceanographic variability in the region, as well as their impacts on biological communities. Daily averaged bottom temperatures and water-column current speeds from two moorings in the inter-island region, between April 2014 and April 2018, present the first continuous set of high temporal resolution in situ measurements at the PEIs. These data 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 0.5-2 degrees C, sometimes lasting longer than 30 days at a time, concomitant with changes in current speed and direction were associated with advection of waters into the shelf region from passing anticyclonic/cyclonic eddies. Similarly, 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 proximity to the islands. When the S-SAF was located north of the PEIs, bottom temperatures were overall lower due to the stronger influx of Antarctic surface and intermediate waters. In contrast, when the S-SAF was close to or south of the PEIs, bottom temperatures were elevated as a result of the occurrence of larger proportions of warmer, more saline surface and intermediate sub-Antarctic and even Subtropical waters. Notably, the 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.