The hydrography and dynamics of the ocean environment of the Prince Edward Islands (Southern Ocean)

Abstract

The Prince Edward Islands lie in the Indian sector of the Southern Ocean at 47°S and 38°E. They lie in the path of the Antarctic Circumpolar Current (ACC), between the Subantarctic Front (SAF) to the north and the Antarctic Polar Front (APF) to the south. Two extensive hydrographic surveys (MOES 2 and MIOS 2) have been carried out to establish for the first time the mesoscale hydrography and dynamics of the oceanic surroundings of these islands. During the MOES 2, the SAF was deflected northward around the islands, while the APF lay south of the survey grid and south of the islands. Water masses in the region changed gradually from Subantarctic Surface Water (SASW) to Antarctic Surface Water (AASW) on crossing the Polar Frontal Zone (PFZ). Downstream of the islands, a wake, resulting in the generation of broad meanders, was formed. As a consequence, water masses, in particular warm SASW, were displaced from north of the SAF across the PFZ, while cooler waters, which have been modified in the transitional band of the PFZ, were displaced northwards. In contrast, during MIOS 2, the surface expression of the SAF formed an intensive frontal band. On approaching the islands, the SAF split into two branches, with a branch deflected northwards around the islands, while a second branch meandered southward. In the downstream region, an intense cold eddy consisting of AASW was observed within the PFZ, displacing the SAF northwards. South of this eddy, a warm patch of SASW water was encountered, its position possibly controlled by the meandering SAF. Evidence from both these surveys demonstrates that the ACC exhibits high degrees of mesoscale variability in the vicinity of the Prince Edward Islands. The displacement of the SAF in both instances was apparent, resulting in the advection or the entrapment of neighbouring water masses into and across the PFZ. The speed of the incident current on approaching the islands may play a role in the degree of mesoscale mixing downstream of the islands.