Ocean fronts and circulation
The northern boundary of the Southern Ocean is defined by the subtropical front (STF) which separates warm, salty, subtropical waters in the north from the colder, subantarctic waters of the south.
Within the Southern Ocean, the vast and highly dynamic Antarctic Circumpolar Current (ACC) circles the globe and dominates the circulation. This current is driven by the globe’s strongest westerly winds between latitudes 45 and 55º S.
The Antarctic Circumpolar Current is associated with several narrow jets or fronts. These frontal regions are characterised by sharp horizontal gradients in hydrographic properties (such as temperature, salinity, density, oxygen and nutrients) that mark the boundaries of different water masses.
Three main fronts are continuous features of the Antarctic Circumpolar Current: the subantarctic front (SAF); the polar front (PF); and a deep-reaching front observed persistently to the south, the southern ACC front (SACCF).
The path of the Antarctic Circumpolar Current is mainly controlled by bottom topography, with major features such as ridges and plateaus acting as barriers that deflect and alter the flow. The Kerguelen Plateau is oriented north-west/south-east along the 70º E meridian and forms a large topographic barrier to the eastward flow of the Antarctic Circumpolar Current.
The subantarctic front and polar front are effectively merged as they pass to the north of the Kerguelen Plateau, while the southern Antarctic Circumpolar current front is deflected around the southern boundary of the Plateau, east of which, it turns northward to form a western boundary current along its eastern flank.
The most important front in the Heard Island region is the Antarctic Polar Front, which is typically defined as the northern limit of a temperature minimum of 2ºC at a depth of 100 to 300 metres.
The Polar Front is most often located just to the north of Îles Kerguelen and as a result that island ’s climate and biogeography is distinctly subantarctic. This is in contrast with Heard Island which is a typically Antarctic island.
The Polar Front follows a meandering course and may in some regions split into two separate jets, occasionally it may also be displaced to the south of Îles Kerguelen which has important implications for the biota and fisheries of the region.
Generally, the most biologically productive waters in the HIMI region are those to the north and east of Heard Island. These areas are high in phytoplankton, which often means that they would be areas high in secondary production such as zooplankton and fish. Highly productive regions like these are the most important local foraging areas for land-based marine predators.