Flow disturbances in the Antarctic Circumpolar Current at the Prince Edward Islands

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The Prince Edward Islands are located in the Indian sector of the Southern Ocean at 46°50’S and 37°50’E. Intensive past investigations have shown that they lie directly in the path of the Antarctic Circumpolar Current (ACC), sandwiched between the Subantarctic Front (SAF) to the north and the Antarctic Polar Front (APF) to the south.

Two mesoscale surveys (MOES 2 and MIOS 2) were designed to study the impact these islands may have on the spatial and temporal variability of the oceanic environment in this sector of the Southern Ocean

Investigations carried out during MOES 2 showed that the SAF, influenced by the shallow topography, was deflected around the islands, while the APF remained distinctly south of the survey grid. Water masses in this region were shown to modify gradually from Subantarctic Surface Water (SASW) (7°C) to Antarctic Surface Water (AASW) (5°C) as the Polar Frontal Zone (PFZ) was crossed. Downstream of the islands a wake, resulting in the generation of broad, cross-frontal meanders was formed. As a consequence, warm SASW from north of the SAF was advected southwards across the PFZ, while cooler waters which had been modified in the transitional band of the PFZ were advected northwards (Figure 1).

In comparison, during MIOS 2, the surface expressions of the SAF and the APF appeared to combine forming an intensive frontal feature upstream of the islands. Closer to the islands, these fronts separated, resulting in the SAF moving northwards around the islands, while the APF meandered south of the islands. In the downstream region, an intensive cold-core eddy consisting of AASW (<5°C) was observed within the PFZ, displacing the SAF further northwards. South of the eddy, a warm patch of SASW water was observed (>7°C), its position possibly controlled by the meandering APF which lay on either side (Figure 2). The exact genesis of both eddies is unknown, however it is possible that they have been generated from instabilities within the meandering SAF and APF.

Evidence from the 2 surveys in the upstream and downstream regions of the islands have shown the ACC to exhibit signs of mesoscale temporal and spatial variability. Under both conditions enhanced cross frontal mixing were apparent, resulting in the advection or the entrapment of neighbouring water masses into and across the PFZ.

 

 

 

 

 

Figure 1: Horizontal distribution of subsurface (200 m) temperatures observed during MOES 2. The meandering wake in the downstream region is clearly evident. Black circles indicate the position of hydrographic stations. The 6°C isotherm representing the subsurface core of the SAF is highlighted.

 

Figure 2: Horizontal distribution of subsurface (200m) temperatures observed during MIOS 2. The positions of the warm and cold core eddies in the downstream region are clearly evident. Black circles indicate the position of hydrographic stations. The 6°C isotherm representing the subsurface core of the SAF is highlighted.