Circulation pathways and transports of the Southern Ocean in the vicinity of the Southwest Indian Ridge

Abstract

Circulation pathways and transports of the Antarctic Circumpolar Current (ACC) and Agulhas Return Current (ARC) around the complex bathymetry of the Southwest Indian Ridge and Del Caño Rise have been established from two hydrographic surveys and two‐year long moorings. After crossing the Southwest Indian Ridge in a 110 Sv (1 Sv = 106 m3s−1) flow concentrated at 48°S, the ACC fragments before being reconcentrated into several branches by the bathymetry. In particular 30–40 Sv turns north around the eastern flank of the Del Caño Rise, turns west as part of anticyclonic flow round the Del Caño Rise finally returning eastward as part of the Crozet Front. Eddy variability from both satellite data and moorings is low over the bathymetry of the Del Caño Rise, so the strong currents found between the Del Caño Rise and the Crozet Plateau are believed to be permanent features. This largely barotropic, time‐independent transport is the major pathway for the ACC to enter the Crozet Basin. The anticyclonic flow also transfers water from the Mozambique Basin across the Southwest Indian Ridge to the south side of the Del Caño Rise. Given the evidence for fragmentation of the ACC where it crosses the Southwest Indian Ridge and the concentration of several jets into the Crozet Front from the ACC, Subtropical Front and Agulhas Return Current, it is misleading to relate standard frontal definitions such as the Polar Front and Subantarctic Front to the major transport pathways.