Structure and transport of the Antarctic Circumpolar Current and Agulhas Return Current at 40°E

Abstract

Hydrographic data are presented from a near‐meridional section across the Agulhas Return Current, the Subtropical Front and the Antarctic Circumpolar Current (ACC) in the Southwest Indian sector of the Southern Ocean. The location and characteristics of the fronts are examined and the water masses are defined. The Agulhas Return Current emerges as a strong current as far east as 40°E which forms a marked front that is separate from the Subtropical Front, and can be distinguished by a separate water mass. Comparisons are made with a similar hydrographic section at the Greenwich Meridian and several notable differences are found. The Subantarctic Front at the Greenwich Meridian is a pronounced feature carrying a large proportion of the geostrophic transport of the ACC. On the Southwest Indian section, however, the Subantarctic Front is merged with the Polar Front at 48°S, where the flow associated with the ACC is concentrated. Geostrophic calculations show a large volume transport, 84 to 103 Sverdrup (1 Sv = 106 m3 s−1), associated with the Agulhas Return Current at 40°S. This is quite separate from the transport of the ACC which totals 138 Sv south of the Subantarctic Zone and the Crozet Plateau, or 150 Sv including flow in the Subantarctic Zone. The transport of the ACC out of the South Atlantic sector appears to be higher than the transport into the South Atlantic sector at Drake Passage, with the extra transport most probably carried by upper layer and intermediate waters. Eddies are seen to be an important feature of the Subantarctic Zone where they help to modify the water masses. Antarctic Intermediate Water appears in two forms, a saline (34.4) Indian Ocean variety and a fresh (34.2) Atlantic Ocean variety. The Atlantic variety is freshest at the Polar Front, and occupies the southern half of the Subantarctic Zone. Within the eddies, in the central Subantarctic Zone, there is interleaving between these two forms of Antarctic Intermediate Water. North Atlantic Deep Water is unable to cross the Southwest Indian Ridge but can pass south of the Crozet Plateau in the form of Circumpolar Deep Water, which is slightly more saline (34.75) than at Drake Passage (34.73).