Circulation of Antarctic intermediate water in the South Indian Ocean

Abstract

Chlorofluorocarbon (CFC) and hydrographic data collected on the R.R.S. Charles Darwin Cruise 29 along 32°S during November-December 1987, are used to examine the circulation in the South Indian Ocean. The emphasis is on Antarctic Intermediate Water (AAIW); bottom waters and mode waters are also examined. Bottom waters entering in the western boundary of the Crozet Basin (about 60°E) and in the Mozambique Basin (about 40°E) have low concentrations of anthropogenic CFCs. The rest of the bottom and deep waters up to about 2000 m have concentrations that are below blank levels. Above the intermediate waters there are injections of mode waters, which are progressively denser in the eastward direction. They form a broad subsurface CFC maximum between 200 and 400 m. The injections of recently ventilated (with respect to CFCs and oxygen) Subantarctic Mode Waters (SAMWs) at different densities indicate that there is considerable exchange between the subtropical and subantarctic regions. The tracer data presented show that the circulation of AAIW in the South Indian Ocean is different from that in the South Atlantic and South Pacific oceans in several ways. (1) The most recently ventilated AAIW is observed in a compact anticyclonic gyre west of 72°E. The shallow topography (e.g. that extending northeastward from the Kerguelen Plateau) may deflect and limit the eastward extent of the most recently ventilated AAIW. As a consequence, there is a zonal offset in the South Indian Ocean of the location of the most recently ventilated SAMW and AAIW, which does not occur in the other two oceans. The strongest component of SAMW is in the east, while the AAIW is strongest in the western-central South Indian Ocean. The offset results in a higher vertical gradient in CFCs in the east. (2) The Agulhas Current may impede input of AAIW along the western boundary. (3) Tracers are consistent with an inter-ocean flow from the South Pacific into the Eastern Indian Ocean, similar to the South Atlantic to Indian linkage. (4) It appears that the high wind stress curl forces an equatorward component of the circulation that is strongest around 60°E. As a result the highest concentrations of CFCs and oxygens in bottom waters, AAIW, and the lightest component of SAMW are co-located along the 32°S track at about 60°E. Thus, the most recently ventilated circumpolar waters, participating in both the wind driven and the thermohaline circulations, follow similar paths equatorward into the subtropical Indian Ocean.