CIRCULATION OF ANTARCTIC INTERMEDIATE WATER IN THE SOUTH INDIAN-OCEAN

Chlorofluorocarbon (CFC) and hydrographic data collected on the R.R.S. Charles Darwin Cruise 29 along 32 degrees S during November-December 1987, are used to examine the circulation in the South Indian Ocean. T he emphasis is on Antarctic Intermediate Water (AAIW); bottom waters a nd mode waters are also examined. Bottom waters entering in the wester n boundary of the Crozet Basin (about 60 degrees E) and in the Mozambi que Basin (about 40 degrees E) have low concentrations of anthropogeni c CFCs. The rest of the bottom and deep waters up to about 2000 m have concentrations that are below blank levels. Above the intermediate wa ters there are injections of mode waters, which are progressively dens er in the eastward direction. They form a broad subsurface CFC maximum between 200 and 400 m. The injections of recently ventilated (with re spect to CFCs and oxygen) Subantarctic Mode Waters (SAMWs) at differen t densities indicate that there is considerable exchange between the s ubtropical and subantarctic regions. The tracer data presented show th at 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 anticyc lonic gyre west of 72 degrees E. The shallow topography (e.g. that ext ending northeastward from the Kerguelen Plateau) may deflect and limit the eastward extent of the most recently ventilated AAIW. As a conseq uence, there is a zonal offset in the South Indian Ocean of the locati on 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 ea st, while the AAIW is strongest in the western-central South Indian Oc ean. The offset results in a higher vertical gradient in CFCs in the e ast. (2) The Agulhas Current may impede input of AAIW along the wester n boundary. (3) Tracers are consistent with an inter-ocean flow from t he South Pacific into the Eastern Indian Ocean, similar to the South A tlantic to Indian linkage. (4) It appears that the high wind stress cu rl forces an equatorward component of the circulation that is stronges t around 60 degrees 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 degrees S track at about 60 degrees E. Th us, 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.