A hydrographic section across the subtropical South Indian Ocean

Geostrophic transport figures are presented based on a zero-velocity surface constructed along the section from the tracer-property evidence of where deep water was moving northward and where southward. Ekman transport, deduced from shipboard acoustic-Doppler profiler measurements, as well as synoptic and historical wind stress data, is found to be small (about 1 × 106 m3 s−1 northward). Net transport (geostrophic and Ekman) across the section is estimated to be 7 × 106 m3 s−1 southward, which implies a similarly sized Indonesia throughflow. Ambiquity in the geostrophic referencing scheme, and the magnitude of baroclinic eddy noise on the section, suggest this figure in uncertain by at least ±10 × 106mm3 s−1. The calculations obtain a figure for net transport of water below 2000 dbars of 27 × 106 m3 s−1 northward, which specifies an average upwelling speed at the 2-km level north of 30°S of 6.9 × 10−5 cm s−1. This estimate, perhaps uncertain by 20-30%, nonetheless contributes to growing evidence for an anomalously vigorous meridional circulation in the Indian Ocean. The associated calculations of heat and fresh water flux divergences demonstrate that the Indian Ocean thermohaline circulation essentially expresses a conversion of bottom and deep water to mid-depth thermocline, and near-surface water.