Measurements within the Pacific-Indian oceans throughflow region

Abstract

Two hydrographic (θ, S, O 2) and trichlorofluoromethane (F-11) sections were carried out between the Australian continental shelf and Indonesia, in August 1989, on board the R.V. Marion Dufresne. The sections lie in the easternmost part of the Indian Ocean where the throughflow between the Pacific Ocean and the Indian Ocean emerges. They allow us to describe the features of the water-property and circulation fields of the throughflow at its entrance in the Indian Ocean. Between the Australian continental shelf and Bali, the Subtropical and Central waters are separated from the waters of the Indonesian seas by a sharp hydrological front, located around 13°30 S, below the thermocline down to 700 m. Near the coast of Bali, upwelling occurs in the near-surface layer under the effect of the southeast monsoon; at depth, between 300 m to more than 800 m, a water mass of northern Indian Ocean origin was present. From the characteristics of the bottom water found in the Lombok basin, the maximum depth of the Java ridge which separates the Lombok basin from the Northwest Australian basin lies around 3650 m. Off Sumba, Savu, Roti and Timor channels a core of low salinity and high oxygen content near-surface water was found in the axis of each channel, which suggests strong currents from the interior Indonesian seas towards the Indian Ocean. The entrance of the deep water flowing in the opposite direction, from the Indian Ocean to the Timor basin, was marked below 1400 m to the sill depth, through an increase of salinity and oxygen content. The flow reversal, observed briefly by a Pegasus direct current profiler in the Timor strait, was located at 1200 m depth. During the southeast monsoon, the net (geostrophic + Ekman) transport calculated on the section Australia-Bali give an estimate of the throughflow between 0 and 500 m of 22 ± 4 × 10 6 m 3 s −1 towards the Indian Ocean, with a concentration of the transport in the upper layers (19 × 10 6 m 3 s −1 in 0–200 m) and near the Indonesian coast, north of 13°30 S. In this region of intense mixing, attempts to make a salinity budget were inconclusive but did not imply any reduction in estimated throughflow transport. Below 500 m the net transport is of the order of the uncertainty. The total estimated transport (0–1900 dbar, deepest sill depth) is 18.6 × 10 6 m 3 s −1 (±7) with a mean temperature of 23°C and a mean salinity of 34.0 psu (but may be as large as 23 × 10 6 m 3 s −1, with mean temperature of 20°C and mean salinity of 34.1 psu).