North Atlantic deep water in the south-western Indian Ocean

The circulation of deep water in the south-western Indian Ocean has been studied from hydrographic observations and current measurements, obtained during the Dutch–South African Agulhas Current Sources Experiment programme, and from similar public data from the World Ocean Circulation Experiment. The three major water masses involved are the saline North Atlantic deep water (NADW), its derivative in the Antarctic circumpolar current, lower circumpolar deep water (LCDW), and the aged variety of deep water, North Indian deep water (NIDW). Although bound by the shallow topography near Madagascar, about 2×106 m3/s from the upper half of the NADW core appears to flow across the sill in the Mozambique Channel into the Somali Basin, while the remaining NADW flows east at about 45°S and is transformed to LCDW by lateral and diapycnal mixing. East of Madagascar the deep circulation is dominated by the southward flow of NIDW. Northward inflow of LCDW into the Indian Ocean therefore can take place only in the eastern half of the Indian Ocean, along the Southeast Indian Ridge and the Ninetyeast Ridge.