A near-synoptic survey of the Southwest Indian Ocean

This study focused on the southwest region of the Indian Ocean, where the poleward-directed Agulhas Current is born, and where dense waters filter through fractures in the Southwest Indian Ridge to form an equatorward-directed deep boundary current east of Madagascar. Both represent major circulation features of the Indian Ocean: the Agulhas in its role as a western-boundary current closes the wind-driven subtropical gyre; the deep western-boundary current renews the bottom waters of the Madagascar, Mascarene, and Somali basins to the north. onal, quasi-synoptic survey of the Southwest Indian Ocean carried out as part of the World Ocean Circulation Experiment Hydrographic Program from May through July of 1995 occupied a cruise track that comprised a closed “box” in the Southwest Indian Ocean. Full-depth water properties and direct-velocity measurements were used to diagnose the circulation patterns as a function of depth and to estimate the transports of the major currents. The synoptic circulation was quantified by the construction of a referenced geostrophic velocity field. Water-mass distributions, direct-velocity measurements, and mass conservation within bounded regions guided the placement of the level of no motion. Errors in the reference scheme and from synoptic-scale circulation features such as eddies and internal waves that are aliased by the hydrographic sampling led to uncertainties in the transport estimates. per-ocean Agulhas transport (neutral density, γn<27.96 kg m−3, depths≲2000 m) was estimated to be 76×106 m3 s−1. Contributions to the Agulhas consisted of 29×106 m3 s−1 from the westward limb of the subtropical gyre south of 25°S, 20×106 m3 s−1 from the poleward flow east of Madagascar, which subsequently turns to move west at about 25°S, and an additional poleward flow of 18×106 m3 s−1 through the Mozambique Channel. Bathymetry strongly controls the deep and bottom circulation: the African Coast, the Mozambique Plateau, and the Madagascar Ridge support deep western-boundary currents along their eastern margins; however, the Natal Valley and the Mozambique Basin are blocked to meridional flow beneath 2800 and 3000 m, respectively. East of the Madagascar Ridge, net northward deep transport is small. In the Madagascar and Mascarene Basins, a deep cyclonic circulation is present: high-latitude deep waters move northward along the western boundary, while North Indian Deep Water flows southward in the basin interior. The deep western-boundary current along the east coast of Madagascar carries about 3×106 m3 s−1 of bottom water (γn>28.11 kg m−3) northward.