Flow at intermediate depths around Madagascar based on ALACE float trajectories

Chapman, P., Marco, S.F.D., Davis, R.E. and Coward, A.C. (2003) Flow at intermediate depths around Madagascar based on ALACE float trajectories. Deep Sea Research Part II: Topical Studies in Oceanography, 50 (12-13), 1957-1986. (doi:10.1016/S0967-0645(03)00040-7).

Abstract

During 1994–1996, 215 Autonomous Lagrangian Circulation Explorers (ALACE floats) were released at a nominal 900 m depth in the Indian Ocean as part of the World Ocean Circulation Experiment. Of these, 66 entered the region around Madagascar (2–30°S, 35–55°E), generally at a depth of 800–900 m. Floats approaching the island from the east were deflected either northward or southward depending on latitude, with the bifurcation point being near 20°S. Mean southward velocities in the western boundary current were 8.5 cm s-1 during each 25-day observation period, with mean northward velocities 7.7 cm s-1. Speeds past Cape Amber were about 11 cm s-1. These figures are comparable to those obtained from in situ current meter measurements. Floats rounding the island to the north frequently drifted north of the Comores for many months with no discernable pattern, before exiting the region either to the north in the East African Coastal Current and the equatorial current system or to the south via the Mozambique Channel. Flow rates in this region were highly skewed towards low (less than 5 cm s-1) velocities.
Floats passing south of Madagascar showed little northward movement into the Mozambique Channel, but tended to move steadily westwards towards the African coast, becoming entrained in the Agulhas Current and its recirculation gyres near 28°S. Similarly, floats released within the southern portion of the Channel all tended to move to the south and west despite occasional entanglement with eddies. Mean flow in the Agulhas region was about 18 cm s-1, with maximum velocities over 25 days of up to about 35 cm s.
All flows were extremely variable because of the ubiquity of eddies. In essence, the flow observed near 800 m resembled closely that seen at the surface from TOPEX/POSEIDON altimetry and at 845 m in the OCCAM global model. Temperature data collected by the floats were used to determine when a given float was being acted on by an anticyclonic eddy. Deviations, which were typically 0.5°C or more from the background temperature field, were found only in eddies south of 12°S and occurred in about 9% of float records within the Mozambique Channel. The results support the idea of a net southward flow of water through the Channel from the tropics to the Agulhas Current, with the eddies playing a major role in the transport in the southern part of the region. In the northern and eastern parts of the region, however, eddies were far less prevalent, and topographic steering by the Mascarene Ridge became important.

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