Observations of the velocity structure of the Agulhas Current

Abstract

The Agulhas Current is the Western Boundary Current (WBC) of the Southwest Indian Ocean and is the second largest WBC in the world's oceans. The full depth velocity field of the Agulhas Current has been directly measured for the first time using a lowered acoustic Doppler current profiler (LADCP), a technique not used before by British scientists. The technique involves a 150kHz ADCP in a 6000m pressure casing which is lowered through the water column to obtain an absolute velocity profile. Fifteen combined CTDO2/LADCP stations were occupied across the Agulhas Current at 32°S in February-March 1995. Careful comparisons with shipboard ADCP, bottom tracking data and geostrophic estimates suggest that LADCP profiles are accurate to within about 6cms^-1.

From LADCP velocities the deep dynamics of the Agulhas Current were found to be very different than previously estimated using geostrophic estimates. In particular, traditional geostrophic calculations generally use near-horizontal levels of no motion from which to obtain absolute velocities, but LADCP velocities show that there is a V-shaped pattern for the level of no motion in the WBC. As a result the LADCP data reveals an Agulhas Undercurrent flowing equatorward below 800m depth directly underneath the core of the Agulhas Current. This Undercurrent is estimated to carry 6Sv of primarily North Atlantic Deep Water into the Indian Ocean.

The volume transport of the Agulhas Current is estimated to be 71Sv using a combination of LADCP velocities and geostrophic estimates. This is 14Sv less than the most recent estimate in the literature and the difference is principally due to the Undercurrent. Using the LADCP section in combination with a transindian hydrographic section across 32°S from November 1987, and taking a climatological Ekman flux of 0.5Sv and an Indonesian Throughflow of 6.4Sv, the heat flux divergence over the Indian Ocean north of 32°S is calculated to be 1PW. The interior transport across 32°S, below 2000db is estimated to be 17Sv equatorward, requiring a basin-averaged upwelling rate of 4x10^-7 ms^-1, close to half the most recent estimate.

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