The velocity and vorticity structure of the Agulhas Current at 32°S

Abstract

The full depth velocity field of the Agulhas Current, the Western Boundary Current of the southwest Indian Ocean, has been directly measured using a lowered acoustic Doppler current profiler (LADCP). Fifteen combined conductivity‐temperature‐depth (CTD) and LADCP stations were occupied across the current at 32°S off the east coast of South Africa in February‐March 1995, as part of the World Ocean Circulation Experiment Indian Ocean program. The deep velocity structure of the Agulhas Current was found to be very different than previously described using geostrophic estimates. In particular, LADCP results reveal a V‐shaped pattern for the level of no motion across the Agulhas Current and an Agulhas Undercurrent is observed flowing equatorward below 800 m depth, directly beneath the surface core of the poleward flowing Agulhas Current. Comparisons of direct and geostrophic velocities suggest that the velocity structure of the Agulhas Current is essentially geostrophic below about 200 m depth, where differences are generally less than estimated errors. Above these depths the LADCP‐measured shears and geostrophic shears exhibit differences and velocities can diverge. Shipboard ADCP data support LADCP results near the surface, indicating that both instruments are measuring real signals. The geostrophic and LADCP‐measured shears are well matched away from the surface, and a depth‐averaged fit (below 200 m) between them exhibits small standard deviations. The geostrophic volume transport of the Agulhas Current, as referenced to LADCP, is 73 Sv, and is just 3% different from the direct LADCP transport estimate. The most recent full depth estimate of Agulhas Current transport in the literature is given by Toole and Warren [1993], who estimate a geostrophic volume transport of 85 Sv. The baroclinic velocity structures from their section and from this work are similar, indicating that the difference in the transport estimates is due to the choice of geostrophic reference level and ultimately to the presence of the previously unobserved Agulhas Undercurrent. Combined CTD/LADCP data are very useful for potential vorticity analysis, since velocity and density measurements are simultaneous and coincident. The vorticity structure of the Agulhas Current suggests the presence of a “mixing boundary” at middepths, similar to features observed in the Gulf Stream. The feature appears to be associated with a sharp upturn in isopycnals, coincident with the foot of the continental slope. The boundary inhibits cross‐stream mixing of intermediate water masses and may help explain the appearance of a discrete filament of Red Sea Water inshore of Antarctic Intermediate Waters in the same density layer, despite intense cross‐stream shears.