As part of the WOCE Indian Ocean
project, simultaneous, full depth hydrographic and velocity data
were collected in the Agulhas Current at 32S off the east coast
of South Africa in March 1995. This unique data set enabled us
to compare geostrophic estimates of Western Boundary Current structure
with direct measurements of the flow field from Lowered ADCP.
Results indicate that the baroclinic structure of the current
is essentially geostrophic below about 200m depth where direct
and geostrophic velocities exhibit differences that are generally
within estimated errors. Near surface differences can reach over
20cms-1, are patchy, and flow curvature
alone does not account for them, but a combination of sampling
biases and ageostrophic flow could explain them.
The barotropic component of the ocean current as measured by LADCP is one of the techniqueís most robust results, and the success of referencing geostrophic currents using the depth-integrated velocity from LADCP reflects this: the correlation of the matched profiles is high and absolute geostrophic bottom velocities exhibit the same pattern of deep flow as the direct measurements. In addition the geostrophic volume transport estimated in this way differs from a direct estimate by just 3%. In contrast, when geostrophic shears are matched to shipboard ADCP data the results are difficult to reproduce and bottom velocities bear little similarity to the measured flow.
LADCP measurements, in combination with CTD data, have also been used to calculate potential vorticity (PV) throughout the Agulhas Current. PV was estimated as three components: a planetary component, and vertical and horizontal shear components. Isopycnic gradients of PV indicate that while there is relatively free exchange of deep waters across the Agulhas Current, the mixing of intermediate water masses appears to be constrained.