BACKGROUND

Geostrophy and Transport

The geostrophic velocity of the Agulhas Current and Undercurrent was carefully examined by Beal and Bryden (1999). Looking at the terms in the Navier-Stokes equations, they found from scale analysis that the geostrophic estimation is valid everywhere in the Agulhas Current and AUC for along-shore flow, despite the large interfacial velocity shears.
Moreover, although the along-shore velocity decreases rapidly with depth, implying that the geostrophic balance may be disrupted deeper in the current (and in the Undercurrent), the core of the WBC moves offshore with depth, thereby increasing the cross-stream scale of the flow and helping to maintain the geostrophic balance.
Encouragingly, as predicted from the results of the scale analysis, geostrophic and LADCP velocity shears were found to be in good agreement, except over the upper 200 m of the Agulhas Current, where Ekman, inertial and internal tide motions are large.
LADCP data was successfully implemented as a reference for the geostrophic shear, producing a geostrophic transport within 2 Sv of the direct velocity transport estimate (Beal and Bryden, 1999).
Of further note is that the geostrophic shears and thus the density structure of the AC and AUC has been similar from one occupation to the next, suggesting that fluctuations in transport may result primarily from changes in the barotropic component of the flow.