BACKGROUND

Velocity

LADCP observations of the velocity structure of the Agulhas Undercurrent (Beal and Bryden, 1997) show a bottom intensified slope current with northeastward velocities up to 30 cm/sec.

The AUC has a vertical extent of 2000 m, from a depth of 800 m down to 2900 m at the foot of the slope.

Horizontally the Undercurrent is some 30 km wide from the slope to the zero-crossing point, where the velocity field passes into the main Agulhas Current.

Above the AUC, in the southwestward flowing Agulhas Current, the vertical velocity shear is dramatically high, as much as 100 cm/sec over 400 m, but in the Undercurrent itself the vertical shear is almost zero.

Figure 1. Vertical section of LADCP velocities (cm/sec) across the Agulhas Current. Positive velocities flow to the southwest. Bathymetry is shaded black.

Velocity vectors show that there is no gradual rotation of the flow with depth as it makes the transition from WBC to AUC, rather the vectors suddenly reverse as the along-shore flow passes through zero.

This is an indication of strong topographic control of the flow field.

Figure 1c. Velocity vectors as a function of depth for Agulhas Current Experiment (ACE). Profiles within the undercurrent have very little vertical shear