BACKGROUND

Water Masses and Potential Vorticity

Water Masses

The AUC carries approximately 2 Sv of Antarctic Intermediate Water (AAIW) and 4 Sv of North Atlantic Deep Water (NADW) equatorward (Beal and Bryden, 1997).

It entrains some remnants of Red Sea Water (RSW) into its core, but the net transport of these waters is poleward in the Agulhas Current.

In all three datasets RSW occurs as a distinct filament, despite the enormous distance from its source (Donohue et al., 2000).

It's position is coincident with the high shear interface between the two currents and AAIW is found further offshore within the same density layer.

Potential Vorticity Structure

On the nearshore side of the current, relative vorticity from horizontal and vertical velocity shear represent significant contributions to the PV, in addition to the planetary component.

Beal and Bryden, 1999 observed a strong potential vorticity front, coincident with the offshore edge of the AUC, between depths of 400 and 1400 m.

The front is associated with the strong velocity shears above the Undercurrent, but primarily it is due to sharply inclined isopycnals and thus diminishing layer thicknesses.

The PV front is suggestive of a barrier between the cyclonic side of the Agulhas Current and water farther offshore, which impedes lateral mixing of water masses. Such a barrier perhaps explains why RSW and AAIW remain so distinct along the same isopycnal, despite strong horizontal shear.

However, the relation between water masses and the velocity and PV fields remains unclear without measurements up or down stream to examine the evolution of these fields.

Figure 3. Contoured section of potential vorticity (10^-10m^-1s^-1)
across the Agulhas Current. Contours of along-stream LADCP
velocity (cm/sec) are overlaid as thick lines and are
positive to the southwest. Dashed lines depict isotherms.