ANALYSIS

Evolution of velocity and potential vorticity fields

Potential vorticity is an important quantity in ocean dynamics because it combines two basic laws: the conservation of mass and the conservation of angular momentum. If dissipation and diabatic processes are insignificant, then PV is conserved following the motion of the fluid and can be used effectively as a tracer. In the Agulhas Undercurrent, where the flow is heavily topographically controlled and velocity shears are high, dissipation could be important. PV can be expressed in a natural co-ordinate system that reflects the geometry of the Agulhas Current. Using Ertel vorticity (Pedlosky, 1986), neglecting unimportant terms by scale analysis, expressing the vertical density gradient in terms of buoyancy frequency N, and the horizontal density gradient in terms of vertical shear, PV can be written,

The three components are planetary vorticity, relative vorticity from horizontal shear, and relative vorticity from vertical shear. The strength of simultaneous CTD/LADCP measurements is that these components can be calculated directly and without interpolation. Beal and Bryden (1999) have shown that the relative vorticity terms in the Agulhas Current are large at intermediate depths close to the slope and contribute significantly to the overall PV field.

By studying the evolving velocity and PV fields in the Undercurrent using the along-stream survey and the four cross-stream sections we will be able to assess the important forces driving and shaping the flow. In addition, we will explore the relation of the intermediate water masses, RSW and AAIW, to the interface of the two currents and to the region of high PV gradients.