CRUISE PLAN

Along-stream survey

An along-stream survey will establish the presence of equatorward flow of intermediate and deep waters over the continental slope along the length of the WBC path (first objective). Station spacing of 100 km is adequate for along-stream resolution, where horizontal velocity shear is order 10 times smaller than in the cross-stream flow. Sixteen stations will be occupied between 26 and 37° along the 1800 m isobath. This is not expected to sample the full depth of the AUC, but to sample close to the velocity maxima, assuming its path to be strongly topographically controlled. From the four detailed cross-sections, we can test whether potential vorticity in a layer corresponding to the Undercurrent is a constant (and hopefully monotonic) function of streamfunction. If it is, the along-stream stations will also be used to define the potential vorticity in the Undercurrent layer at each particular location, and thus to define the trajectory of the Undercurrent more extensively along the continental slope. This method is similar to Gill's (1977), who determined the potential vorticity as a function of streamfunction for a particular layer of the Agulhas Current. From two sections, Gill showed the function was similar for the 16° C to 20° C layer and hence the trajectory of this water layer could be followed alongshore. The along-stream survey will begin at 26° S, after completion of the northern-most cross-stream section, since it should be conducted drifting southwestward within the surface jet of the Agulhas Current. Data from the cross-stream sections will allow anchoring of the survey to the core of the Undercurrent at four latitudes. Time allowing, sampling will continue around the tip of Africa into Cape Town, where a slope current was observed in the Atlantic, and the experiment will terminate there.
Figure 5. Bathymetric map with proposed initial along-stream stations (dots) and eddy-resolving sections of stations (lines). Bathymetry is shaded in steps of 1000 m.