CRUISE PLAN

Cross-stream sections

It is vital to have extremely close station spacing over the continental slope, in order to properly resolve the cyclonic side of the WBC and the Undercurrent. For each cross-stream section, we occupy six stations over the slope, no matter what its width, and then space the remaining ten stations with increasingly more distance as the line progresses offshore. This sampling strategy follows the observation that the eddy scale of the flow increases with distance from the coast. Webb (1999) has shown that the velocity structure of the Agulhas Current close to the surface is well described as a WBC with constant viscosity on the cyclonic edge, and with viscosity linearly increasing away from the boundary on the anticyclonic side of the current. Thus, we have chosen constant, close station spacing on the cyclonic side of the current over the slope, and linearly varying spacing on the offshore, anticyclonic side of the current. The close spacing over the slope is, of course, optimum for high sampling of the Undercurrent. Each section will be terminated beyond the offshore edge of the Agulhas Current. This will provide full coverage of the western boundary current system and thus enable calculations of water mass entrainment, heat and salt fluxes in addition to an assessment of the evolving structure of both the Undercurrent and the main current, fulfilling our second and third scientific objectives.

An offshore section, joining the ends of the cross-stream sections, will close the survey area into three boxes. Station spacing on this offshore section can be increased, although we expect this to be an energetic eddy field with along-stream scales smaller than in the core of the WBC jet. Stations every 50 km are proposed and will be completed in between the cross-stream sections to reduce steaming time and increase synopticity. This section closes off the experiment area, allowing for absolute calculations of the fluxes.
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.