CRUISE OUTLINE

Four cross-stream CTD/LADCP sections will (Figure 5) measure the evolving structure of the AUC (second objective). These will be perpendicular to topography and of high resolution, less than 5 km spacing over the slope, to capture the narrow slope current.

A short mooring array will be deployed in the AUC to measure the variability of its flow and accurately assess its mean transport (objective four). The array will be placed along the 32° S section, where its structure is known and the height and positions of the moorings can be carefully planned. The array will consist of three moorings of three instruments each, with a maximum height of 1000 m off the sea bed (Figure 6). The array duration will be up to two years, which will adequately cover the expected variability, which peaks at 30-40 days.

A hydrographic section parallel to topography, with 50km station spacing, linking the offshore ends of the four cross-stream sections will enclose the work area (Figure 5) and allow for a quantitative analysis of the WBC system. Through consideration of mass and geostrophic balance, it will enable the quantification of the heat and freshwater budgets associated with the Agulhas Current and the Undercurrent, therefore fulfilling our third scientific objective.

Finally, to fulfill the first objective, an investigation of the along-stream extent of the AUC is proposed as a string of CTD/LADCP stations along the South African continental slope, between the Natal Bight at 26° S and just north of the retroflection region at 37° S (Figure 5). This survey is designed under the assumption that, along a streamline the AUC is conserving potential vorticity, so that one may expect to find it at a constant depth along its path. In fact, over the latitude of the survey, this predicted depth will change by order 500 m, due to the change in Coriolis parameter, from approximately 2000 m at 37° S to 1500 m at 26° S. However, considering non-linear effects, such as lateral friction and entrainment, which may cause a sinking of the Undercurrent along its path, we propose that sampling along the 1800 m isobath is the optimum strategy for this initial survey. The already-completed cross-stream sections will help to anchor the along-stream survey to the observed core of the Undercurrent.

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.
Figure 6. The Agulhas Undercurrent Experiment mooring array at 32° S