PROJECT SUMMARY

It is rare to identify a new ocean current. In March 1995, as part of the Agulhas Current Experiment aboard RRS Discovery at 32° S, the first observations of the deep velocity structure of the Agulhas Current were made using a Lowered Acoustic Doppler Current Profiler (LADCP). During the experiment a deep counter-current was measured beneath the Agulhas Current, a feature not previously observed and now known as the Agulhas Undercurrent Beal and Bryden 1997. The Undercurrent was strong (30 cm/sec) and of sufficient size to reduce transport estimates of the Agulhas Current by more than 12 Sv, profoundly affecting the derived heat budget of the Indian Ocean. Since then, the Undercurrent has only been measured at 32° S and just one current meter record has captured it, showing it to be a persistent feature of the circulation there. Thus, little is known about the Agulhas Undercurrent as yet, only that it is a real and long-lived feature of the circulation and one worthy of research.

The primary reason that the Agulhas Undercurrent is of interest to oceanographers and climatologists is its potential role in the global thermohaline circulation. At 32° S it carries North Atlantic Deep Water (NADW) and Antarctic Intermediate Water (AAIW) quickly northward. It is possible that there is a continuous current facilitating rapid inter-ocean exchange of NADW around the African Cape, since a slope current has also been observed off Cape Town. Even more significant is the recent observation of a northward current of AAIW and NADW in the Mozambique Channel, strongly indicating that the western boundary is indeed an important region for ventilation of the Indian Ocean. But without measurements, we can only speculate the Undercurrent's role. In addition to fundamental questions of its global importance, there are many specific questions about its dynamics to be addressed. With observations of undercurrents now made in the Gulf Stream, the Brazil, East Australia, Mozambique, Somali and Agulhas Currents it seems likely that they are ubiquitous to western boundary current systems. What is the mechanism for this? How similar is the Agulhas Undercurrent to these other bottom trapped flows? Does it entrain waters, rise or deepen as it flows northward? Does its transport vary over time? Is its speed related to the steepness of the slope, and is it always attached to the slope? Does the Undercurrent conserve potential vorticity along its path? What happens to the north as the continental shelf widens and the Natal Valley shallows?

Shipboard hydrographic work is proposed off the east coast of South Africa, between 26° and 37° S, in order to address all these questions. The most pressing question is that of the Undercurrent's role in ventilating the Indian Ocean, in other words as a component of the global thermohaline circulation. This will be answered by conducting a combination of alongslope and cross-stream sections. The along-slope survey will follow the path of the Undercurrent, assuming it to be strongly topographically controlled, with a series of combined Conductivity-Temperature-Depth-Oxygen (CTDO2) and LADCP stations at 100km intervals. Thus, it will define the latitudinal extent of the Undercurrent and its role in transporting deep and intermediate waters northward. Four high resolution, cross-stream sections, will address all the questions above concerned with the structure and evolution of the Undercurrent. An offshore section, closing the survey region into three boxes, will allow the calculation of entrainment, heat and freshwater budgets for the entire WBC system, quantifying the thermohaline fluxes due to both the Agulhas Current and its Undercurrent. Finally the variability and long-term mean transport of the Undercurrent, which cannot be assessed from one-time surveys, will be monitored through the deployment of a moored current meter array.