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Purpose of ACSEX-I cruise

- The purpose of the cruise is 1) to obtain hydrographic observations of the Mozambique current between Madagascar and Mozambique and 2) to obtain observations on the magnitude of the horizontal currents in the Mozambique current by deploying an array of moorings on a section across the Mozambique channel, roughly at its shallowest width. These moorings will be recovered in February - March 2001. During the cruise some short-term (one week) moorings will be deployed on the same cross-section.

General description of ACSEX-I cruise

- The climatologically important interbasin leakage of Indian Ocean water into the South Atlantic is thought to be largely controlled by the inflows into the Agulhas Current on its upstream edge. In the far field this inflow is fed by two sources, the Mozambique Current and the East Madagascar Current. Both branches exceed a high variability, as observed by satellite altimetry. In spite of their global significance surprisingly little observations have been taken of the currents and hydrographic structure in these source regions. As part of the `Pelagia around Africa' programme the Pelagia will be in the area in March-April 2000, affording a special opportunity to carry out an Agulhas Current Sources Experiment (ACSEX). The main observational aim of ACSEX is to determine as much as possible of the strength, variability and hydrographic structure of one of the two sources of the Agulhas: the Mozambique Current in the sea area between Mozambique and Madagascar. It involves a detailed hydrographic survey to determine whether the Mozambique Current is a continuous current or whether it exists merely of a train of eddies, as the altimetric data suggest. In addition an array of current meter moorings will be placed across the narrows of the Mozambique Channel to monitor the transport and its variability over the subsequent year (2000-2001).

The observational methods to be applied during the hydrographic survey will be the lowering of a CTD in order to measure temperature and salinity profiles. Attached to the CTD frame is a rosette sampler which will be used to obtain samples for the determination of nutrients and sea salt. A ship-mounted ADCP will be used to determine the surface (0-500m) velocity structure. The preliminary design for the moored array consists of the deployment of 2 bottom mounted upward-looking ADCP's on either side of the Mozambique Channel at the 600 m isobath and of 5 conventional current meter moorings, to be deployed more or less evenly distruted between both ADCP moorings. Each current meter mooring has 4 current meters. The exact position of the current meters will be determined during the cruise when part of the hydrographic data have been analyzed.

The data, obtained during the cruise, will also be used for implementation in numerical models of the circulation around Africa. The results of these models will be used in a later stage for the evaluation of large scale general ocean circulation models.
The intended research is part of the Dutch contribution to the international CLIVAR programme. The Netherlands Institute of Sea Research and Utrecht University as well as the Department of Oceanography from the University of Cape town from South Africa participate in this study.

Details of moored equipment

- The array of moorings will be deployed on a transect across a narrow reach of the Mozambique Channel, roughly between 16�30'S-40�E on the Mozambique side and 17�30'S-43�20'E on the Madagscar side (see figure). On the 600 m isobath at both sides of the Channel a bottom-mounted upward looking Acoustic Doppler Current Profiler will be deployed. On a transect in between both moorings we intend to deploy 5 conventional current meter moorings, more or less evenly distributed across the Channel. Each of these moorings consists of 4 current meters. The vertical position of the current meters is at 250 m, 500 m and 750 m and 1000 m from the surface. In water depths greater than 1500 m one additional current meter will be deployed at 250 m above the bottom. The exact position of these moorings will be determined during the cruise.

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