Benguela Source and Transport (BEST)

by S.L. Garzoli, A.L. Gordon and D. Pillsbury
Published in WOCE Notes, Vo. 6, No. 1, pp. 10-11,15 1994

I. Introduction and Objectives:

Wind-driven ocean circulation models, as well as field observations reveal significant transfer of Indian Ocean Central (thermocline) Water into the South Atlantic Ocean. The transfer is accomplished by an eddy shedding process at the western end of the Agulhas Retroflection and by intermittent streams or plumes of Indian Ocean water injected into the Benguela Current. The heat content, salt content and rotational energy of the transferred water influence the state of the Atlantic Ocean. Perhaps the most profound effect is that the Atlantic's salinity is increased by drawing salty water from the evaporative Indian Ocean. A saltier Atlantic may be linked with increased formation of North Atlantic Deep Water and the associated vigor of the climatic important global thermohaline circulation.
The BEST (Benguela Source and Transport) project was designed to investigate the form of the Benguela Current and ratio of the Agulhas Current and South Atlantic Current source waters masses. The transport of the Benguela Current and its variability is measured by a moored array of instruments as well as analysis of the TOPEX/POSEIDON satellite altimeter and CTD density field observations. The source of the water is determined by analysis of the water mass properties measured from CTD observations.
To accomplish the BEST objectives, an extensive field program was carried out between June 1992 and November 1993. The field work consisted of three hydrographic surveys and a moored array of inverted echo sounders (IES), inverted echo sounders with pressure gauges (PIES) and current meter moorings (CMM). The BEST mooring array was deployed in June 1992 and, during October/November 1993, 4 PIES and 4 current meter moorings (CMM) along 30S, and 2 IES at the edges of the line Cape Town 37.5S ,12.3E were recovered. The location of the moorings as well as the CTD and XBT stations obtained during the three BEST cruises are shown in Figure 1.
The BEST objectives contribute to the WOCE international plan. The 30S array will provide the transport information of the WOCE ACM-4 mooring , as called for in the WOCE science plan, 1988. Paired with ACM-3 in the Brazil Current (part of the German WOCE program) the BEST 30S mooring array will aid in accomplishing the WOCE South Atlantic heat flux line objectives. The IES deployed off Cape Town will help to relate the Algulhas Retroflection characteristics to the transport across 30S. The main hydrographic work in the Agulhas Retroflection region and in the intervening region will define the water mass characteristics of the Benguela Current and assess the relative contributions of South Atlantic and Indian Ocean water to the Benguela Current. In addition, BEST has mutually beneficial interaction with other WOCE South Atlantic subtropical activities: the German program of monitoring the poleward flux of the Brazil Current (WOCE mooring ACM-3), the South Atlantic drifter program, occupying a hydrographic line (WOCE WHP A-10) along 30S; the UK WOCE program of WHP A-11 (40S) and work south of Africa; the French WOCE program of WHP A - 13, A-14; the South African WOCE study of the Agulhas Retroflection and the WOCE Deep Basin Experiment. The DBE will provide detailed information on the Brazil Current and its re-circulation pattern. The relationship of the deep flow and of the Brazil Current to variations in the eastern subtropical South Atlantic can be inspected by comparison of the BEST data to the DBE.

II. The BEST cruises

Three BEST cruises have been completed:

- BEST 1 aboard the R/V Africana Voyage 105; Cape Town to Cape Town, June 16 to July 5, 1992. G. Bailey (SFRI, South Africa) and S. Garzoli (LDEO,U.S.), Chief Scientists

The main objective of the BEST 1 cruise was to deploy the BEST moored array and to obtain the first realization of the hydrographic conditions along the line of deployments for further calibration and interpretation of the time series to be obtained. The BEST moored array was deployed at the positions given in Figure 1 . Twenty nine hydrographic stations were obtained. During the transit between the northern leg of deployments and the southern leg, an intensive XBT survey was carried out to search for eddies (Shillington and Duncombe Rae, South Africa). A ring was detected at the mid point of the southern leg. CTD and XBT measurements were obtained to define the characteristics of the ring. The horizontal extent of the ring was 300 km (E-W) x 260 km (N-S). The temperature anomaly of the center with respect to the edge was 4C at 500m. The ring showed an isothermal surface layer of 16.45C extending to 280 m depth (Duncombe Rae, 1993, personal communication).

- BEST 2, aboard the R.R.S. Discovery, Voyage 202; Cape Town to Cape Town, 7 May to 3 June 1993. A. L. Gordon, Chief Scientist (LDEO, U.S.).

This cruise is the primary CTD component of the BEST project. Figure 1 shows the CTD and XBT station array. High quality ADCP data was obtained along the ship track with excellent navigation control with GPS. The two basic objectives of the hydrographic work were to: - Obtain CTD stations over the BEST moorings. These CTD stations provide a "snap-shot" view of the thermohaline and oxygen stratification to help interpret the moored array time series by exhibiting how specific features are manifested in the PIES and current meter records; and - Survey Agulhas eddies and their surrounding waters to better define the distribution of Indian Ocean (Agulhas Current and Retroflection) and South Atlantic (South Atlantic Current) water within the Benguela Current.

The ship's RD Instruments' ADCP (Acoustic Doppler Current Profiler) whose data are logged with GPS (Global Positioning System) and gyro readings, provided a valuable view of the currents of the upper 200 meters to as deep as 400 meters, depending on sea conditions. An Agulhas Eddy Survey (CTD Stations 33-58 and XBT grid) was carried out to observe the thermohaline and dynamic form of the resident eddies. The long NW-SE line of XBTs is along a TOPEX/POSEIDON satellite altimeter line. This will allow for direct comparison of the thermal and ADCP data with sea level as detected by the satellite. A transect between the southern IES mooring was undertaken to support the moored instrument array and to inspect for streams of South Atlantic Current and Agulhas Current directed into the Benguela Current, and to investigate their relationship to the eddy field.

Preliminary analysis of the May 1993 CTD survey of DISCOVERY cruise 202 revealed a complex array of eddies with varied scales, ages and sources. A possible Brazil Current eddy was surveyed near the Walvis Ridge at 34S. Three Agulhas eddies were explored: a winter-altered eddy between 30 and 33S near 6E; one eddy which had not yet experienced a winter between 32 and 34S near 9E; and a vigorous newly formed eddy centered near 37S and 15E. The Agulhas eddies are carriers of Indian Ocean water, but they also determine the ratio of Atlantic to Indian contribution to the Benguela Current as they direct streams of regional current systems (South Atlantic Current and Agulhas Current) into the Benguela Current.

- BEST 3, aboard the R/V Maurice Ewing Voyage 9308; Cape Town to Cape Town, October 24 to November 11, 1993. Silvia .L. Garzoli, Chief Scientist (LDEO. U.S.).

The objective of this cruise was to recover the moorings deployed during BEST 1 and to obtain a CTD station at the location of the deployment for further calibration of the records. The hydrographic work consisted of a CTD cast and a rosette cast at each mooring site as well as salinity samples and nutrient samples for analysis ashore (C. Duncombe Rae, SFRI, South Africa). XBTs were launched between the moorings and also in transit between the mooring lines. An eddy was located centered at approximately 36 20'S 10 30'E. It was sampled with XBT probes and CTD stations were obtained at the edges, middle and center. Simultaneously and during the survey, ADCP data collected. The eddy has a thermostat of 13 to 14C and salinity that suggest the possibility of being a Brazil Current eddy.

III. Future Analysis

The main tasks now are to jointly analyze and synthesize the data collected during the BEST Program. In addition to the observations made from the ship and the time-series collected by the BEST mooring array (CMM, IES/PIES, CTD/XBT surveys and ADCP), the data from the TOPEX/POSEIDON altimeter and NOAA infrared satellite images of sea surface temperature will be incorporated to the analysis.

CTD, XBT and ADCP measurements allow us to describe the horizontal and vertical scale of the eddies which can then be related to the time series mooring and satellite data. Comparison of BEST data with that previously collected will illuminate differences and similarities between the three Agulhas eddies surveyed and Agulhas eddies of previous years, providing clues to variability of the eddy field.