Benguela Current Sources and Transports

Publised in U.S. WOCE Report, July 1995

Contributors: Silvia L. Garzoli, Arnold L. Gordon 1 and Christopher Duncombe Rae 1, 2

1. Lamont-Doherty Earth Observatory, Palisades, NY, USA) 2. Sea Fisheries Research Insitute, P. Bag X2, Rogge Bay, 8012 Cape Town, South Africa

The heat and salt fluxes of the Benguela Current are important components of the South Atlantic meridional fluxes, balancing to some extent the poleward movement of the warmer, saltier thermocline water within the Brazil Current. At 30S in the South Atlantic the heat flux is toward the equator (Saunders and King, in press) and forms part of the thermohaline ciculation assocated with North Atlantic Deep Water formation (Gordon, 1985). Water within the Benguela Current has several sources (Gordon et al. 1992): the South Atlantic Current (the southern limb of Circumpolar Current and the Agulhas Current (the western boundary current of the southern Indian Ocean). By combining geography and the large-scale wind field, the opportunity exists in the South Atlantic for linking with the western boundary of an adjacent ocean, which may help explain the anomalous meridional heat flux (Gordon, 1985). The primary means by which Agulhas water enters the Benguela Current is the shedding of large eddies at the Agulhas retroflection, though there also may be quasi-stationary branching of the Agulhas Current. While the excess heat of this water is quickly lost to the atmosphere (Olson et al., 1992), its salt content, a product of the Indian Ocean evaporation, may be important to the salinity (and other property budgets) of the South Atlantic and perhaps to the enter Atlantic basin.

The sources and transport of the Benguela Current were the focus of a cooperative 1992 1993 filed program called Benguela Sources and Transports (BEST) between U.S. (i.e. A. Gordon, S. Garzoli, D. Pillsbury) and South African (i.e., C. Duncombe Rae, F. Shillington) scientists. The main objectives of BEST were to measure the transport of the Benguela Current, monitor its variability, and determine its origins. Numerous CTD (conductivity, temperature, density) and expendable bathythermograph stations were occupied during three cruises between July 1992 and November 1993. An array of moored current meters and inverted echo sounders (IES) was deployed. The IESs were equipped with bottom pressure gauges and ambient noise detectors.

The patterns of volume transport in the area as obtained from the IES measurements is shown schematically in Figure 1. The mean (16-month) transport across 30S between the South African Coast and the Walvis ridge (2 to 3E) was 13 Sv. East of 5E and offshore of the coastal current, mean transport was mostly toward the north. Maximum northward values (20 Sv) were observed during June 1993. We call the shaded area the Agulhas Eddy Corridor because there the transient eddy fields stirs water masses contributed to the Beguela Current from the South Atlantic and Indian oceans.

The eddy variability is illustrated by the three-dimensional representation of the dynamic height across 30S obtained from the instruments moored along this parallel (Figure 2). In this figure, the eddy corridor is seen clearly in the western part of the section, less variabilty being evident to the east along the African continental slope. From the instruments moored southeast of Cape Town, it was found that four to six eddies per year entered the Cape Basin, most likely from the Agulhas retroflection (duncombe Rae et al., 1995). However, the Agulhas is not the only eddy source for the region. During the hydrographic surveys, two eddies originating in the Brazil Current were encountered and measured, in addition to the five Agulhas eddies examined. A comparison of th vertical temperature and salinity profiles fo these two types of eddy is shown in Figure 3.


Duncombe Rae, C., S.L. Garzoli & A.L. Gordon (1995) The eddy field of the south-east Atlantic Ocean: a statistical census from the BEST Project (submitted JGR)

Garzoli, S.L. & A.L. Gordon, (1995) Origin and variability of the Benguela Current (submitted JGR)

Gordon, A.L. (1985) Indian-Atlantic Transfer of Thermocline Water at Agulhas Retroflection, Science, 227(4690): 1030-1033.

Gordon, A.L., Weiss, R. F., W. M. Smethie, Jr. and M. J. Warner (1992) Thermocline and intermediate water communication between the South Atlantic and Indian Oceans. J. Geophys. Res. 97(C5): 7223-7240.

Olson, D. B., R. A. Fine and A. L. Gordon (1992) Convective Modifications of Water Masses in the Agulhas. Deep-Sea Research, 39(Suppl. 1A): S163-S181.

Saunders, P.M. and B.A. King. Oceanic fluxes on the WOCE A11 Section, J. Phys. Oceanogr., in press.

Stramma, L. and R.G. Peterson (1990) The South Atlantic Current. J. Phys. Oceanogr. 20(6): 846-859.

Captions for the Figures

Fig. 1. Time average from June 1992 to December 13, 1992 of the transport obtained between station pairs overlapped to a schematic of the circulation in the area (Adapted from Garzoli and Gordon, 1995).

Fig. 2. Time latitude variability of the dynamic height along 30S.

Fig. 3. Comparison between the characteristics (temperature and salinity of a Brazil and Algulhas eddy. The Brazil eddy shown in this picture was observed during the BEST 3 cruise (Adapted from Duncombe Rae, Garzoli and Gordon, 1995).