Computed surface currents off the Cape of Good Hope

Deep-Sea Research, 1963 Vol. 10, pp. 623 to 632. Pergamon Press Ltd. Printed in Great Britain.

Computed surface currents off the cape of Good Hope M O L L I E DARBYSHIRE Department of Oceanography, University of Cape Town

(Received 12 June 1963)

Abstract--Data from six surveys of the region off the Cape of Good Hope and one in the Bcnguela Current have been used to calculatedynamic height anomalies and surface currents. The temperature and salinitydistributionsand the nature of the water masses are also discussed. Current velocities up to 70 cm/scc were found. The pattern of surface currentsvaried with each survey but a predominant featurein all was the large scaleanticycloniceddies off the Cape.

INTRODUCTION

THE South Atlantic and Antarctic oceans have been studied by a number of workers and their general characteristics are well known. The Indian ocean, however, has been much less adequately covered. Water from these three oceans contribute to the water masses found off the coasts of southern Africa. CLOWES(1950) has made a study of the hydrology of South African waters between the limits of 20 ° and 45°S, and 5°W and 45 °E using data from Discovery Investigations and the Meteor Expedition. Other workers, notably DEFANT (1935) and HART and CURR~ (1960), have made detailed studies of the Benguela Current, off the west coast of southern Africa. Much attention is being focused on the Indian ocean at the present time, during the International Indian Ocean Expedition. A detailed survey of the Agulhas Current system, off the cast coast of southern Africa, is now being prepared as part of this programme. The Benguela Current, flowing northwards, is charactcriscd by cool water upwelling near the coast, whereas the Agulhas Current flowing south and then southwest, in much the same latitudes, is associated with waters of considerably higher temperatures. The area off the Cape of Good Hope, considered here, is a transitional zone between these two current systems and a study of the water masses and currents of this area should provide a valuable link between them. Published data of temperature and salinity arc available in Annual Reports of the South African Division of Fisheries (1953, 1958, 1960) and from these the temperature distribution, temperature-salinity relationships, dynamic height anomalies at various depths, and surface current velocities can be found. Lines of stations have been worked at various seasons of the year and six sets of data have been used. The area covered and the position of the stations can be seen in Fig. 1. The 200 and 1000 m depth contours are marked to indicate the dimensions of the continental shelf. In addition, one set of data from the Benguela Current region given in the Division 623

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of Fisheries, Annual Report (1961) has also been studied and this area and the station positions are shown in Fig. 2.

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DISTRIBUTION

The distribution of surface temperature at different seasons is shown in Fig. 1. The most striking features are the upwelling of cool water near the coast, most marked in summer, and the movement westwards in summer of the tongue of warm water, over 24°C, which forms the western limit of the Agulhas Current system. The lowest temperatures recorded, about 12 °, occurred in summer along the west coast from Table Bay northwards. There is very little variation in surface temperature in winter as shown on the charts for June and September. HART and CtrRgIE (1960) apply the name "Benguela C u r r e n t " to the region of cool upwelled water between 15 ° and 34°S, within 100 sea-miles of the coast. In their paper they give two diagrams of surface temperature, one in March, the other in September. A similar chart for January 1959 shown here in Fig. 2, agrees well with the first of these. The September surface temperatures were a good deal lower. The Cape region thus includes the southern part of the Benguela system and, in summer, the extreme south-west of the Agulhas system. The upwelling often continues beyond 34°S, and 35°S would seem to be a better limit in general. This allows for upwelling east of Cape Point, particularly in summer, and to a lesser degree east of Cape Agulhas.

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At the deeper stations, temperature and salinity measurements were carried out to a depth of about 2000 metres. Vertical temperature sections along some of the lines of stations are shown in Fig. 3 (a). This shows that most of the upwelling occurs in the surface layer and that below this there is an almost horizontal stratification. DISTANCE

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At depths greater than 400 m temperatures are fairly uniform throughout the year over the whole area from 17°S to 38°S. More expanded sections of the surface layer along the same lines are shown in Fig. 3 (b). The amount of upwelling varies considerably and is greatest between 32 ° and 34°S. HART and Ctrggm give several vertical sections for latitudes between 20 ° and 29°S for both March and September and again there is good agreement with sections for similar latitudes presented here. 21°% J AN.

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Temperature-salinity curves for all stations studied here follow the same trend. There is little seasonal variation and only a slight change with latitude. T-S curves for four fairly representative lines of stations are shown in Fig. 4, different symbols

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being used to indicate depth classes. Water density is also shown. Several water masses may be identified. A linear relationship between 6 °, 34.4~oo and 16 °, 35.5%0, as found by CLOW~ (1950) is very marked. This water mass is generally known as the South Atlantic Central Water and in this area occupies the depths between about 100 and 700 m. Below this there exists a marked salinity minimum which is associated with the Antarctic Intermediate Water. Although the mean depth of this layer is between 700 and 900 m, there can be considerable variation from 300 m to over 1000 m. Charts showing the depth of the salinity minimum at different times are shown in Fig. 5. Where the sounding is less than about 1000 m this water mass cannot be identified. Below the Antarctic Intermediate Water is a layer of water of lower temperature and higher salinity, the North Atlantic Deep Water. This normally contains a salinity maximum but our observations do not go deep enough to include it. T-S curves in the surface and sub-surface layers show more scatter as might be expected. Below 16 ° the points lie on the straight line already mentioned. This is the upwelled water near the coast which, having come from the South Atlantic Central Water further from the shore, has the same T-S characteristics. The surface water of higher temperatures has a much slower rate of salinity increase. From 16 ° to 25°C at the meridian of 20°E in February 1957 the salinity is constant. This probably indicates a mixing in the surface layer between the South Atlantic Central Water and less saline sub-tropical water in the Agulhas Current.

DYNAMIC

HEIGHT

ANOMALIES

The dynamic height anomalies for all available depths have been calculated and 1000 m was selected as a reasonable level of no motion. The dynamic height differences between the surface and 1000 m have been plotted in Fig. 6. Extrapolation was necessary for the stations of depth less than 1000 m and the method of HELLANDHANSEN (1918) was used. The required difference was obtained from the adjacent station of greater depth in each case. The computed dynamic heights for shallow water stations must be treated with some reserve but they are useful in indicating the probable pattern near the shore. The contours have been drawn at intervals of 0.05 dynamic metres. The arrows indicate the direction of motion of the geostrophic current at the surface. The scale for conversion from contour spacing to current speed is given in the diagram. There is a good deal of variation in the current pattern at different seasons but there are some features in common. Inshore the currents are generally parallel with the coast in a north-westerly direction. Further out are a series of eddies, some anticyclonic and some cylonic. The anticyclonic are much the stronger, currents of 70 cm/sec or greater being quite common. On the north the current sets generally northwards in the main direction of flow of the Benguela Current. On the east is the western limit ot the Agulhas system in winter but there is not sufficient of this area shown to determine its characteristics. The eddies appear to be formed in the area of transition between the two current systems. The dynamic height anomalies with reference to the 1000 decibar surface decrease in a regular manner from the surface downwards. The currents at intermediate depths will be similar to the surface ones but of lesser strength. Another way of

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Fig. 7. Dynamic height anomalies at the surface with respect to the 1000 decibar surface inthe Benguela Current area. Contours are drawn at intervals of 0.05 dynamic metres.

showing currents at sub-surface depths was given in Fig. 5. This represents the depth of the Antarctic Intermediate Water or approximately the depth of the 27.25 at surface. This is nearly always above the selected level of no-motion. The direction of the current should follow the contours, being anticyclonic where the level is deep and cyclonic where it is shallow. A comparison between Figs. 5 and 6 shows this to be true. Both methods have their advantages. The dynamic height anomalies give the horizontal current velocities more accurately but show no indication of vertical movement whereas the contours of a particular at surface illustrate the three-dimensional motion of water of that density. Figure 7 shows the dynamic height anomalies at the surface in the Benguela Current area with reference to the 1000 decibar plane. The flow is generally from south to north with a number of irregularities, in good agreement with similar charts shown by HART and CURRIE. Current speeds of up to 25 cm/sec occur. Currents in these regions, although more regular in direction, are much weaker than those in the eddies off the Cape. CONCLUSIONS

Currents off the Cape of Good Hope form a series of eddies between the more stable regions of the Benguela and Agulhas Currents. The most marked are anticyclonic, the position of the centres varying considerably with time. Upwelling

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MOLLIEDARBYSHIRE

of the South Atlantic Central Water is the most important feature near the coast. The surface current system is closely associated with the vertical movement of the Antarctic Intermediate Water and from this level down to 2000 m there appears to be little motion.

Acknowledgement--The author wishes to thank the South African Council for Scientificand Industrial Research for funds provided for this research. REFERENCES

CLOWn, A. J. (1950) An introduction to the hydrology of South African waters. Fish and Mar. BioL Surv., Union orS. Africa, lnv. Rep. No. 12. D~ANT, A. (1936) Das Kaltwasserauftriebsgebiet vor der Ktiste Stidwesafrikas. Landerkdl. Forseh.; Festchr. N. Krebs., pp. 52,-66. HART, T. J. and CURRIE, R. I. (1960) The Benguela Current. Discovery Rep. 31, 123-298. HELLAND-HANSEN,B. (1918) Nogen hydrografiske metoder. Forh. skand, naturf. Mote, 16, 357-9. UNION OF S. AFRICA, DEPARTMENTOF COMMERCEAND INDUSTRIES,DIVISIONOF FISHERIES (1953, 1958, 1960, 1961) Annual Report.