Satellite infra-red images in the Agulhas Current System

Deep-SeaResearch,Vol 25, lap 543 to 548 © PergamonPressLtd 1978 Printedm Great Britain

0011-7471/78/0601-0543 ~0200/0

NOTE

Satelfite infra-red images in the Agulhas Current System T F W.

HARRIS,*

R

LEGECKISt

and D VAN FOREST*

(Recewed 14 July 1977, in rewsedJorm 15 November 1977, accepted 25 November 1977) Abstract--Infra-red images from the satellites, NOAA 4 and 5 show that surface temperature differences m the Agulhas Current System are sufficiently contrasting to enable meso-scale features to be identified when cloud cover is absent Those revealed by images made m 1976 to 1977 include sources of the current, time dependent wave-hke structures, sharp current retroflexlons, and the contribution of the Agulhas Current to the South Atlantic The techmque allows the varying carculatlon modes to be studied

INTRODUCTION

IN 1976 to 1977 recordings were made over the southwest Indian Ocean (and part of the southeastern Atlantic) by the 1 r. sensor aboard the satelhtes N O A A 4 and 5 The recordings, made possible by the cooperation of National Oceamc and Atmospheric AdministrationNational Environmental Satelhte Service (NOAA/NESS), were started as an experiment to see if sea surface temperature contrasts were sufficient to allow features of the current to be identified When it was found that th~s was indeed so and that structures of considerable interest were revealed, the experiment was continued. In this paper the more immediate results of the study are reported and a selection of the images reproduced, including those relating to the sources of the Agulhas Current, deflections, retroflexions, and wave-hke features in the course of its passage along the South African coast and also m the Agulhas Return Current The locations of the features discussed have been sketched m Fig. 1 THE DATA

The Very High Resolution Radiometer (VHRR) aboard N O A A ' s polar orbiting satelhte (SCHwALB, 1972) was used to collect the thermal 1 r (10.5 to 12.5#m) and the visible (0 6 to 0 7 #m) data with a spatial resolution of 1 km The data were recorded on the satelhte during a pass over the Agulhas Current and were later transmitted to a receiving station In the United States Because of the hmited onboard recording capability as well as N O A A ' s obligations to obtain satelhte data at other geographic locations, the data were only collected at intervals of several days. F r o m July 7, 1976 to April 16, 1977 a total of 67 pairs of i.r. and vistble ~mages were obtained Approximately 25~/o of the Lr. images were sufficiently cloud free to reveal the sea surface temperature expression of the Agulhas Current System. Selected images were retained on dtgital tapes to allow further data manipulation and the images in this report * Department of Oceanography, University of Cape Town, Rondebosch 7700, South Africa t Environmental Soences Group, NOAA/NESS, Washington D C, 20233, U S A

543

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(unless otherwise indicated) have been geometrically corrected for the Earth's curvature (LEGECKIS and PRITCHARD, 1976) and have been computer enhanced for the range of sea surface temperatures. In all the images the higher temperatures are represented by the darker end of the grey scale IN T H E S O U R C E A R E A

According to classical concepts, a branch of the South Equatorial Current (the East Madagascar Current) passes south of Madagascar to form a tributary to the Agulhas Current From volume flux budget analyses, DUNCAN (1970) and HARRIS (1972) deduced that a substantial contribution from this tributary was required. On the other hand, LUTJEHARMS (1976) could find no evidence of there being the required surface flow m the regaon in the summer months. BANG, DUNCAN and LUTJEHARMS (m preparation) from bathythermograph data, concluded that there was an east to west surface flow ~mmedmtely south of Madagascar. An i_r linage obtained on 10 July is reproduced an Fig. 2. In ~t there is a band of warmer water (darker shade) originating on the east side of Madagascar, meandering poleward just to the south of Madagascar to about 28~S, before curving northwestwards towards the centre of the Mozambique Channel, and finally recurwng to the southwest. Also visible on the western side of the Mozambique Channel is an element of the Mozambique Current passing close to Cape Correntes. IN T H E M A I N C U R R E N T

The behaviour of the Agulhas Current after it has passed East London has not been mtensively studied, but known features are the cold dynamically upwelled water often found along the coast downcurrent of East London (BANG, 1970), the offshore deflection of water m the region of Port Elizabeth reported by HARRIS and BANG (1974) and evident in ship's drift data (British Admiralty Charts), and the retroflectlon of the major

Fig 2 Infra-red Image of 10 July showmg the East Madagascar Current (the dark shade 1s the warmer) rounding the southern tip of Madagascar The Mozambique Current appears on the west side of the Mozambique Channel (Images not rechfied or enhanced-tape not avallable)

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nnage of 27 July 1976 showmg oRshore deflection of the Agulhas Current Fig 3 Infra-red between East London and Port Ehzabeth (top) and subsequent movement of the features downcurrent, 4 August (bottom)

Fig 4(a) Elizabeth apparent

Infra-red image of A wave form m the To the north of the east, and

30 November 1976 showmg the Agulhas Current passmg close to Port current and the remams of a zonal wave feature (lower oentre) are also wave feature IS a probable tributary to the Agulhas Current from the associated with it a cold core eddy (right oentre)

Fig 4(b) Infra-red Image of 11 December 1976 showmg a wave form m the current, slmllar to that of 4(a) but displaced to the south Also shown 1s a branch of the Agulhas Current passmg mto the South Atlantic, and the mltlal stages of upwelhng along the Cape Coast (both absent on 30 November)

Fig 6 Infra-red image of 9 February 1977 showmg the more westward mode of Agulhas Current retroflenon, An mtruslon of cold water from the south may be cuttmg off the warm mass to the west Well-developed upwelhng 1s apparent along the Cape Coast

Fig 7 Infra-red image of 4 November 1976 showmg the Agulhas Current retroflerlon and a topographic planetary wave feature (lower centre) The southern tip of Africa 1s at the extreme left centre

545

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Sketches of waves in the current on 30 November and 11 December.

part of the Agulhas Current back into tile Indian Ocean (DIETRICH, 1935). The i.r. images are helpful in furthering our understanding of these features. Figure 3 shows an occasion in July to August when a marked offshore deflection was apparent. This deflection was first recorded as starting near East London on 27 July, and by 4 August the point of departure from the coast had moved towards Port Elizabeth, a distance of some 100 km, i.e. the feature had moved at a mean speed of 20 cm s- I. In the subsequent images in November the feature was not present, e.g. on November 30 nor on 11 December (see Fig. 4a and b). Figure 4(a) and (b) show a possible wave-like teature in the Agulhas Current. Figure 5 contains sketches of the course of the current on these two occasions. The wave appears to have travelled with a phase velocity of about 23 cm s- 1 which compares with an average phase velocity of 39 cm s-1 found in the Gulf Stream (LEGECKIS, 1975). Wave lengths were 300 and 330 + 30 km. Figure 4(b) also indicates a branch of Agulhas Current water flowing into the South Atlantic and the first stages of upwelling along the Cape Peninsula and close inshore along the coast north of Cape Town, where departures of the upwelled water at coastal discontinuities are evident. It is notable that during November and the first week of December (1976) the seasonal southeasterly winds were suppressed by an abnormally frequent succession of fronts in the westerlies. This would account for the absence of cold upwelled water in Fig. 4(a). The cold water, in the first stages of upwelling in Fig. 4(bK

546

T . F . W . HARRIS, R. LEGECKISand D. VAN FOREEST

appeared after the southeasterly winds started again on December 6. Because the flow of the Agulhas Current into the South Atlantic also only appears in Fig. 4(b), it may also depend on the stress of the southeasterly winds. RETROFLECTION

One of the remarkable characteristics of the Agulhas Current is its retroflexion. The positions at which this 'turnaround' occurred could be discerned in a number of images, and the longitude of it is set out in Table 1. Two figures are given for most days--the Table 1. Longitude of the retroflexion of the surface waters of the Agulhas Current. Date

Longitude (~E) Relic current

Longitude (~E) current

4.11.76 6.11.76 17.11,76 19.11,76 21.11.76 24.11,76 29.11,76 30.11,76 2.12,76 11.12.76 8.1.77 9.2.77 25.2.77 15.3.77

-13 -13 13 14 13 13 13 -12 14 12 --

19 19 20 19 18 20 18 20 19 21 19 18 20 17

one labelled 'current' is the westward limit of the warmest water, and the 'relic current' is the limit of the less warm water whose temperature was clearly distinguishable from the colder water to the west. The mean longitude for the 'current' is 19°E_+ 1.5 °, and that of the 'relic current' is 13°E _+ 1°. The latter is consistent with the terminus of the trajectory of a satellite drifter reported by GRUNDLINGH (in preparation). Figure 6 shows the two modes of retroflexion. A warm mass from the more westerly penetration appears here to be in the process of being isolated by an intrusion of colder sub-antarctic water, THE R E T U R N A G U L H A S C U R R E N T

The month of November, when the bulk of the images were obtained, was notable for the number of fronts in the westerly winds. Consequently, cloud covered much of the Agulhas Return Current on most occasions. However, a gap on 4 November revealed an interesting wave form (Fig. 7). After retroflexion the current moves equatorward and then poleward, delineating an intrusion of cold water. The procedure was repeated downstream to the east at least once--and, from the pattern in the clouds, perhaps twice. The first cold intrusion was over the Agulhas Plateau, a large feature rising about 2000 m above the surrounding deeps and about 2500m deep at its shallowest. The wave-like feature is probably a topographic Rossby wave occasioned by the current encountering the plateau. Its possible existence on hydrographical and theoretical grounds was noted by HARRIS (1970) and DARBYSHIRE(1972), and subsequently GRUNDLINGH (in preparation) reported that the trajectories of a satellite tracked drifter were consistent with it. On the 30th

547

Satellite infra-red images in the Agulhas Current System 20"E

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the tops of the cold water intrusions slewed westward, suggesting a breakup of the wave form into warm core vortices. Sketches of the wave form on several occasions are shown in Fig. 8. It appears that there is sometimes an eastward return current passing south of the Agulhas Plateau. Notable in Fig. 4(a) is the warm curving filament just to the north, which could be a manifestation of a tributary from the east, joining the Agulhas Current south of Durban. What appears to be a cold core cyclonic vortex is also delineated. DISCUSSION

In the account given above it has been tacitly assumed that the i.r. radiation, which has its origin in the surface skin of the sea, is representative of a substantial subsurface water body. Some measure of justification for this assumption comes from two sources. PEARCE (in preparation) concluded from studies of the Agulhas Current off Durban that the warmer surface water in the current is associated with the higher velocities. HARRIS and VAN FOREEST (1978) have demonstrated the vertical coherence of both the Agulhas Current and the Agulhas Return Current. If the assumption is accepted, the following conclusions are drawn for the occasions sampled: (a) There can be a flow of surface water from the South Equatorial Current westward around the southern tip of Madagascar. (b) Ephemeral, large offshore deflections occur in the Agulhas Current south of East London, associated with which is the presence of cold water between the deflecting current and the coast. The deflections move downstream with a phase velocity of about

548

T . F . W . HARRIS, R. LEGECKISand D. VAN FOREEST

20cms-X. Wave forms in the current have similar velocities and directions of propagation. (c) The Agulhas Current appears to have two modes of retroflexion, one at about 14E and one (perhaps more commonly) further to the east, at about 20°E (the longitude of Cape Agulhas). (d) Apart from the more westward retroflexion mode in (c), there is evidence that a branch of the Agulhas Current penetrates into the South Atlantic close to the continent. This was absent during November and early December 1976--when the summer southeasterly winds were abnormally suppressed--but appeared when the southeasterly winds resumed. (e) Planetary waves, probably induced by the presence of the Agulhas Plateau, occur in the Agulhas Return Current. In one instance the wave form appeared to be disintegrating 16 days after it was first recorded. (f) An alternate route for the Agulhas Return Current can apparently be to the south of the Agulhas Plateau. (g) In the initial phases of the well-known upwelling off the Cape coast, the jet of cold upwelled water moving under the influence of the southeasterly winds does not adhere to the coast at coast discontinuities. As a general conclusion it seems that a study of the mesoscale features of the current system and their time-dependent regimes is likely to be beyond the capabilities of classical ship oceanography alone, and that satellite i.r. measurements can be of great value in such investigations. Aeknowled.qements The original i.r. images were supplied by the Environmental Products Division of NOAA/ NESS in the United States. The cooperation of Mr. RUSSl~LLKOVVLERin arranging for the satellite recorders to be switched on is gratefully acknowledged. REFERENCES BANG N. D. (1970) Dynamic interpretations of a detailed surface temperature chart of the Agulhas Current retroflexion and fragmentation area in March 1969. South African Geographical Journal, 52, 67-76. BANG N. D., C. P. DUNCAN and J. R. E. LU'rJFHARMS. Use of controlled XBT technique to obtain first detailed geostrophic specifications of currents east and south of Madagascar. In preparation. DARBVSHIRE J. (1972) The effect of bottom topography on the Agulhas Current. Pure and Applied Geophysics, 101, 1972/IX, 208-220. DJETRICH G. (1935) Au~au und Dynamik des sudlichen Agulhasstrom-gebietes. Ver~!ffentlichun.qendes ln.stitut.s .fur Meereskunde an dem Universitiit Berlin, A27, 1-79. DUNCAN C. P. (1970) The Agulhas Current. Doctoral Dissertation, University of Hawaii, 76 pp. GRUNDLINGH M. L. Drift of a satellite tracked buoy in the southern Agulhas Current and Agulhas Return Current. In preparation. HARRIST. F. W. (1970) Planetary type waves in the south west Indian Ocean. Nature, 227(5262), 1043 1044. HARRIST. F. W. (1972) Sources of the Agulhas Current in the spring of 1964. Deep-Sea Research, 19, 633-650. HARRIS T. F. W. and BANG N. D. (1974) Topographic Rossby waves in the Agulhas Current. South AJrican Journal of Science, 70, 212-214. HARRtST. F. W. and D. VAN FOREEST(1978) The Agulhas Current in March 1969. Deep-Sea Research, 25, 549- 561. LEGECKIS R. and J. PRITCHARD(1976) Algorithm for correcting the VHRR imagery for geometric distortions. NOAA Technical Memorandum NESS 77. U.S. Department of Commerce. LEGECKIS R. (1975) Application of synchronous meteorological satellite data to the study of time dependent sea surface temperature changes along the boundary of the Gulf Stream. Geophysical Research Letters, 2, 435-438. LUTJEHARMS J. R. E. (1976) The Agulhas Current during the north-east Monsoon. Journal of Phi,sical Oceanography, 6, 655-670. PEARCI~A. F. Some features of the upper 500 m of the Agulhas Current. In preparation. SCHWALB A. (1972) Modified version of the Improved TIROS Operational Satellite. NOAA Technical Memorandum NESS 35. U.S. Department of Commerce.