A description of inshore current reversals off Richards Bay based on airborne radiation thermometry

Deep-Sea Research, 1974,Vol. 21, pp. 47 to 55. PergamonPress.Printedin Great Britain.

A description of inshore current reversals off Richards Bay based on airborne radiation thermometry M. L. GRUNDLINGH* (Received 16 August 1972; in revisedform 27 March 1973; accepted7 August 1973) Abstract

The infra-red radiation thermometry done over the coastal waters off Richards Bay is

described. A model concerning the local current behaviour is derived from these results as well as measurements from a research vessel. It is postulated that the atmospheric forcing of the Agulhas Current at nodal points along the South African East Coast has direct bearing on the creation of inshore turbulence and reversals regularly experienced along this coast. INTRODUCTION

THE OCEANOGRAPHYDivision of the National Physical Research Laboratory (Council for Scientific and Industrial Research) has been using an infra-red radiation thermometer (or radiometer) since June 1965, in the determination of sea surface temperatures off the East Coast of South Africa. The radiometer is used in conjunction with the investigation into currents and water properties carried out from the C.S.I.R.R.V. Meiring Naud~. The Barnes Radiation Thermometer, model PRT 14-313, employs a thermistor bolometer and lens/filter assembly providing a spectral passband from 8 to 14/~m. The factors affecting the registered temperature in this region are mainly water vapour and other gases between the sensor and the target, and radiation reflected by the sea surface. These subjects have, however, been dealt with quite fully in the literature, both theoretically (e.g. SAUNDERS and WILKrNS, 1966; SAUNDERS, 1967, 1970; KIMORA and MISAWA, 1970; LORENZ, 1970; and others) and empirically (RICHARDSONand WILKINS, 1958 ; PICKETT, 1966 ; and others). The corrections are, to a large extent, only necessary when a precise absolute temperature is required, as in air-sea interaction studies and comparisons between radiometer measurements and sea-surface temperatures obtained in situ. From the results of the authors listed above, it seems that at 150 m altitude during windy, overcast conditions, the maximum error is less than 1°C. For the type of research presently done by the Oceanography Division, this error has been accepted to be within the required tolerance, and no further adjustments were made to the results apart from the routine calibrations before and after each flight (GR~INDLINGH and SNYMAN, 1972). The radiometer, power supply and recorder have been assembled in a container which is easily installed in the back compartment of a twin-engined Cessna, along with sufficient spares and standby lead-acid batteries. The radiometer head views the sea surface vertically downwards through a hole in the deck. The aircraft flies at *Oceanography Division, National Physical Research Laboratory, South African Council for Scientific and Industrial Research, Congella, Natal, South Africa. Present address: Institut fiir Meereskunde an der Universitiit K i d , 23 Kiel, Diisternbrookerweg 20, Germany.

47

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M . L . GRONDLINGH

approximately 150 m altitude, and at this height the area viewed by the radiometer is about 25 m 2. Taking into account the reaction time of the recorder and an airspeed of 200 km h -1, the radiometer has a spatial resolution of better than 100 m. MEASUREMENTS

AT RICHARDS

BAY

Between March, 1970, and March, 1972, the Oceanography Division was engaged on a project to study the coastal waters off Richards Bay in view of its intended development as a major South African harbour and industrial centre, and because results from previous flights off the Natal coast (SNVMAN, 1969) have indicated inshore turbulence and possible current reversals in that area (Fig. 1, inset). Fifteen station positions were routinely occupied every day by the R.V. Meiring Naud~ for seven consecutive days every alternate month (Fig. 1). Weather permitting, three radiometer flights were then undertaken on a selection of the days of the ship's survey period. Standard meteorological parameters as well as the current speed vector and chemical properties at different subsurface levels were measured from the Meiring Naudd (STAVROPOULOS, 1971). In some radiation thermometer results obtained during the warm summer months under conditions of intense solar heating, low wind speed and calm sea conditions, the Agulhas Current could not be distinguished. These occasions were, however, very rare, and normally good agreement was found between the position of the current as indicated by radiometer temperatures and the current measurements made from the Meiring Naudd (Fig. 2). On the 18th March, 1971, the Meiring NaudOwas anchored off Richards Bay and on the 19th was allowed to drift approximately 50 km down the coast (Fig. 3), while the aircraft executed a reduced rectangular grid. The steep gradient in the sea surface temperature indicates a high current speed, and at 10 m depth a maximum current speed of 1.98 m s -1 was recorded during the two days. On the 19th an 8-m cargo parachute was used to lock the vessel to the flow. The drift track [Fig. 3(b)] indicates that the ship was closely following the surface temperature contours, and current measurements made during the period of the ship's drift yielded more or less steady values. These examples seem to indicate that a critical examination of the airborne radiometer results will yield a realistic description of the current pattern. The purpose of this paper is to suggest an explanation of the current reversals at Richards Bay using the radiometer data. Due to the sparse time coverage of the radiometer flights, a complete current reversal was never recorded during any set of three flights, but different stages of the process have been recognized from the results of several surveys. They have been extracted and those records which illustrate the main features of the model most clearly will be discussed. To overcome the resulting lack of continuity in the atmospheric pressure and wind diagrams (Fig. 6) accompanying the radiometer results (Fig. 5), a time series of pressure and wind is presented (Fig. 4), where it has been indicated how the current patterns would fit into a real time scale. RESULTS AND DISCUSSION The Agulhas Current originates at the confluence of the Mozambique Current and that part of the South Equatorial Current deflected down the eastern coast of

Inshore current reversals off Richards Bay

REPUBLIC of SOUTH AFRICA

49

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Fig. 1. Airborne radiation thermometer track chart used at Richards Bay: rectangular grid (lines 1-12) was used from March, 1970, to October, 1971 (25 flights); triangular grid (lines I-X) from December, 1971, to March, 1972 (9 flights). Flights were executed daily between 07 h 00 and 11 h 00, while the ship measurements started at 06 h 00 and ended at 19 h 00, working from North to South. (Ship station positions are indicated "). Inset: Sea surface temperatures from a radiometer flight off the coast of Natal in October, 1968, showing turbulence inshore of the Agulhas Current (after StCX'MA~,1969).

50

M.L. GRUNDLINGH

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Madagascar (DARBYSHIRE,1964), and thereafter follows the East Coast of southern Africa quite closely up to south of Port Elizabeth. From the analysis of previous flights and cruises off the coast of Natal, it has been realized that 6ome relationship seems to exist between the position and movement of the Agulhas Current, the inshore current reversals and the regular passing of low pressure cells that move up the East Coast of South Africa (ANDERSON, 1967; STAVROPOULOS,1967). Although no mention is made of reversals, WEaSTER 0 9 6 1 ) reported similar phenomena from observations of the Gulf Stream. Because the wind and pressure are inter-related, the influence of the low pressure cells on the current is augmented by the northeast wind preceding the arrival of the cell and the southwest wind following it (Fig. 4). It is, in addition, quite possible that the bottom topography has an important influence on the current (WARREN, 1963), as the direction of the continental shelf break varies in the Richards Bay area, and the shelf width increases southwards into a plateau at the Tugela Bight (Fig. 1).

51

Inshore current reversals off Richards Bay

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Inshore current reversals offRichards Bay

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In the steady-state flow condition, the Agulhas Current passes Cape St. Lucia close inshore in a southwest direction, and is about 10-20 km offshore opposite Richards Bay, with core speeds between 1 and 3 m s-1 [Fig. 5(a)]. During this time, the atmospheric pressure fluctuates diurnally and a variable northeast wind is blowing at approximately 5 m s-1 [Fig. 6(a)] before 10 h 00. As a coastal low pressure cell moving in a northeast direction approaches the area, the atmospheric pressure starts falling, the wind becomes steady northeast [Fig. 6(a)]: wind 6 m s-1 northeast; Fig. 4(a) and (b): wind 5-8 m s-1 northeast and the diurnal pressure variation becomes less noticeable (SMITH, 1961). With the atmospheric pressure steady [Fig. 6(b)] or still dropping [Fig. 4Co)] the laminar flow of the Agulhas Current is perturbed in the area off Cape Vidal and Cape St. Lucia, resulting in an eddy off Richards Bay [Fig. 5('o)]. It is possible that, under steady flow, the current is unstable and that a change in the atmospheric pressure and wind speed in conjunction with the clockwise change in the coastline and continental shelf direction (34 ° and 15°, respectively), provides the stimulus to disrupt the equilibrium. The correlation between the radiometer sea surface temperature and 10 m ship current vectors proves to be oven better when taking the 4-h lapse between aircraft and ship survey (in that area) into consideration. In the TS section for the upper 50 m at the southernmost station [Fig. 7(e)], considered from the aircraft results to have been close to the centre of the cyclonic gyre, the stratified layers of the northern offshore stations [Fig. 7(a--d)] have disappeared, thereby strongly suggesting upwelling. Similar sized eddies were detected during other ship and aircraft surveys (e.g. 19 May, 1970; 8 December, 1971), but some larger whirls which have been observed (a ship cruise off Cape St. Lucia failed to encounter the Agulhas Current as far as 25 km offshore) indicate that if the theory suggested above holds, at least one other nodal point north of Cape St. Lucia exists from which eddies are created.

54

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Fig. 7. TS sections for the upper 50 m at the offshore 5 ship stations (21 March, 1972), taken from north to south. During the passing of the low pressure cell, the wind speed drops as the atmospheric pressure gradient disappears [Fig. 6(b): wind 3 m s -1 northeast; Fig. 4(c): wind 5 m s -1 northeast]. The pressure then rises sharply, followed by a southwest wind [Fig. 6(c): wind 5-6 m s -1 west; Fig. 4(c): wind 8-14 m s -1 southwest], and the inshore current is almost completely reversed in the area covered by the ship [Fig. 5(c)]. Unfortunately, due to equipment trouble, no currents were measured below 50 m on that particular day, but current profiles from 11 o f the 15 stations revealed a reversed current (northeast direction) down to this depth. (Current measurements made under similar conditions on other cruises have shown a reversed current down to at least 150 m at the offshore stations.) After the atmospheric perturbation has passed, the wind speed returns to normal and the wind direction becomes more variable [Fig. 6(d), (e) and (f), and Fig. 4(d) and (e)]. The current appears to restore itself, the eddy has moved southwards [Fig. 5(d) and (e)] and the current increases its speed to normal, until the situation is more or less the same as before the reversal [Fig. 5(t")].

tnslaore current reversals off Richards Bay

55

COMMENTS AND CONCLUSIONS The southwest wind which occurred on 24 January, 1972, while the current was directed southwards [Fig. 6(e) and 5(e)], as well as the fact that the current reversed on the 21 March, 1972, before the wind had swung to southwest [Fig. 6(b) and 5(b)], suggests that the wind plays a minor role in the process. The whole procedure, from the initiation of the eddy until the flow is steady again, normally lasts 2-5 days. No relationship has as yet been found between the intensity of the low pressure cells and the turbulence itself or its duration. With the aid of current recorders placed close to the sea floor about 6 km off Richards Bay, 18 current reversals were registered over a period of 102 days of recording between May and August, 1971. Sixteen of these reversals coincided with the passing of a low pressure cell. The qualitative model proposed here does obviously not explain all the observed current patterns at Richards Bay, largely because of the varying frequency of the low pressure cells. The model can, however, provide a basis for describing occurrences at Richards Bay and similar areas elsewhere where comparable conditions exist.

A cknowledgements--The author would like to thank Messrs F. P. ANDERSON,Head of the Oceanography Division, C. C. STAVROPOULOSand A. F. P~ARCEfor their discussion and useful suggestions in the preparation of this paper. REFERENCES ANDERSONF. P. (1967) Time variations in the Agulhas Current near Durban. Oceanography Division, C.S.I.R., 16 pp. (Unpublished manuscript.) DARBYSHIREJ. (1964) A hydrological investigation of the Agulhas Current area. Deep-Sea Research, 11,781-815. GRONDLINGHM. L. and C. G. SNYMAN(1972) Use of a radiation thermometer in oceanography. C.S.LR. Symposium on remote sensing. Pretoria. 3-5 May, 1972. KIMURA R. and N. MISAWA(1970) Observation of sea surface temperature by infra-red radiation thermometer. Journal of the Oceanographic Society of Japan, 26(1), 22-37. LORENZD. (1970) Zur Methodik der Oberfl~ichentemperaturmessung yon Wasser mit InfrarotStrahlungstermometern. Deutscher Wetterdienst, Offenbach Am Main, 69 pp. PICKETT R. L. (1966) Accuracy of an airborne radiation thermometer. U.S. Naval Oceanographic Office, 13 pp. (Unpublished manuscript.) RICHARDSONW. S. and C. H. WILKINS(1958) An airborne radiation thermometer. Deep-Sea Research, 5, 62-71. SAUNDERS P. M. (1967) Aerial measurement of sea surface temperature in the infra-red. Journal of Geophysical Research, 72, 4109-4117. SAtrNDERSP. M. (1970) Corrections for airborne radiation thermometry. Journal of Geophysical Research, 75, 7596-7601. SAUNDERSP. M. and C. H. WILKINS(1966) Precise airborne radiation thermometry. Proceedings of the fourth symposium on remote sensing of the environment, Institute of Science and Technology, University of Michigan, pp. 815-826. SMITHA. J. J. (1961) Meteorological aspects, especially surface winds and associated weather along the Natal coast. Marine studies offthe Natal coast, C.S.I.R. Symposium, pp. 10-21. SNYMANC. G. (1969) Radiation thermometry offthe Natal coast during the year 1968. Oceanography Division, C.S.I.R., 45 pp. (Unpublished manuscript.) STAVROPOULOSC. C. (1967) Present status and future prospects of knowledge of the sea off the Natal coast. Oceanography Division, C.S.I.R., 9 pp. (Unpublished manuscript.) STAVROPOULOSC. C. (1971) Data acquisition on the R.V. Meiring Naudd. Electronics and Instrumentation, May 1971, pp. 11-14. WARREN B. A. (1963) Topographic influences on the path of the Gulf Stream. Tellus, 15, 167-183.

WEBSTERF. (1961) A description of Gulf Stream meanders off Onslow Bay. Deep-Sea Research, 8, 130-143.