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Cyclogenesis in the mozambique ridge current
Deep-SeaResearch.Vol. 34, No. 1, pp. 89-103. 1987.
0198-0149/87$3.(1()+ 00.0 © 1987 PergamonJournals Ltd,
Printed in Great Britain.
Cyclogenesis in the Mozambique Ridge Current MARTEN L.
GRONDLINGH*
(Received 24 July 1985; in revised form 22 April 1986; accepted 6 June 1986) A b s t r a c t - - T h e quadrilateral 25-33°S, 35-43°E in the southwest Indian Ocean seems to have a circulation characterized by the frequent occurrence of cyclonic eddies. Data from three hydrographic cruises, combined with the results of satellite-tracked buoys, delineated the process whereby the Mozambique Ridge Current is induced by the topography of the Mozambique Ridge to spawn eddies at 30°S, 37°E. Evidence was found in 1979-1980 of two eddies existing simultaneously: T h e first transported up to 28 × l0 n m 3 s ~ with kinetic energy of 17 × 10 H J (relative to 1000 db). The second, which was estimated to have been more than 7 m o n t h s old, transported 13-18 x l06 m 3 s -j with kinetic energy of 3-4 x 1014 J. T h e existence of occluded eddies in this area suggests a m e c h a n i s m by which the subtropical contribution into the Agulhas Current may be varied or diverted.
INTRODUCTION
THE RESULTSof the International Indian Ocean Expedition (IIOE, s e e D U N C A N , 1970; WYRTKI, 1971) indicated that the flow pattern of the Agulhas Current System between 25 ° and 35°S, and 30° and 45°E consists of (a) the Agulhas Current flowing southwestwards close to the African coast; (b) water recirculated in a large anticlockwise gyre (the so-called "Agulhas eddy") from the southern to the northern parts of the Agulhas Current; (c) the Mozambique Current (flowing southwards along the African coast in the Mozambique Channel) and (d) the westward contribution from the East Madagascar Current into the Agulhas Current. The variability of the Agulhas Current System is by and large unknown ( D U N C A N , 1970; LUTJEHARMS,1972; GRIJNDLINGH,1980; PEARCEand GRI~INDL1NGH,1982). The first observation of eddy activity in the deep sea area beyond the Aguihas Current was made in 1962 ( H A R R I S , 1970); this finding differed strongly with the image of semistagnant character of the flow in this region that emerged from IIOE data. The enigma presented by the existence of the eddy was firmly established by subsequent reports of abundant mesoscale activity in the area, both in the northern Agulhas Current ( G R U N D L INGHand PEARCE, 1984) as well as in the vicinity of the Mozambique Ridge ( G R U N D L I N G H , 1977, 1983a, 1984a, 1985a). This region is further characterized by a strong but variable current that can be manifested within a period of weeks (GRISNDLINGH, 1985b). During three cruises of the R.V. Meiring Naudd in the austral summer 1979-1980, two eddies were located simultaneously. An unexpected bonus was acquired in the form of some satellite-tracked buoys that drifted into the area and extended the ship's coverage.
*National Research Institute for Oceanology, CSIR, Stellenbosch 7600, South Africa. 89
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M . L . GRUNI)IJN(;II
Although the bouy data became available only much later and thus obviated any realtime advantage that could be derived from the buoy's drift patterns, the tracks provided an important insight into the larger-scale circulation of which the eddies formed part. In this paper the relevant results are presented, and an attempt is made to reconstruct the mechanism by which the eddies were generated. DATA
The first of the three cruises by the Meiring Naudd (28 November-6 December 1979) was aimed at locating and surveying any cyclonic eddies existing in the region of 30°S, 35-40°E (roughly the location of previous eddies). An eddy was discovered and a freedrifting array with 5 current meters was deployed and tracked for about 40 h in the eddy (GRONDLINGH, 1984b). The second (16-19 January 1980) and the third (20-22 March 1980) surveys were planned to relocate and track the eddy observed in December, but due to equipment malfunction their data yield was lower. Altogether 88 CTD stations were occupied successfully (Fig. 1). Some satellite-tracked buoys deployed east of Madagascar (EE~M, 1979; LUTJEtlARMSet al., 1981) entered the southwest Indian Ocean and fortuituously delineated the largerscale circulation associated with the eddies. These buoys were drogued, but it is uncertain how long the drogues remained intact. In addition, the temperature sensors on the buoys failed before the end of 1979, obviating any information on the water masses the buoys were following.
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Fig. 1. Hydrographic station positions executed on three cruises of the R.V. Meiring Nuudd in N o v e m b e r - D e c e m b e r 1979, January 198(/ and March 19811. A free-drifting array was tracked between Stas 33 and 49 in D e c e m b e r 1979.
Cyclogenesis in the Mozambique Ridge Current C R U I S E IN N O V E M B E R - D E C E M B E R
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1979
Thermohaline characteristics of eddy Fred During the cruise in December 1979, an eddy (code named Fred) was discovered by two transects across the eddy centre (Fig. 2). Most of the isotherms below 300 db showed vertical displacements of several hundred metres, the maximum elevation of about 400 db having been that of the 9°C isotherm. Judging from the shape of the deeper isotherms, the eddy extended well below the maximum measuring depth of 1000 db. In the surface layers (0-200 db) the distortion of the isotherms was much less, suggesting that thermally the eddy was mainly a subsurface feature. Considering the subsurface nature of Fred, the mid-thermocline 10°C isotherm was chosen to represent the shape and size of the eddy. The topography of this isotherm (Fig. 3a) indicates that the eddy was elliptical, with its major axis orientated northwestsoutheast. An indication of the size of the eddy is found from the 10°C/650 db intersection, which is normally situated at a depth of about 740 m in this region (WYRTKI, 1971; GRUNDLINGH, 1985b). According to this definition, the eddy dimensions were roughly 170 x 240 kin. Although the vertical sections showed that the main perturbance associated with the eddy was located deeper than 200 db, the surface T-S characteristics (Fig. 3b) indicate the presence of at least two separate water masses: In its northern and northeastern sectors, the eddy contained water of tropical origin, designated by the low salinity and high temperature recorded at Stas 24-29. Some of this water was also observed in the vicinity of Sta. 19. In contrast, Fred's interior consisted mostly of subtropical water, while to the south it was bordering on water with a relatively cold subtropical, almost Central Indian Water character.
Dynamics of Fred The geopotential topography of the sea surface relative to 1000 db showed that the eddy maintained a dynamic profile of about 20-40 dyn cm (2-4 x 103 J kg-1) below the level of the surrounding water. The gradient current distribution (Fig. 2) showed a filament of high speed current between Stas 23 and 25 in the area north of the eddy centre, and continuing to intensify while passing between Stas 29 and 30 in the eastern part. Comparison with the sea surface T-S characteristics indicated that this high-speed filament coincided more or less with the positions of strong thermohaline fronts at the surface. The maximum current velocities in the northern (60 cm s-l) and eastern (100 cm s-l) parts were located at the surface, but became submerged to 200-300 db in the southern (60 cm s-l) and western (40 cm s-l) sectors. Considering that Fred extended well below 1000 db, it can be assumed that velocity maxima in all sectors of the eddy probably approached or exceeded 100 cm s-l. The gradient velocities were used to compute the volume transport (Fig. 3c) and the kinetic energy distribution of the flow in the eddy (Fig. 3d). The eddy volume transport was greatest in the southern part and smallest in the northern part (the inverse was true of the gradient current maxima), with values ranging from 17 to 28 x 106 m 3 s-~ (the tolerance on these values is estimated at 10%). In addition, the volume transport distribution did not display the same asymmetry as the topography of the 10°C isotherm (Fig. 3a). The kinetic energy was contained mainly in an elliptic ribbon around the eddy. The positions of maximum energy (Fig. 3a) revealed a close relationship with the
92
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Cyclogenesis in the Mozambique Ridge Current
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position of the 10°C/550 db intersection. The total kinetic energy (relative to 1000 db) of Fred amounted to 1.74 × 10 ~5 J. In May-June 1978, an eddy similar to Fred was observed in approximately the same position (GRONDLINGH,1985a). The cyclonic eddy was smaller ( - 1 2 0 x 240 km) than Fred, but compatible in transport ( - 2 1 - 2 6 x 106 m 3 s-j relative to 1000 m). The possibility that the eddy observed in 1978 was Fred was ruled out because of the time interval ( - 1 8 months) between the two sets of observations. The observations do, however, confirm the impression that cyclonic eddies and their generating current(s) are regular features in this area. The objectives of the deployment of the current meter array were two fold. First, the drift of the array would provide some idea of the true currents in the eddy because of its large ratio of submerged area to exposed area. Second, it was hoped that the current meters at various depths would provide some insight into the vertical structure of the
94
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velocities inside the eddy. The position of the array relative to the eddy centre can be seen in Fig. 2. Details of the design of the array and an assessment of the data quality have been given elsewhere (GRONDLINGH, 1984b). The array initially drifted inside the current core at 50 cm s-I (150°T), later slowing down to 29 cm s-l (176°T). The relative current velocities (uncorrected for the drift of the array) were only slightly larger than the tolerance arising from the array motion. The velocity profile (corrected for the drift of the array) compared favourably with the geostrophic profile between stations in the vicinity of the drift track (Fig. 4), although the current meter velocities exceeded the gradient velocities by approximately 20 cm s-~. This can be attributed to the shallow (1000 db) reference level of the geostrophic computation. It was concluded that valuable data can be gleaned from such an experiment (for example, verification of the subsurface jets or eddy-environment mass exchange) provided that the design of the array is improved (to reduce the inaccuracies of the data) and the array allowed to orbit the eddy a few times. C R U I S E S IN J A N U A R Y AND MARCII 1980
The cruises in January and March 1980 were attempts to relocate Fred and thus derive some insight into its possible advection rate. Had the satellite buoy tracks been available for real-time use at the time, their results would have profoundly influenced the design of the cruises. The initial transect in January passed close enough to Fred's centre to verify its position (Fig. 5a), while further to the southeast a second eddy (codenamed Golf) was observed with its centre located more or less at 31°20'S, 38°E. The depth of the isotherm in January suggested that Fred had remained in about the same position between the surveys. However, it seems that during the intervening 6 weeks Fred had either significantly reduced its size, or had rotated its axes. It was considered unlikely that the eddy's surface area had shrunk within this period to less than one third of its original size, and it was concluded that Fred had rotated its axes, a feature that seems not uncommon in eddies of this kind (e.g. SPENCEand LEGECKIS, 1981; GRONDLINGH, 1985a).
95
Cyclogenesis in the Mozambique Ridge Currcnt
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Eddy Golf was situated approximately 200 km southeast of Fred. This was only the second time that two eddies had been observed simultaneously in this area (in August 1975, the co-existence of two eddies was postulated by GRUNDLINGIt, 1977). The shape of the 10°C topography (Fig. 5a) suggested that Sta. 15 of the January cruise was situated close to Golf's centre. The volume transport of Golf was about 12 x 10~' s-l, while the total kinetic energy value of 3 x 1014 J was obtained by rough extrapolation in view of the partial coverage. The cruise in March 1980 could find no evidence of Fred in the area where it had been observed in December and January (Fig. 5b). As such a rapid decay was not considered realistic, it was concluded that Fred had moved from its original position. Farther to the southeast an eddy was located and partially surveyed at 31°20'S, 38°45'E, where the 10°C isotherm rose to 460 db. The proximity of this eddy to the position where Golf had been observed 2 months previously suggested they were the same. If this was the case, it had moved approximately 70 km to the east since January, and its diameter was now an estimated 120 km. Although the gradient velocity maxima in Golf was reduced to 20-30 cm s-~ in March (compared to 20-40 cm s-~ during January), the volume transport (14-18 x 106 m 3 s-1) and total kinetic energy ( - 4 x 1014 J) were higher than in January. However, in the light of the incomplete coverages on both occasions, these slight variations were not considered significant. S A T E L L I T E BUOY T R A C K S
The period during which the buoys (code numbers 14621, 14622 and 14627) were active in the area can be separated into two parts. The first part covers the period from the time the buoys entered the area (Fig. 6) up to 31 November 1979; this was chosen to represent the circulation preceding the cruises of the R.V. Meiring Naud(. The second period (Fig. 7), from 1 December 1979 to 31 January 1980, provided insight into the flow patterns
96
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reigning at the time of the hydrographic surveys, as well as into the possible generating mechanism of the eddies. The buoys under discussion either failed or left the area between or soon after 31 January 1980.
Period I: March-November 1979 (Fig. 6) Buoy 14627 was trapped in a translating cyclonic eddy south of Madagascar during May and June 1979 before leaving the eddy at the beginning of July. The eddy seems to have originated from the area southeast of Madagascar (possibly generated by the East
97
Cyclogenesis in the Mozambique Ridge Current
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Madagascar Current), and the buoy displayed a regular rotation rate of once every 4-6 d a y s (LuTJEHARMSet al., 1981). The buoy was subsequently carried northwestwards into the Mozambique Channel before turning southwards along the African continent. Its course resembled the path of a satellite-tracked buoy deployed in 1975 (GRt)NOLtNGH, 1977). As in the case of that buoy, 14627 did not follow the "coastal" route but moved away from the coast at 28°S and drifted southwards along the Mozambique Ridge. During October the buoy became entrained in a cyclonic eddy east of the Mozambique Ridge at 31°S, 38°E, and completed
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three revolutions at an average speed of 55 cm s-' before leaving the eddy in a northeasterly direction in November. The orbits of the buoy coincided closely (Fig. 8) with the position where Golf was observed in January 1980 (that is, 2.5 months later). Considering the slow movement of Golf between January and March, it was concluded that buoy 14627 had in fact been trapped in Golf in October 1979. This would suggest that Golf was at least 5 months old at the time of the March 1980 cruise. Buoy 14622 completed a large oval-shaped circuit at 29°S, 43°E in April 1979 before drifting steadily westwards over the Mozambique Ridge. It remained there for 2 months, moving in a seemingly irregular pattern at first and then in a general westerly direction toward the African continent. Apart from the delineation of Golf, of importance here is the behavior of the buoys in a particular area shared by boti] buoys but at different times. Toward the end of April 1979, buoy 14622 drifted through the region of 30°S, 37°E (Fig. 6) that was also traversed by buoy 14627 in almost the same direction in October. The flow direction (southeast) derived from these buoy tracks at this point was exactly opposite to the direction of the current (northwest) recorded by the ship survey of Fred in December (e.g. Fig. 8). This
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Cyclogenesis in the Mozambique Ridge Current
99
contradiction suggested that Fred had not been present during either April or October 1979, and only appeared in the period between October and December 1979.
Period II: December 1979-January 1980 (Fig. 7) After buoy 14627 had been ejected from Golf at 31°S, 38°E, it moved northeastwards towards Madagascar in November and then accelerated rapidly westwards in December. By coincidence, buoy 14621 was in the same area at that time (having entered the area from the east), and the two buoys followed more or less the same route during the following 2 months. As both buoys drifted westwards onto the Mozambique Ridge at 27°S, they turned anticlockwise through almost 180° before leaving the Ridge into the Mozambique Basin in an eastsoutheast direction. This was followed by a clockwise circuit at the end of December, through which the buoys returned westwards onto the Ridge in January 1980. The separation of the buoys during their drift in December could be accounted for by a current about 100 km wide. Figure 7 shows that the drift tracks of these two buoys formed an envelope around Fred and Golf. Both buoys passed through the northeastern sector of Fred but failed to become entrained into the eddy itself. Combination of the buoy tracks and the hydrographic results allows inference of some tentative conclusions about the origins and existence of the eddies: Fred. According to the hydrographic data, Fred's peak 10°C elevation was at 350 db, which was higher than any of the eddies observed in this region between 1975 and 1978 (GRuNDLINGH, 1977, 1983a, 1984a, 1985a). This information does not provide a gauge for Fred's age, but it is felt that Fred was generated not long before it was first observed in December 1979. To a certain extent this is corroborated by the failure of the buoys to provide any evidence of Fred in October. It is therefore suggested that Fred had not merely drifted into the position where it was observed in December, but had actually been generated there in the 2-month interval between 1 October and 1 December 1980. Golf. As Golf was observed in October (from the buoy tracks), January and March (from the hydrographic results), it was obviously in existence during the passage of the buoys in December-January, even though its position was not clearly delineated by the buoy tracks (nor, for that matter, was the position of eddy Fred). According to the observations presented above, Golf underwent only minor changes in its position between October and March; it probably remained in approximately the same position during the passage of the buoys through the area. Third eddy. The buoy tracks suggested the existence of a third eddy situated to the east of and adjacent to Golf, at 31°S, 40°E. However, the evidence is considered too scant for unequivocal identification. THE M O Z A M B I Q U E R I D G E C U R R E N T
The buoy tracks seemed to indicate the existence of a current (referred to as the Mozambique Ridge Current, MRC) that is manifested as a quasi-zonal jet flowing westward over the northern Mozambique Basin before meandering in an "S" shape over the Mozambique Ridge. The first (indirect) reference to the MRC was made by HARRIS(1972), who found that a portion of the westward flow between the Madagascar Ridge and the African continent appeared to be diverted by the Mozambique Ridge and recirculated anticyclonically.
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HARRISand VAN FOREEST(1977) reported further evidence of an easterly tributary to the Agulhas Current south of 30%; they estimated its magnitude at 11-19 x 10 6 m 3 s 1 (relative to 2000-4000 m). Applying the inertial-jet model of WARREN(1963) they were able to show the tendency for water emerging southward from the Mozambique Channel to follow either the "coastal" route, leading to the type of observations reported by PEARCE (1977), PEARCEand GRONDLINGtt(1982) and GRUNDLINGH(1983b) off the South African east coast between 28° and 30°S, or an offshore, circuituous path along the Mozambique Ridge to 30°S. Here the MRC turns inshore (that is, westward) to join the coastal branch. GRUNDEINGH (1985b) indicated that the MRC transported about 36 × 10¢' m 3 s-~ in 1981 (confirming that this current unambiguously has a sufficient dynamic signature to create intense eddies) and suggested that topographic induction by the Mozambique Ridge could explain the existence of eddies in this region. The influence of the Mozambique Ridge on the circulation in the southwest Indian Ocean seems to be indicated quite clearly by the pattern of the satellite buoy tracks in the area. The S-shaped meandering of the tracks (see Fig. 6 and especially 7 and 8) is reminiscent of a steady-state current (MRC) in which potential vorticity is conserved while it flows over significant variations of the bottom depth: as the current flows westward (at 27°S) across the Mozambique Ridge (depth -2000 m) the vorticity of the current decreases (that is, it becomes more anticyclonic) and the current turns anticlockwise back into the Basin, The sharp increase of the depth from the Ridge into the Basin (-5000 m) increases the vorticity, causing the current to return clockwise onto the Ridge. This stationary situation obviously oversimplifies the characteristics of a variable flow regime, but nevertheless elucidates some of the factors controlling the circulation. To test the hypothesis of topographic control on such a current, and thus to indicate that the buoy tracks could be a realistic representation of the MRC, the simple model of WARREN (1963) was applied. This model was also employed by DARBYSHIRE(1972), HARRIS and VAN FOREEST (1977), GRONDLINGH (1978) and LUTJEttARMSand VAN BALLEGOOYEN (1984) to explain some of the features of the Agulhas Current System. The model has some well-known deficiencies, amongst others the consideration of only steady-state conditions. It therefore provides merely a first-order insight into the true circulation, and cannot address more detailed aspects of the flow. In the model, the curvature C(x,y) of an inertial current at co-ordinates (x,y) is related to the curvature G~ at a previous point (x0,yo), the volume transport V, the momentum transport M and the volume transport T across the unit depth near the bottom, the Coriolis parameterf and its change with latitude [3 = Of/Oy, the initial depth Do(xo,yo) and a subsequent depth D(x,y) at the position under discussion by C(x,y) = Co(xo,yo) - f$[V(y - Yo) + fT(D-Do)]/M. Using the Runge-Kutta method, the curvature was derived numerically and then integrated piecemeal from (xo,y0) onwards to produce the current path. In the present study, values of ~1/M and TIM were not available a priori, and were chosen more or less arbitrarily to simulate the drift tracks of the buoys during December 1979 and January 1980 (Fig. 8). The computation was initiated at 27°30'S, 41°E, where the tracks seemed to show a point of inflection (that is, Co = 0) and the "best fit'! of the model to the observed paths of the buoys was obtained visually (Fig. 8). It was found, firstly, that the model could successfully simulate most of the buoy tracks during December and January. This confirmed that the buoys had been embedded in a
Cyclogenesisin the MozambiqueRidge Current
101
current (the MRC) that conformed more or less to the requirements of an inertial jet, and were not merely blown around by the wind to (fortuitously) produce the resulting tracks. Secondly, two sets of parameters were required to duplicate the MRC: North of 29°S, T/M and V/M equalled 0.55 x 10 -4 ( m 2 s-I) -1 and 3.5 (m s-l) -I respectively, while south of this latitude the values were 1.2 x 10-a (m 2 s-l) -l and 3.5 (m s-l) -l, respectively (Fig. 8). Thirdly, the model was unable to simulate the tracks of the buoys in the elongated cyclonic loop at 30°-33°S, 38°-42°E for any values of V/M and T/M. It was therefore concluded that the southeasterly, cyclonic loop of the buoys' drift was incompatible with the predicted course of the MRC. The model predicted a much tighter clockwise turn at 30°S, 38°E where eddy Fred was observed. A plausible explanation for the behaviour of the buoys (that is, their separation from the predicted course of the MRC) could be situated in the presence of an additional eddy besides Fred. The proximity of such an eddy, possibly combined with other factors such as wind, could have caused the buoys to be detrained from the MRC and entrained around the second eddy (Golf). This hypothesis is supported by the sudden change in drift course of buoy 14627 in the eastern perimeter of Fred (Figs 7 and 8). From hydrographic sections of the R.V. Meiring Naudd obtained in 1981 (see GRt?NDUNGH, 1985a), values of V (26 x 10 6 m3 s-I) and M (107 m 4 S-2) were estimated; they produced a slightly lower value of V/M (2.5 m-~ s) than the one used in the model. Using the velocity distribution of NIILER and ROBINSON (1967), the model predicted surface velocities of 1.0-1.2 m s-~, which are higher but of the same order as the buoy drift speeds (Fig. 8) that varied mainly between 30 and 100 cm s-I (Fig. 7). It is possible at this stage to combine the different sets of observations and to consider a coherent method of cyclogenesis for eddies such as Fred and Golf. By exploiting some of the points of agreement between Gulf Stream rings and the eddies described here, an indication of the generating mechanism of eddies such as Fred and Golf may be gleaned. The process by which cyclonic Gulf Stream rings are created has been well documented (e.g. FUGHSTER, 1972; GOTTHARDT, 1973; DOBLAR and CHENEV, 1977) and consists basically of the occlusion of an extended loop of the Gulf Stream. This process has also been observed in the Brazil Current (LEGECKISand GORDON, 1982) and in the southern Agulhas Current (LuTJEttARMS, 1981). As far as the results of the present study are concerned, there is no reason to believe that the current loops (both cyclonic and anticyclonic) visible in Fig. 8 cannot be similarly occluded. Occlusion of the clockwise loop in Fig. 8 would sever a cyclonic eddy (e.g. Fred) from the MRC, after which MRC retracts onto the Mozambique Ridge. This hypothesis is supported by the track of buoy 14622 in the period 1 April-1 July 1979 (Fig. 6). This buoy described a track similar to those of the other buoys, with an important exception: it remained on the Mozambique Ridge at 31°S, 36°E. Buoy 1116, tracked in 1975 (GR~JNDLINGH, 1977) described a very similar pattern to that of buoy 14622 (Fig. 8), with an eddy located in almost the same position as Fred. At the time, the relation between the eddy (which was delineated by ship observations) and the drift pattern of the buoy on the Ridge was not recognized, but the composite ship and buoy results (Fig. 8) now seem to present a logical arrangement of the flow patterns. In the light of the results presented above it is believed that there is strong evidence suggesting that the eddies are generated by an occlusion of a cyclonic loop of the MRC. Anticyclonic eddies originating from the northerly part of the Spattern should also be possible, in fact, they have been observed (GRt?NDLINGH, in preparation). These results, along with those of GRONDHNGH(1985b) and other s, support
102
M . L . GR()NDLIN¢Itt
an in situ generation mechanism for these cyclonic eddies, and tend to exclude the possibility of a remote cyclogenesis (e.g. southeast of Madagascar or from the Agulhas Return Current). If the arguments above are valid, a rough estimate can be made of the age of Golf. Assuming that the drift of buoy 14622 between April and July 1979 delineated a manifestation of the MRC from which an eddy has been separated through occlusion, the eddy itself would have been situated approximately where Golf was observed from the buoy tracks in October (Fig. 6). It is therefore quite possible that Golf was the same eddy generated by the MRC in the period April-July 1979. For this to have occurred, the eddy created in April-July would have had to survive for at least 6 months. Although dissipation rates are not available for these eddies, Gulf Stream rings have been found to collapse their elevated cold-water core at a rate of 0.4-1.0 m day ~ (PARKER, 1971; GOTTttARDT, 1973; CnENEY and RICItARDSON, 1976). In terms of kinetic energy dissipation, CHENEYand RICHARDSON(1976) found a decay rate of 2 x 10 t2 J day-j for Gulf Stream rings. Assuming that Golf initially has the same density structure and energy as Fred, it would have been 80-200 days old (based on core collapse rates) and about 700 days old (based on energy dissipation rates) at the time of its observation in January 1980. It is, therefore, quite possible that Golf was occluded from the MRC in April-July 1979, as postulated by the buoy tracks. CONCLUSIONS
In previous surveys (e.g. GR13NDLINGtt, 1977, 1983a) during which cyclonic eddies were located in this region, the impression was gained that the eddies occur only one at a time. This illusion can probably be ascribed to the limited coverage possible with a small vessel. The present results focused attention on the main eddy, Fred, but managed to delineate the presence of at least another eddy in the close proximity. With lifetimes of several months, more eddies could exist simultaneously, and theoretically these eddies could have an abundance compatible with that of Gulf Stream rings (e.g. PARKER, 1971). However, no information is available to determine the persistence of the MRC, and it is suspected that the eddy generation rate may be low (a few eddies per year). Whether or not Golf was as old as suggested above, the existence of intense eddies can have a profound influence on the circulation in the southwest Indian Ocean. The data presented above suggest that the MRC is discontinuous, which may lead to a situation where occluded eddies completely "absorb" the flow of the MRC. The propagation and fate of the eddies are therefore important factors to determine the amount of MRC water that reaches the Agulhas Current. According to DUNCAN(1970) and WYRTKI(1971), the steady-state flow in this region is toward the northeast. The results of GRONDLINGtt (1985b) point toward a much more variable flow. By trapping an amount of water originating from the MRC, eddies can thus prevent the contribution from this current to reach the Agulhas Current or hold it in abeyance for some time. Acknowledgements--1 thank the typing and draughting groups of the NRIO and especially the officers and crew of the Meiring Naudd for their support. REFERENCES CItENE-Y R. E. and P. L. RICHARDSON (1976) Observed decay of a cyclonic Gulf Stream ring. Deep-Sea Research, 23, 143-155.
Cyclogenesis in the Mozambique Ridge Current
103
DARB"tStlIRI-;J. (1972) The effect of bottom topography on the Agulhas Current. Pure and Applied Geophysics, 101,208-220. DOBLAIt R. A. and R. E. CttENEY (1977) Observed formation of a Gull Stream cold-core ring. Journal <~/
Physical Oceanography, 7,944--946. DtJNOAN C. P. (1970) The Agulhas Current. Ph.D. Thesis, University of Hawaii, 76 pp. EERM (Establissement d'Etudes et de Rechcrches Metcorologiques) (1979) Project Marisondc, Second dossier. Direction de la Meteorologic, Ministere des Transportes, France, 53 pp. Ft~GI.ISHZR F. C. (1972) Cyclonic rings lk)rmed by the Gulf Stream. In: Studies in physical oceano~raplLv, A. L. G()RI)ON, editor, Gordon and Breach, London, pp. 137-168. Go-H IIARDT G. A. (1973) Gulf Stream eddies in the western North Atlantic. US Nawil Oceanographic Oflice Report TN 6150-11>-73, 46 pp. GRON~)LINGli M. L. (1977) Drift obserwltions from NIMBUS VI satellite-tracked buoys in the southwest Indian Ocean. Deep-Sea Research, 24, 9/)3-913. GRtTNnLIN(Itt M. L. (1978) Drift of a satellite-tracked buoy in the southern Agulhas Current and Agulhas Return Current. Deep-Sea Research, 25, 1209-1224. GRLTNDLIN(III M. L. (1980) On the volume transport of the Agulhas Current. De¢7~-SeaResearch, 27,557-563. GRUNI)LINGit M. L. (1983a) Eddies in the southern Indian Ocean lind the Agulhas Current. In: Eddies in marine science, A. R. ROBINSON, editor, Springer Verlag, Berlin, pp. 245-264. GI'~.tINDLING]t M. L. (1983b) On the course of the Agulhas Current. Soltlh A.l)ican Geographical Jottrnal, 65, 49-57. GRONDLINGH M. L. (1984a) An eddy over the northern Mozambique Ridge. South African Journal ~fScience, 80,324-329. GRUNDI.INGII M. L. (1984b) A free-drifting current meter array in a cyclonic eddy of the Mozambique Ridge Current. South African CSIR Technical Report T/SEA 8501, 24 pp. GR~rNI)I.INGII M. L. (1985a) An intense cyclonic eddy east of the Mozambique Ridge. Jottrnal (>["Geophysical Research, 90, 7163-7167. GRUNI)LINGH M. L. (1985b) Features of the circulation in the Mozambique Basin in 1981. Journal ~>[Marine Research, 43,779-792. GR{!NDLINGtI M. L. and A. F. PEARCE (1984) Large vortices in the northern Agulhas Current. Deep-Sea Research, 31, 1I49-1156. HARRIS T. F. W. (1970) Features of the surface currents in the southwest Indian Ocean. CSIR Symposium Oceanography in South Africa, Durban, 13 pp. HARRIST. F. W. (1972) Sources of the Agulhas Current in the spring of 1964. Deep-Sea Research, 19,633-65/). HARRIS T. F. W. and D. VAN FOREL-ST(1977) The Agulhas Current system. Department of Oceanography, University of Cape Town, 38 pp. LE~G[~CKISR. lind A. L. GOROON (1982) Satellite observations of the Brazil lind Falkland Currents, 1975-1976. and 1978. Deep-Sea Research, 29, 375-4[)1. LtIT,n~HARMSJ. R. E. (1972) A quantitative assessment of year-to-ye~lr w~riability in water movement in The southwest Indian Ocean. Nature. Physical Science, 239, 59-60. Ltrr,H~ltARMS J. R. E. (1981) Features of the Southern Agulhas Current circulation from satellite remote sensing. South African Journal ~r Science, 77,231-236. LUT.n~tiARMSJ. R. E. and R. VAN BALLEGOOYEN(1984) Topographic control in the Agulhas Current system. De~7~-Sea Research, 31, 1321-1337. LUTJNtARMS J. R. E., N. D. BANGand C. P. DUNCAN ( 1981 ) Characteristics of the currents south and east of Madagascar. Deep-Sea Research, 28,879-899. NULt~R P. P. and A. R. ROB[NSON(1967) The theory of free inerti~ll jets. A numerical experiment for the path of the Gulf Stream. Tellus, 19, 6/)1-619. PARKt~RC. E. (1971) Gulf Stream rings in the Sargasso Sea. Deep-Sea Research, 18, 981-994. PliAR('~ A. F. (1977) Some features of the upper 500 m of the Agulhas Current. Joitr#tal of Marine Research, 35,731-753. PI~ARCt-~A. F. and M. L. GR'~INDLINGIt(1982) Is there a seasonal wlriation in the Agulhas Current'? Journal ~J[" Marine Research, 40, 177-184. SVt~NCi~T. W. lind R. Ll-:C;t~CKiS( 1981 ) Sate[litc and hydrographic observations of low-frequency wave motions associated with a cold-core Gulf Stream ring. Journal ~["Geophysical Research, 86, 1945-1953. WARRrN B. A. 11963)Topographic influences on the path of the Gulf Stream. Tellus, 15, 167-183. WYRTKI K. 11971) Atlas of the International Indian Ocean Expedition. National Science Foundation, Washington, DC, 531 pp.
0198-0149/87$3.(1()+ 00.0 © 1987 PergamonJournals Ltd,
Printed in Great Britain.
Cyclogenesis in the Mozambique Ridge Current MARTEN L.
GRONDLINGH*
(Received 24 July 1985; in revised form 22 April 1986; accepted 6 June 1986) A b s t r a c t - - T h e quadrilateral 25-33°S, 35-43°E in the southwest Indian Ocean seems to have a circulation characterized by the frequent occurrence of cyclonic eddies. Data from three hydrographic cruises, combined with the results of satellite-tracked buoys, delineated the process whereby the Mozambique Ridge Current is induced by the topography of the Mozambique Ridge to spawn eddies at 30°S, 37°E. Evidence was found in 1979-1980 of two eddies existing simultaneously: T h e first transported up to 28 × l0 n m 3 s ~ with kinetic energy of 17 × 10 H J (relative to 1000 db). The second, which was estimated to have been more than 7 m o n t h s old, transported 13-18 x l06 m 3 s -j with kinetic energy of 3-4 x 1014 J. T h e existence of occluded eddies in this area suggests a m e c h a n i s m by which the subtropical contribution into the Agulhas Current may be varied or diverted.
INTRODUCTION
THE RESULTSof the International Indian Ocean Expedition (IIOE, s e e D U N C A N , 1970; WYRTKI, 1971) indicated that the flow pattern of the Agulhas Current System between 25 ° and 35°S, and 30° and 45°E consists of (a) the Agulhas Current flowing southwestwards close to the African coast; (b) water recirculated in a large anticlockwise gyre (the so-called "Agulhas eddy") from the southern to the northern parts of the Agulhas Current; (c) the Mozambique Current (flowing southwards along the African coast in the Mozambique Channel) and (d) the westward contribution from the East Madagascar Current into the Agulhas Current. The variability of the Agulhas Current System is by and large unknown ( D U N C A N , 1970; LUTJEHARMS,1972; GRIJNDLINGH,1980; PEARCEand GRI~INDL1NGH,1982). The first observation of eddy activity in the deep sea area beyond the Aguihas Current was made in 1962 ( H A R R I S , 1970); this finding differed strongly with the image of semistagnant character of the flow in this region that emerged from IIOE data. The enigma presented by the existence of the eddy was firmly established by subsequent reports of abundant mesoscale activity in the area, both in the northern Agulhas Current ( G R U N D L INGHand PEARCE, 1984) as well as in the vicinity of the Mozambique Ridge ( G R U N D L I N G H , 1977, 1983a, 1984a, 1985a). This region is further characterized by a strong but variable current that can be manifested within a period of weeks (GRISNDLINGH, 1985b). During three cruises of the R.V. Meiring Naudd in the austral summer 1979-1980, two eddies were located simultaneously. An unexpected bonus was acquired in the form of some satellite-tracked buoys that drifted into the area and extended the ship's coverage.
*National Research Institute for Oceanology, CSIR, Stellenbosch 7600, South Africa. 89
90
M . L . GRUNI)IJN(;II
Although the bouy data became available only much later and thus obviated any realtime advantage that could be derived from the buoy's drift patterns, the tracks provided an important insight into the larger-scale circulation of which the eddies formed part. In this paper the relevant results are presented, and an attempt is made to reconstruct the mechanism by which the eddies were generated. DATA
The first of the three cruises by the Meiring Naudd (28 November-6 December 1979) was aimed at locating and surveying any cyclonic eddies existing in the region of 30°S, 35-40°E (roughly the location of previous eddies). An eddy was discovered and a freedrifting array with 5 current meters was deployed and tracked for about 40 h in the eddy (GRONDLINGH, 1984b). The second (16-19 January 1980) and the third (20-22 March 1980) surveys were planned to relocate and track the eddy observed in December, but due to equipment malfunction their data yield was lower. Altogether 88 CTD stations were occupied successfully (Fig. 1). Some satellite-tracked buoys deployed east of Madagascar (EE~M, 1979; LUTJEtlARMSet al., 1981) entered the southwest Indian Ocean and fortuituously delineated the largerscale circulation associated with the eddies. These buoys were drogued, but it is uncertain how long the drogues remained intact. In addition, the temperature sensors on the buoys failed before the end of 1979, obviating any information on the water masses the buoys were following.
30 °
35OE
__
30 ° $
30 ° S
l 30 °
35 °E
40 °
Fig. 1. Hydrographic station positions executed on three cruises of the R.V. Meiring Nuudd in N o v e m b e r - D e c e m b e r 1979, January 198(/ and March 19811. A free-drifting array was tracked between Stas 33 and 49 in D e c e m b e r 1979.
Cyclogenesis in the Mozambique Ridge Current C R U I S E IN N O V E M B E R - D E C E M B E R
91
1979
Thermohaline characteristics of eddy Fred During the cruise in December 1979, an eddy (code named Fred) was discovered by two transects across the eddy centre (Fig. 2). Most of the isotherms below 300 db showed vertical displacements of several hundred metres, the maximum elevation of about 400 db having been that of the 9°C isotherm. Judging from the shape of the deeper isotherms, the eddy extended well below the maximum measuring depth of 1000 db. In the surface layers (0-200 db) the distortion of the isotherms was much less, suggesting that thermally the eddy was mainly a subsurface feature. Considering the subsurface nature of Fred, the mid-thermocline 10°C isotherm was chosen to represent the shape and size of the eddy. The topography of this isotherm (Fig. 3a) indicates that the eddy was elliptical, with its major axis orientated northwestsoutheast. An indication of the size of the eddy is found from the 10°C/650 db intersection, which is normally situated at a depth of about 740 m in this region (WYRTKI, 1971; GRUNDLINGH, 1985b). According to this definition, the eddy dimensions were roughly 170 x 240 kin. Although the vertical sections showed that the main perturbance associated with the eddy was located deeper than 200 db, the surface T-S characteristics (Fig. 3b) indicate the presence of at least two separate water masses: In its northern and northeastern sectors, the eddy contained water of tropical origin, designated by the low salinity and high temperature recorded at Stas 24-29. Some of this water was also observed in the vicinity of Sta. 19. In contrast, Fred's interior consisted mostly of subtropical water, while to the south it was bordering on water with a relatively cold subtropical, almost Central Indian Water character.
Dynamics of Fred The geopotential topography of the sea surface relative to 1000 db showed that the eddy maintained a dynamic profile of about 20-40 dyn cm (2-4 x 103 J kg-1) below the level of the surrounding water. The gradient current distribution (Fig. 2) showed a filament of high speed current between Stas 23 and 25 in the area north of the eddy centre, and continuing to intensify while passing between Stas 29 and 30 in the eastern part. Comparison with the sea surface T-S characteristics indicated that this high-speed filament coincided more or less with the positions of strong thermohaline fronts at the surface. The maximum current velocities in the northern (60 cm s-l) and eastern (100 cm s-l) parts were located at the surface, but became submerged to 200-300 db in the southern (60 cm s-l) and western (40 cm s-l) sectors. Considering that Fred extended well below 1000 db, it can be assumed that velocity maxima in all sectors of the eddy probably approached or exceeded 100 cm s-l. The gradient velocities were used to compute the volume transport (Fig. 3c) and the kinetic energy distribution of the flow in the eddy (Fig. 3d). The eddy volume transport was greatest in the southern part and smallest in the northern part (the inverse was true of the gradient current maxima), with values ranging from 17 to 28 x 106 m 3 s-~ (the tolerance on these values is estimated at 10%). In addition, the volume transport distribution did not display the same asymmetry as the topography of the 10°C isotherm (Fig. 3a). The kinetic energy was contained mainly in an elliptic ribbon around the eddy. The positions of maximum energy (Fig. 3a) revealed a close relationship with the
92
M. L, GRONDI.INGI]
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Cyclogenesis in the Mozambique Ridge Current
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position of the 10°C/550 db intersection. The total kinetic energy (relative to 1000 db) of Fred amounted to 1.74 × 10 ~5 J. In May-June 1978, an eddy similar to Fred was observed in approximately the same position (GRONDLINGH,1985a). The cyclonic eddy was smaller ( - 1 2 0 x 240 km) than Fred, but compatible in transport ( - 2 1 - 2 6 x 106 m 3 s-j relative to 1000 m). The possibility that the eddy observed in 1978 was Fred was ruled out because of the time interval ( - 1 8 months) between the two sets of observations. The observations do, however, confirm the impression that cyclonic eddies and their generating current(s) are regular features in this area. The objectives of the deployment of the current meter array were two fold. First, the drift of the array would provide some idea of the true currents in the eddy because of its large ratio of submerged area to exposed area. Second, it was hoped that the current meters at various depths would provide some insight into the vertical structure of the
94
M.L. GRONDIJN(;II
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velocities inside the eddy. The position of the array relative to the eddy centre can be seen in Fig. 2. Details of the design of the array and an assessment of the data quality have been given elsewhere (GRONDLINGH, 1984b). The array initially drifted inside the current core at 50 cm s-I (150°T), later slowing down to 29 cm s-l (176°T). The relative current velocities (uncorrected for the drift of the array) were only slightly larger than the tolerance arising from the array motion. The velocity profile (corrected for the drift of the array) compared favourably with the geostrophic profile between stations in the vicinity of the drift track (Fig. 4), although the current meter velocities exceeded the gradient velocities by approximately 20 cm s-~. This can be attributed to the shallow (1000 db) reference level of the geostrophic computation. It was concluded that valuable data can be gleaned from such an experiment (for example, verification of the subsurface jets or eddy-environment mass exchange) provided that the design of the array is improved (to reduce the inaccuracies of the data) and the array allowed to orbit the eddy a few times. C R U I S E S IN J A N U A R Y AND MARCII 1980
The cruises in January and March 1980 were attempts to relocate Fred and thus derive some insight into its possible advection rate. Had the satellite buoy tracks been available for real-time use at the time, their results would have profoundly influenced the design of the cruises. The initial transect in January passed close enough to Fred's centre to verify its position (Fig. 5a), while further to the southeast a second eddy (codenamed Golf) was observed with its centre located more or less at 31°20'S, 38°E. The depth of the isotherm in January suggested that Fred had remained in about the same position between the surveys. However, it seems that during the intervening 6 weeks Fred had either significantly reduced its size, or had rotated its axes. It was considered unlikely that the eddy's surface area had shrunk within this period to less than one third of its original size, and it was concluded that Fred had rotated its axes, a feature that seems not uncommon in eddies of this kind (e.g. SPENCEand LEGECKIS, 1981; GRONDLINGH, 1985a).
95
Cyclogenesis in the Mozambique Ridge Currcnt
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Eddy Golf was situated approximately 200 km southeast of Fred. This was only the second time that two eddies had been observed simultaneously in this area (in August 1975, the co-existence of two eddies was postulated by GRUNDLINGIt, 1977). The shape of the 10°C topography (Fig. 5a) suggested that Sta. 15 of the January cruise was situated close to Golf's centre. The volume transport of Golf was about 12 x 10~' s-l, while the total kinetic energy value of 3 x 1014 J was obtained by rough extrapolation in view of the partial coverage. The cruise in March 1980 could find no evidence of Fred in the area where it had been observed in December and January (Fig. 5b). As such a rapid decay was not considered realistic, it was concluded that Fred had moved from its original position. Farther to the southeast an eddy was located and partially surveyed at 31°20'S, 38°45'E, where the 10°C isotherm rose to 460 db. The proximity of this eddy to the position where Golf had been observed 2 months previously suggested they were the same. If this was the case, it had moved approximately 70 km to the east since January, and its diameter was now an estimated 120 km. Although the gradient velocity maxima in Golf was reduced to 20-30 cm s-~ in March (compared to 20-40 cm s-~ during January), the volume transport (14-18 x 106 m 3 s-1) and total kinetic energy ( - 4 x 1014 J) were higher than in January. However, in the light of the incomplete coverages on both occasions, these slight variations were not considered significant. S A T E L L I T E BUOY T R A C K S
The period during which the buoys (code numbers 14621, 14622 and 14627) were active in the area can be separated into two parts. The first part covers the period from the time the buoys entered the area (Fig. 6) up to 31 November 1979; this was chosen to represent the circulation preceding the cruises of the R.V. Meiring Naud(. The second period (Fig. 7), from 1 December 1979 to 31 January 1980, provided insight into the flow patterns
96
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reigning at the time of the hydrographic surveys, as well as into the possible generating mechanism of the eddies. The buoys under discussion either failed or left the area between or soon after 31 January 1980.
Period I: March-November 1979 (Fig. 6) Buoy 14627 was trapped in a translating cyclonic eddy south of Madagascar during May and June 1979 before leaving the eddy at the beginning of July. The eddy seems to have originated from the area southeast of Madagascar (possibly generated by the East
97
Cyclogenesis in the Mozambique Ridge Current
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Fig, 7. Drift tracks of satellite-tracked buoys in the southwestern Indian Ocean in December 1979 and January 1981). Station positions of the Meiring Naudd during these months have been included.
Madagascar Current), and the buoy displayed a regular rotation rate of once every 4-6 d a y s (LuTJEHARMSet al., 1981). The buoy was subsequently carried northwestwards into the Mozambique Channel before turning southwards along the African continent. Its course resembled the path of a satellite-tracked buoy deployed in 1975 (GRt)NOLtNGH, 1977). As in the case of that buoy, 14627 did not follow the "coastal" route but moved away from the coast at 28°S and drifted southwards along the Mozambique Ridge. During October the buoy became entrained in a cyclonic eddy east of the Mozambique Ridge at 31°S, 38°E, and completed
98
M.L. Gr0ni>t.In~;H
three revolutions at an average speed of 55 cm s-' before leaving the eddy in a northeasterly direction in November. The orbits of the buoy coincided closely (Fig. 8) with the position where Golf was observed in January 1980 (that is, 2.5 months later). Considering the slow movement of Golf between January and March, it was concluded that buoy 14627 had in fact been trapped in Golf in October 1979. This would suggest that Golf was at least 5 months old at the time of the March 1980 cruise. Buoy 14622 completed a large oval-shaped circuit at 29°S, 43°E in April 1979 before drifting steadily westwards over the Mozambique Ridge. It remained there for 2 months, moving in a seemingly irregular pattern at first and then in a general westerly direction toward the African continent. Apart from the delineation of Golf, of importance here is the behavior of the buoys in a particular area shared by boti] buoys but at different times. Toward the end of April 1979, buoy 14622 drifted through the region of 30°S, 37°E (Fig. 6) that was also traversed by buoy 14627 in almost the same direction in October. The flow direction (southeast) derived from these buoy tracks at this point was exactly opposite to the direction of the current (northwest) recorded by the ship survey of Fred in December (e.g. Fig. 8). This
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Fig. 8. Selected tracks of buoys deployed in 1975 (1116, see GRUNDI,IN(IH, 1977) and 1979~ superimposed on the 10°C topography of eddies Fred (December 1979, Fig. 3a) and Golf (January 1980, Fig. 5a). The dashed line represents the computed track of a steady-state inertial jet with conservative potential vorticity. The dot on this track indicates the separation point of two sets of model parameters (see text for further details).
Cyclogenesis in the Mozambique Ridge Current
99
contradiction suggested that Fred had not been present during either April or October 1979, and only appeared in the period between October and December 1979.
Period II: December 1979-January 1980 (Fig. 7) After buoy 14627 had been ejected from Golf at 31°S, 38°E, it moved northeastwards towards Madagascar in November and then accelerated rapidly westwards in December. By coincidence, buoy 14621 was in the same area at that time (having entered the area from the east), and the two buoys followed more or less the same route during the following 2 months. As both buoys drifted westwards onto the Mozambique Ridge at 27°S, they turned anticlockwise through almost 180° before leaving the Ridge into the Mozambique Basin in an eastsoutheast direction. This was followed by a clockwise circuit at the end of December, through which the buoys returned westwards onto the Ridge in January 1980. The separation of the buoys during their drift in December could be accounted for by a current about 100 km wide. Figure 7 shows that the drift tracks of these two buoys formed an envelope around Fred and Golf. Both buoys passed through the northeastern sector of Fred but failed to become entrained into the eddy itself. Combination of the buoy tracks and the hydrographic results allows inference of some tentative conclusions about the origins and existence of the eddies: Fred. According to the hydrographic data, Fred's peak 10°C elevation was at 350 db, which was higher than any of the eddies observed in this region between 1975 and 1978 (GRuNDLINGH, 1977, 1983a, 1984a, 1985a). This information does not provide a gauge for Fred's age, but it is felt that Fred was generated not long before it was first observed in December 1979. To a certain extent this is corroborated by the failure of the buoys to provide any evidence of Fred in October. It is therefore suggested that Fred had not merely drifted into the position where it was observed in December, but had actually been generated there in the 2-month interval between 1 October and 1 December 1980. Golf. As Golf was observed in October (from the buoy tracks), January and March (from the hydrographic results), it was obviously in existence during the passage of the buoys in December-January, even though its position was not clearly delineated by the buoy tracks (nor, for that matter, was the position of eddy Fred). According to the observations presented above, Golf underwent only minor changes in its position between October and March; it probably remained in approximately the same position during the passage of the buoys through the area. Third eddy. The buoy tracks suggested the existence of a third eddy situated to the east of and adjacent to Golf, at 31°S, 40°E. However, the evidence is considered too scant for unequivocal identification. THE M O Z A M B I Q U E R I D G E C U R R E N T
The buoy tracks seemed to indicate the existence of a current (referred to as the Mozambique Ridge Current, MRC) that is manifested as a quasi-zonal jet flowing westward over the northern Mozambique Basin before meandering in an "S" shape over the Mozambique Ridge. The first (indirect) reference to the MRC was made by HARRIS(1972), who found that a portion of the westward flow between the Madagascar Ridge and the African continent appeared to be diverted by the Mozambique Ridge and recirculated anticyclonically.
100
M.L. GRUNDLINGII
HARRISand VAN FOREEST(1977) reported further evidence of an easterly tributary to the Agulhas Current south of 30%; they estimated its magnitude at 11-19 x 10 6 m 3 s 1 (relative to 2000-4000 m). Applying the inertial-jet model of WARREN(1963) they were able to show the tendency for water emerging southward from the Mozambique Channel to follow either the "coastal" route, leading to the type of observations reported by PEARCE (1977), PEARCEand GRONDLINGtt(1982) and GRUNDLINGH(1983b) off the South African east coast between 28° and 30°S, or an offshore, circuituous path along the Mozambique Ridge to 30°S. Here the MRC turns inshore (that is, westward) to join the coastal branch. GRUNDEINGH (1985b) indicated that the MRC transported about 36 × 10¢' m 3 s-~ in 1981 (confirming that this current unambiguously has a sufficient dynamic signature to create intense eddies) and suggested that topographic induction by the Mozambique Ridge could explain the existence of eddies in this region. The influence of the Mozambique Ridge on the circulation in the southwest Indian Ocean seems to be indicated quite clearly by the pattern of the satellite buoy tracks in the area. The S-shaped meandering of the tracks (see Fig. 6 and especially 7 and 8) is reminiscent of a steady-state current (MRC) in which potential vorticity is conserved while it flows over significant variations of the bottom depth: as the current flows westward (at 27°S) across the Mozambique Ridge (depth -2000 m) the vorticity of the current decreases (that is, it becomes more anticyclonic) and the current turns anticlockwise back into the Basin, The sharp increase of the depth from the Ridge into the Basin (-5000 m) increases the vorticity, causing the current to return clockwise onto the Ridge. This stationary situation obviously oversimplifies the characteristics of a variable flow regime, but nevertheless elucidates some of the factors controlling the circulation. To test the hypothesis of topographic control on such a current, and thus to indicate that the buoy tracks could be a realistic representation of the MRC, the simple model of WARREN (1963) was applied. This model was also employed by DARBYSHIRE(1972), HARRIS and VAN FOREEST (1977), GRONDLINGH (1978) and LUTJEttARMSand VAN BALLEGOOYEN (1984) to explain some of the features of the Agulhas Current System. The model has some well-known deficiencies, amongst others the consideration of only steady-state conditions. It therefore provides merely a first-order insight into the true circulation, and cannot address more detailed aspects of the flow. In the model, the curvature C(x,y) of an inertial current at co-ordinates (x,y) is related to the curvature G~ at a previous point (x0,yo), the volume transport V, the momentum transport M and the volume transport T across the unit depth near the bottom, the Coriolis parameterf and its change with latitude [3 = Of/Oy, the initial depth Do(xo,yo) and a subsequent depth D(x,y) at the position under discussion by C(x,y) = Co(xo,yo) - f$[V(y - Yo) + fT(D-Do)]/M. Using the Runge-Kutta method, the curvature was derived numerically and then integrated piecemeal from (xo,y0) onwards to produce the current path. In the present study, values of ~1/M and TIM were not available a priori, and were chosen more or less arbitrarily to simulate the drift tracks of the buoys during December 1979 and January 1980 (Fig. 8). The computation was initiated at 27°30'S, 41°E, where the tracks seemed to show a point of inflection (that is, Co = 0) and the "best fit'! of the model to the observed paths of the buoys was obtained visually (Fig. 8). It was found, firstly, that the model could successfully simulate most of the buoy tracks during December and January. This confirmed that the buoys had been embedded in a
Cyclogenesisin the MozambiqueRidge Current
101
current (the MRC) that conformed more or less to the requirements of an inertial jet, and were not merely blown around by the wind to (fortuitously) produce the resulting tracks. Secondly, two sets of parameters were required to duplicate the MRC: North of 29°S, T/M and V/M equalled 0.55 x 10 -4 ( m 2 s-I) -1 and 3.5 (m s-l) -I respectively, while south of this latitude the values were 1.2 x 10-a (m 2 s-l) -l and 3.5 (m s-l) -l, respectively (Fig. 8). Thirdly, the model was unable to simulate the tracks of the buoys in the elongated cyclonic loop at 30°-33°S, 38°-42°E for any values of V/M and T/M. It was therefore concluded that the southeasterly, cyclonic loop of the buoys' drift was incompatible with the predicted course of the MRC. The model predicted a much tighter clockwise turn at 30°S, 38°E where eddy Fred was observed. A plausible explanation for the behaviour of the buoys (that is, their separation from the predicted course of the MRC) could be situated in the presence of an additional eddy besides Fred. The proximity of such an eddy, possibly combined with other factors such as wind, could have caused the buoys to be detrained from the MRC and entrained around the second eddy (Golf). This hypothesis is supported by the sudden change in drift course of buoy 14627 in the eastern perimeter of Fred (Figs 7 and 8). From hydrographic sections of the R.V. Meiring Naudd obtained in 1981 (see GRt?NDUNGH, 1985a), values of V (26 x 10 6 m3 s-I) and M (107 m 4 S-2) were estimated; they produced a slightly lower value of V/M (2.5 m-~ s) than the one used in the model. Using the velocity distribution of NIILER and ROBINSON (1967), the model predicted surface velocities of 1.0-1.2 m s-~, which are higher but of the same order as the buoy drift speeds (Fig. 8) that varied mainly between 30 and 100 cm s-I (Fig. 7). It is possible at this stage to combine the different sets of observations and to consider a coherent method of cyclogenesis for eddies such as Fred and Golf. By exploiting some of the points of agreement between Gulf Stream rings and the eddies described here, an indication of the generating mechanism of eddies such as Fred and Golf may be gleaned. The process by which cyclonic Gulf Stream rings are created has been well documented (e.g. FUGHSTER, 1972; GOTTHARDT, 1973; DOBLAR and CHENEV, 1977) and consists basically of the occlusion of an extended loop of the Gulf Stream. This process has also been observed in the Brazil Current (LEGECKISand GORDON, 1982) and in the southern Agulhas Current (LuTJEttARMS, 1981). As far as the results of the present study are concerned, there is no reason to believe that the current loops (both cyclonic and anticyclonic) visible in Fig. 8 cannot be similarly occluded. Occlusion of the clockwise loop in Fig. 8 would sever a cyclonic eddy (e.g. Fred) from the MRC, after which MRC retracts onto the Mozambique Ridge. This hypothesis is supported by the track of buoy 14622 in the period 1 April-1 July 1979 (Fig. 6). This buoy described a track similar to those of the other buoys, with an important exception: it remained on the Mozambique Ridge at 31°S, 36°E. Buoy 1116, tracked in 1975 (GR~JNDLINGH, 1977) described a very similar pattern to that of buoy 14622 (Fig. 8), with an eddy located in almost the same position as Fred. At the time, the relation between the eddy (which was delineated by ship observations) and the drift pattern of the buoy on the Ridge was not recognized, but the composite ship and buoy results (Fig. 8) now seem to present a logical arrangement of the flow patterns. In the light of the results presented above it is believed that there is strong evidence suggesting that the eddies are generated by an occlusion of a cyclonic loop of the MRC. Anticyclonic eddies originating from the northerly part of the Spattern should also be possible, in fact, they have been observed (GRt?NDLINGH, in preparation). These results, along with those of GRONDHNGH(1985b) and other s, support
102
M . L . GR()NDLIN¢Itt
an in situ generation mechanism for these cyclonic eddies, and tend to exclude the possibility of a remote cyclogenesis (e.g. southeast of Madagascar or from the Agulhas Return Current). If the arguments above are valid, a rough estimate can be made of the age of Golf. Assuming that the drift of buoy 14622 between April and July 1979 delineated a manifestation of the MRC from which an eddy has been separated through occlusion, the eddy itself would have been situated approximately where Golf was observed from the buoy tracks in October (Fig. 6). It is therefore quite possible that Golf was the same eddy generated by the MRC in the period April-July 1979. For this to have occurred, the eddy created in April-July would have had to survive for at least 6 months. Although dissipation rates are not available for these eddies, Gulf Stream rings have been found to collapse their elevated cold-water core at a rate of 0.4-1.0 m day ~ (PARKER, 1971; GOTTttARDT, 1973; CnENEY and RICItARDSON, 1976). In terms of kinetic energy dissipation, CHENEYand RICHARDSON(1976) found a decay rate of 2 x 10 t2 J day-j for Gulf Stream rings. Assuming that Golf initially has the same density structure and energy as Fred, it would have been 80-200 days old (based on core collapse rates) and about 700 days old (based on energy dissipation rates) at the time of its observation in January 1980. It is, therefore, quite possible that Golf was occluded from the MRC in April-July 1979, as postulated by the buoy tracks. CONCLUSIONS
In previous surveys (e.g. GR13NDLINGtt, 1977, 1983a) during which cyclonic eddies were located in this region, the impression was gained that the eddies occur only one at a time. This illusion can probably be ascribed to the limited coverage possible with a small vessel. The present results focused attention on the main eddy, Fred, but managed to delineate the presence of at least another eddy in the close proximity. With lifetimes of several months, more eddies could exist simultaneously, and theoretically these eddies could have an abundance compatible with that of Gulf Stream rings (e.g. PARKER, 1971). However, no information is available to determine the persistence of the MRC, and it is suspected that the eddy generation rate may be low (a few eddies per year). Whether or not Golf was as old as suggested above, the existence of intense eddies can have a profound influence on the circulation in the southwest Indian Ocean. The data presented above suggest that the MRC is discontinuous, which may lead to a situation where occluded eddies completely "absorb" the flow of the MRC. The propagation and fate of the eddies are therefore important factors to determine the amount of MRC water that reaches the Agulhas Current. According to DUNCAN(1970) and WYRTKI(1971), the steady-state flow in this region is toward the northeast. The results of GRONDLINGtt (1985b) point toward a much more variable flow. By trapping an amount of water originating from the MRC, eddies can thus prevent the contribution from this current to reach the Agulhas Current or hold it in abeyance for some time. Acknowledgements--1 thank the typing and draughting groups of the NRIO and especially the officers and crew of the Meiring Naudd for their support. REFERENCES CItENE-Y R. E. and P. L. RICHARDSON (1976) Observed decay of a cyclonic Gulf Stream ring. Deep-Sea Research, 23, 143-155.
Cyclogenesis in the Mozambique Ridge Current
103
DARB"tStlIRI-;J. (1972) The effect of bottom topography on the Agulhas Current. Pure and Applied Geophysics, 101,208-220. DOBLAIt R. A. and R. E. CttENEY (1977) Observed formation of a Gull Stream cold-core ring. Journal <~/
Physical Oceanography, 7,944--946. DtJNOAN C. P. (1970) The Agulhas Current. Ph.D. Thesis, University of Hawaii, 76 pp. EERM (Establissement d'Etudes et de Rechcrches Metcorologiques) (1979) Project Marisondc, Second dossier. Direction de la Meteorologic, Ministere des Transportes, France, 53 pp. Ft~GI.ISHZR F. C. (1972) Cyclonic rings lk)rmed by the Gulf Stream. In: Studies in physical oceano~raplLv, A. L. G()RI)ON, editor, Gordon and Breach, London, pp. 137-168. Go-H IIARDT G. A. (1973) Gulf Stream eddies in the western North Atlantic. US Nawil Oceanographic Oflice Report TN 6150-11>-73, 46 pp. GRON~)LINGli M. L. (1977) Drift obserwltions from NIMBUS VI satellite-tracked buoys in the southwest Indian Ocean. Deep-Sea Research, 24, 9/)3-913. GRtTNnLIN(Itt M. L. (1978) Drift of a satellite-tracked buoy in the southern Agulhas Current and Agulhas Return Current. Deep-Sea Research, 25, 1209-1224. GRLTNDLIN(III M. L. (1980) On the volume transport of the Agulhas Current. De¢7~-SeaResearch, 27,557-563. GRUNI)LINGit M. L. (1983a) Eddies in the southern Indian Ocean lind the Agulhas Current. In: Eddies in marine science, A. R. ROBINSON, editor, Springer Verlag, Berlin, pp. 245-264. GI'~.tINDLING]t M. L. (1983b) On the course of the Agulhas Current. Soltlh A.l)ican Geographical Jottrnal, 65, 49-57. GRONDLINGH M. L. (1984a) An eddy over the northern Mozambique Ridge. South African Journal ~fScience, 80,324-329. GRUNDI.INGII M. L. (1984b) A free-drifting current meter array in a cyclonic eddy of the Mozambique Ridge Current. South African CSIR Technical Report T/SEA 8501, 24 pp. GR~rNI)I.INGII M. L. (1985a) An intense cyclonic eddy east of the Mozambique Ridge. Jottrnal (>["Geophysical Research, 90, 7163-7167. GRUNI)LINGH M. L. (1985b) Features of the circulation in the Mozambique Basin in 1981. Journal ~>[Marine Research, 43,779-792. GR{!NDLINGtI M. L. and A. F. PEARCE (1984) Large vortices in the northern Agulhas Current. Deep-Sea Research, 31, 1I49-1156. HARRIS T. F. W. (1970) Features of the surface currents in the southwest Indian Ocean. CSIR Symposium Oceanography in South Africa, Durban, 13 pp. HARRIST. F. W. (1972) Sources of the Agulhas Current in the spring of 1964. Deep-Sea Research, 19,633-65/). HARRIS T. F. W. and D. VAN FOREL-ST(1977) The Agulhas Current system. Department of Oceanography, University of Cape Town, 38 pp. LE~G[~CKISR. lind A. L. GOROON (1982) Satellite observations of the Brazil lind Falkland Currents, 1975-1976. and 1978. Deep-Sea Research, 29, 375-4[)1. LtIT,n~HARMSJ. R. E. (1972) A quantitative assessment of year-to-ye~lr w~riability in water movement in The southwest Indian Ocean. Nature. Physical Science, 239, 59-60. Ltrr,H~ltARMS J. R. E. (1981) Features of the Southern Agulhas Current circulation from satellite remote sensing. South African Journal ~r Science, 77,231-236. LUT.n~tiARMSJ. R. E. and R. VAN BALLEGOOYEN(1984) Topographic control in the Agulhas Current system. De~7~-Sea Research, 31, 1321-1337. LUTJNtARMS J. R. E., N. D. BANGand C. P. DUNCAN ( 1981 ) Characteristics of the currents south and east of Madagascar. Deep-Sea Research, 28,879-899. NULt~R P. P. and A. R. ROB[NSON(1967) The theory of free inerti~ll jets. A numerical experiment for the path of the Gulf Stream. Tellus, 19, 6/)1-619. PARKt~RC. E. (1971) Gulf Stream rings in the Sargasso Sea. Deep-Sea Research, 18, 981-994. PliAR('~ A. F. (1977) Some features of the upper 500 m of the Agulhas Current. Joitr#tal of Marine Research, 35,731-753. PI~ARCt-~A. F. and M. L. GR'~INDLINGIt(1982) Is there a seasonal wlriation in the Agulhas Current'? Journal ~J[" Marine Research, 40, 177-184. SVt~NCi~T. W. lind R. Ll-:C;t~CKiS( 1981 ) Sate[litc and hydrographic observations of low-frequency wave motions associated with a cold-core Gulf Stream ring. Journal ~["Geophysical Research, 86, 1945-1953. WARRrN B. A. 11963)Topographic influences on the path of the Gulf Stream. Tellus, 15, 167-183. WYRTKI K. 11971) Atlas of the International Indian Ocean Expedition. National Science Foundation, Washington, DC, 531 pp.