Surface currents derived from satellite-tracked buoys off Namibia

Deep-Sea Research II 46 (1999) 453—473

Surface currents derived from satellite-tracked buoys off Namibia Marten L. Gru¨ndlingh* CSIR, P.O. Box 320, Stellenbosch 7599, South Africa Received 22 September 1997; received in revised form 4 February 1998

Abstract To study the flow field off Namibia (20—30°S, 10—15°E), 48 satellite-tracked buoys were deployed and tracked in six bimonthly batches between July 1994 to September 1995. In situ supporting wind information was collected from a weather buoy moored off Lu¨deritz, from coastal stations and from voluntary observing ships. Buoy drift tracks were compared with surface topography data from the TOPEX/POSEIDON satellite and satellite infrared images. Most of the buoys drifted in a northwesterly direction, the buoys deployed in the south generally moving faster and diverging more from the coast than the northern buoys. The overall maximum daily drift velocity was 72 cm s\, but typical speeds were 10—30 cm s\. In the proximity of the coast some buoys experienced transient southward sets associated with the effect of coastal trapped waves, while tracks north of 23°S showed inertial oscillations.  1999 Elsevier Science Ltd. All rights reserved.

1. Introduction Fish production in the Namibian offshore region amounted to more than 200 million tonnes in 1991. In addition, there is continuous exploration for oil and gas at various locations off this coast. The fishing and exploration activities are potentially in conflict because of possible oil spills or other environmental impacts, and detailed environmental information is required for contingency planning. Information on currents and winds is particularly sparse off the Namibian coast. To enhance the knowledge of surface currents in the region, a programme was designed to deploy 48 satellite-tracked buoys at two locations off the Namibian coast and track each of them for 1—2 months. To investigate the forcing of the currents a weather buoy was to be deployed in the proximity of the coast, and wind data from coastal sites obtained. * Fax: 0027 21 888 2693; e-mail: [email protected] 0967-0645/99/$ — see front matter  1999 Elsevier Science Ltd. All rights reserved. PII: S 0 9 6 7 - 0 6 4 5 ( 9 8 ) 0 0 0 9 6 - 4

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2. Study area 2.1. Bottom topography The Namibian coastline runs in a general SSE—NNW direction, with virtually no significant coastal promontories (Fig. 1). A dominant feature of the Namibian offshore area is the Walvis Ridge, which presents a barrier to any meridional water movement below 3000 m (Shannon, 1985; Boyd, 1987). A significant narrowing of the continental shelf north of the present target area (i.e. in the vicinity of the Namibian—Angolan border) allows southward-moving water of equatorial origin to approach close inshore (Boyd et al., 1987). From 20°S the shelf slowly widens southward, reaching a maximum of about 100 km in the vicinity of Walvis Bay (Fig. 1). Off Lu¨deritz the shelf is much narrower, but widens dramatically at Oranjemund to about 200 km. 2.2. Wind The large-scale wind field is caused by the presence of a high pressure over the South Atlantic in conjunction with the pressure field over the southern African subcontinent (Shannon, 1985). On average, the high moves from a southerly position and close to the coast in summer (30°S, 5°W), to further north and offshore in winter (26°S, 10°W) (Van Loon, 1972). The anticyclonic circulation around the high causes

Fig. 1. Chart of the study area off the coast of Namibia. Bottom topography and coastline was derived from Gebco (1994). The nominal deployment positions of the satellite buoys are indicated off Walvis Bay (northern batches) and Oranjemund (southern batches). The position of the weather buoy is indicated off Lu¨deritz.

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south-southeasterly wind (mainly parallel to the coast) along its eastern flank (the coastal region of Namibia). In general, the wind speed is greater further offshore (Shannon, 1985), reaching a maximum about 300 km from the coast. A maximum of about 4 m s\ wind speed occurs in spring (October), with another peak in autumn (April) over much of the region (Boyd, 1987). Winds vary on a 5—12 day cycle (Boyd, 1987; Kamstra, 1987), associated with the generation and movement of coastal lows. These cells of about 100—150 km in diameter are generated mainly along the Namibian coast (Hunter, 1987) and propagate poleward as trapped atmospheric waves (Gill, 1977) at typically 6 m s\ (Presston-Whyte and Tyson, 1973; Jury et al., 1990). Their effect is to interrupt the long-term, southerly wind by introducing transient winds from the north, northwest or west. On a diurnal time scale, a distinct land—sea breeze occasionally extends seaward by more than 100 km (Hart and Currie, cited by Shannon, 1985). 2.3. Upwelling Upwelling plays an important role in the oceanography of the Namibian coast. It is enhanced by steep bathymetry (e. g. at 18°S and 27°S) and restrained by the gradually sloping bottom (e.g. at 22°S). The longshore or ‘‘upwelling’’ wind stress (Boyd, 1987) has two maxima located at 16—18°S, and 24—28°S, respectively. The southern upwelling cell off Lu¨deritz is the main one, occuring '90% of the time and extending to an average of 280 km offshore (Lutjeharms and Meeuwis, 1987; Lutjeharms and Stockton, 1987). Upwelling filaments — surface streamers of cold water typically 50 km wide and extending beyond the normal offshore extent of the upwelling cell — with lifetimes between five days and five weeks, tend to amalgamate to form broader plumes and also to shed eddies along the frontal zones (Stockton and Lutjeharms, 1988). Westward currents up to 40 cm s\ have been reported between the coast and 200 km offshore during events of intense upwelling (Lutjeharms et al., 1991; see also Shillington et al., 1990). 2.4. Currents A comprehensive review of macroscale current conditions (Shannon, 1985) shows a general northwestward flow (the Benguela Current) with typical speeds of 10—20 cm s\ extending northward to about 20°S before veering westward (Fig. 2). This offshore divergence of the flow is also portrayed in geostrophic currents (Stramma and Peterson, 1989). The average drift rate of satellite buoys tracked during the late 1970s in the region 24—28°S, 12—16°S was 18 cm s\ toward 325°T (Piola et al., 1987; Johnson, 1989). The Benguela Current can also form a ‘‘conveyor belt’’ for large anticyclonic eddies generated by the Agulhas Current Retroflection between 35 and 40°S (Duncombe Rae, 1991). These eddies can entrain coastal waters to a considerable distance offshore and are quite intense; surface speeds up to 60 cm s\ have been observed, (Lutjeharms et al., 1991). The effect of the eddies will be to add a sporadic, variable component to the rather sluggish background flow.

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Fig. 2. Surface currents in the southeast Atlantic Ocean (from Shannon, 1985).

Closer inshore, the shelf currents veer offshore at 24°S and onshore at 22°S (north of Walvis Bay), with some eddy motion west of Walvis Bay (Barange and Boyd, 1992). Drogues deployed in the proximity of the coast showed that the dominant drift direction was toward the NNW with velocities up to 40 cm s\ during strong southerly winds, and with more variable direction under low velocities ((20 cm s\)

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(Boyd and Agenbag, cited by Shannon, 1985). The strongest currents were at 18°S and directed slightly more offshore. Small anticyclonic eddies with radii of 25—60 km and current speeds of 15—35 cm s\ have been reported off central and southern Namibia (Salat et al., 1992). These speeds were confirmed by current meter data (Hagen, 1979; Hagen et al., 1981).

3. Methods The field data collection programme extended from August 1994 to August 1995, and consisted of deployment of satellite-tracked buoys and collection of supporting weather and other data. 3.1. Wind A 3 m diameter discus weather buoy was deployed approximately 10 km off Lu¨deritz to obtain wind data in the area beyond the coastal regime (see Fig. 1). The buoy recorded hourly, vector-averaged, wind speed and direction, and 1 min gusts, at a height of 2 m above sea level. Data recording started on 28 September 1994 and continued with interruptions owing to breakages of the moorings until 31 August 1995. To augment the weather data collected in the proximity to the coast, other wind data sets were obtained from automatic weather stations (see Fig. 1) at Elizabeth Bay, Kolmans Kop (a few kilometres inland of Lu¨deritz) and Mo¨we Bay, and from data from voluntary observing ships (VOS). The individual data sets were not adjusted to a common height. 3.2. Free-drifting buoys The Argodrifter buoy, based on the CODE (coastal dynamics experiment) buoy design (Davis, 1985a, b, c) was used to capture the surface (0—1 m) currents. It consists of a 1 m long, 9 cm diameter PVC canister with a satellite transmitter. Attached to the canister in a radial fashion are four vertical drogues of acrylic cloth each supported by PVC struts at the top and the bottom. The buoy’s exposure to wind drag is minimal, it has good surface following capability, a large righting moment, and large vanes to ensure good current-following capabilities at all angles to the current. Comparison with in situ current meter data (Weller and Davis, 1980) indicated that the velocity derived from the tracking of the buoys would be accurate to about 3 cm s\, even under strong wind conditions. The Stokes’ drift for a 3 m, 12 s swell is about the same value, while the buoys’ drifts were an order of magnitude larger (see Section 4.2). It is therefore assumed that the buoys’ drifts were representative of the surface Ekman currents. The ‘‘lifetime’’ of all the buoys used in the investigation was preset to either one or two months, after which transmission stopped automatically. Forty-eight buoys were deployed by ship or helicopter, half off Oranjemund in the south and half off Swakopmund just north of Walvis Bay (Fig. 1). The nominal

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Table 1 Details of the buoys tracke Deployment

Buoy lifetime (days)

Number

Date

5 km

15 km

25 km

35 km

1

N: 1 August 94 S: 27 July 94

58 29

60 29

48 30

60 0

2

N: 9 October 94 S: 5 October 94

6 41

30 40

24 46

25 60

3

N: 5 December 94 S: 29 November 94

5 2

30 3

31 39

31 60

4

N: 6 February 95 S: 16 February 95

2 5

30 30

29 29

30 40

5

N: 9 May 95 S: 18 May 95

30 2

16 19

16 18

17 34

6

N: 1 August 95 S: 7 July 95

12 57

10 20

6 50

1 58

N: Walvis Bay; S: Oranjemund. Nominal deployment distance offshore.

deployment positions were 5, 15, 25 and 35 km offshore, although these, as well as the exact position along the coast, varied depending on conditions at the time. A total of 1349 buoy-days were obtained, or an average of 28 days per buoy (see Table 1). The buoy position fixes were checked by determining the drift rate between successive fixes, and eliminating outliers in the track plots. The average number of position fixes per buoy-day was 7.3 after editing. 3.3. Satellite data Geostrophic currents were derived from the TOPEX/POSEIDON satellite altimeter, which measured the sea-surface elevation at intervals of about 7 km along the satellite track. Tracks are covered once every 10 days (referred to as a cycle) but neighbouring, parallel tracks are 220 km apart in the area of interest, and this negatively affects the spatial resolution that can be obtained from the data. The standard processing of these data was undertaken for the period June 1994—June 1995. Previous analyses of this data type have shown (e.g. Van Ballegooyen et al., 1994) that one year’s data are sufficient to remove the geoidal signal from the data. NOAA infrared satellite imagery has a subsatellite spatial resolution of about 1 km;1 km, and an accuracy of better than 1°C, making it very suitable for detection of sea-surface temperature patterns. The data were received directly from the satellite by the Satellite Applications Centre, Pretoria, where they were processed, calibrated and geometrically corrected. Although satellite images are available daily, a small

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number were selected on the basis of low cloud cover. Of these, only the data of 6 August 1995 (during deployment 6, see Table 1) will be presented. 4. Results 4.1. Spatial and temporal wind variation On average, the wind speeds recorded by the weather buoy and the weather stations at Elizabeth Bay and Kolmans Kop were all very similar (see example, Fig. 3). The VOS data (not shown) were very noisy. The median speed of the VOS data was 7.6 m s\, compared to 7.3 m s\ at the weather buoy and 7.6 m s\ at Elizabeth Bay (Fig. 4). The wind speeds at Mo¨we Bay (see e.g. Fig. 3) were generally lower (median 5.3 m s\), an aspect that also has been reported by Boyd (1987). At Kolmans Kop the median was 5.7 m s\, but it may be that the local topography and the inland location of the site contributed to this lower median. The general wind direction was southerly at all the coastal stations, with little wind in the other sectors. The wind direction at the weather buoy was mostly in 150—160°T (SSE), an anticlockwise shift of about 22° relative to the coastal stations. This swing in the direction at the coast can be ascribed to the refraction of the wind field associated with the coastal topography. The VOS winds were on average slightly more southeasterly than the weather buoy, suggesting that the wind direction rotated from southeasterly over the deep sea to southerly over the land. The data set with the most significant diurnal signal was Kolmans Kop (mainly in speed and to a lesser extent in direction) (e.g. Fig. 3). A typical daily cycle comprised an increase in wind speed starting from a minimum before sunrise, probably when the land temperatures are at a minimum, rising to a maximum at about 15 : 00—16 : 00 local time, before decreasing again. The maximum peak-to-trough amplitude of the wind speed was 14 m s\. The wind speed variation was normally accompanied by wind direction variations of less than 40°. The maximum peak-to-trough diurnal speed variation at Elizabeth Bay was around 6 m s\ (although typical diurnal variations were about half that value) with directional variations similar to those at Kolmans Kop. At Mo¨we Bay the diurnal variations in the wind speed were about 1—2 m s\ (e.g., Fig. 3). The diurnal variations at the weather buoy were weakly defined (about 1—2 m s\), suggesting that the ability of the weather buoy to detect the land—sea breeze may have been impaired by the valleys and ridges of the swell or that the marine boundary layer did not quite reach down to the anemometer height (2 m) on the buoy. 4.2. Combined buoy tracks The general, large-scale tendency of all the buoy drifts was toward the northwest (Fig. 5). The maximum daily drift speeds for the 6 deployments were 34, 32, 64, 72, 40 and 45 cm s\, respectively, all recorded by buoys from the southerly batches.

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Fig. 3. Time series of wind speed and direction for January 1995 from the automatic weather stations at (from top to bottom) Mo¨we Bay, Kolmans Kop (near Lu¨deritz), the weather buoy and Elizabeth Bay (see Fig. 1 for location). In the speed/gust panels, the larger values are the gusts. Vertical time lines are midnight.

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Fig. 4. Time series of air temperature (top), wind speed and direction at Elizabeth Bay, and wind speed and direction at the weather buoy, May 1995. Arrows mark two occasions of catabatic wind conditions.

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Fig. 5. Tracks of all buoys deployed and tracked between July 1994 and September 1995. Six blocks were selected to investigate possible effects of land—sea breezes on buoy tracks in the proximity of the coast.

However, there is considerable variability from buoy to buoy and from batch to batch, and a few of these features can be mentioned: Firstly, the northern buoys tended to reflect far greater coherency in their tracks than the southerly buoys, signifying a much more uniform flow field. This may be ascribed to the southerly buoys drifting through the intense Lu¨deritz upwelling cell, where the wind-driven flow is more dynamic (compared to the northerly area, cf. Section 4.1) and where current filaments are known to carry the water along relatively narrow ribbons into the deep sea. Secondly, there is a tendency of the northern buoy tracks to display small (less than 10 km diameter) cyclic loops (discussed further in Section 4.3). Although these loops are visible along the southern buoy tracks too (e.g., along the buoy track penetrating furthest westward), they are not as prolific. Thirdly, the southern buoys seem to show more ‘‘large-scale’’ (of the order of 100 km) features than the northerly buoys. These are located further offshore and may be purely a consequence of the fact that the southerly buoys penetrated further into the deep sea than the northerly buoys (they had longer lifetimes, cf. Table 1). Fourthly, indications that the southern buoys may have followed ‘‘preferred’’ routes are not considered sufficiently robust, since the ‘‘routes’’ were followed by buoys forming part of the same deployment batch and therefore exposed to the same wind and current conditions. Finally, it is clear that the buoy tracks did not provide adequate insight into the shelf flow in the region between 23° and 26°S. It is nevertheless interesting to note that

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Fig. 6. Flow vectors derived through smoothed gridding of all the daily drift velocities.

one of the buoys of the northern batch (deployment 4) that moved southward in this region, and a southern buoy that moved northward into this area (deployment 6) both swung offshore at the same location, suggesting a degree of confluence (cf. the observations of Barange and Boyd, 1992). The day-by-day drift velocities of each buoy were calculated by fitting straight lines to the changes in latitude and longitude, respectively, that occurred with time over these periods, and combining these north—south and east—west drift components. These components were spatially gridded, using a method of Kriging, on a grid with mesh size of 30;30. The two (N—S and E—W) gridded sets were then recombined into a single vector field, which was considered an objective analysis of the overall flow field based on the buoy drifts (Fig. 6). Vectors in the areas where coverage was too low have been omitted. However, the remaining vectors are still based on varying number of observations per block, and this would detract from their ability to represent the mean flow field. There is a fair agreement between this portrayal and that of Shannon (1985), accepting that both presentations are based on non-synoptic data, and that they were derived from different measuring mechanisms and therefore probably represent different aspects and depths of the flow. Shannon’s vectors are a composite of results of a large number of investigators, and may be descriptive rather than strictly quantitative (as could be expected from a low-velocity, variable flow field). The Shannon vectors running in a northwesterly direction from Lu¨deritz agree well with the corresponding buoy vectors, although the buoy speeds are higher. The coastal flows

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are also in fair compliance, except that the buoys indicate a southward flow immediately south of Walvis Bay (one buoy observation only). Elsewhere, the Shannon vectors reflect a more northerly set, whereas the buoys tended to move in a more northwesterly and even westerly direction. 4.3. Inertial currents Inertial currents follow circular, anticlockwise courses in the Southern Hemisphere. If the velocity of the water is 20 cm s\, the radius of the circle will be 7.3 km in 22°S and the period 32 h. If the circular motion is combined with some other flow, the resulting motion will be cyclic (spiralling) (see analyses of Poulain et al., 1992; Vihma and Launiainen, 1993). Such cyclic motion was observed in many tracks of the northern buoys, and the tracks of two buoys were selected as examples. The latitude and longitude of the buoy positions were treated as separate time series, which were linearly interpolated and subsampled at hourly intervals. Next, each time series was demeaned and linearly detrended, before being zero-padded to yield a total time series length of 1024 h. Each time series was tapered by a cosine bell window prior to being converted to the frequency domain. Spectral amplitude estimates were corrected for the tapering and smoothed. The northerly and easterly spectra (Fig. 7) show a clear peak around 30 h which was taken as confirmation that the cyclic motion was inertial. Virtually no evidence was found of diurnal or semidiurnal variability.

Fig. 7. North and east energy spectra for buoy C22 963 (December 1994, upper panels) and buoy C22 984 (May 1995, lower panels).

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The spectra of the tracks of both buoys were integrated to estimate the mean sideways displacement (deviation from the ‘‘mean’’ path) of the buoys as a result of this periodicity. These zonal and meridional excursions were combined to provide a mean excursion of 5.6 and 4.9 km, respectively, for the two buoys, which compared well with the estimates above. 4.4. Land—sea breeze effect To investigate the possible effect of land—sea breezes further (the spectral analysis, Section 4.3, failed to reveal any such effect), the combined tracks of all buoys were binned into six coastal boxes (see Fig. 5). In each bin the drift speeds were simply grouped into a ‘‘daytime’’ drift (consisting of the drift from 09 : 00—21 : 00) and a ‘‘nighttime’’ drift (21 : 00—09 : 00). The daytime and nighttime occurrence frequency tables for these blocks were calculated. The results revealed no significant day—night difference in the average direction, average speed, modal directions, or the median speed in the modal direction. It was concluded that although the buoy tracks may contain diurnal variations, they can be disregarded for all practical purposes. In a buoy tracking experiment 13 km off Cape Cross, Boyd (1983) found that a buoy with a drogue at 2 m depth virtually reversed its drift direction as a result of the land—sea breeze. This effect was not reflected by the present data. To determine a possible relation between the wind and the buoy drifts in the coastal region, the modal drift direction and the median drift speed in that direction were derived for the two blocks closest to the weather buoy. The median drift speed was 28 cm s\. Comparison of the modal drift direction (322°T) of the buoys and the modal direction of the wind (155°T) suggests that the buoy drifts were offset by 13° to the left of the direction of the wind stress. This was also reflected by a satellite-tracked buoy (Harris and Shannon, 1979) of which the average drift rate south of 25°S was 24 cm s\ in 328°T. This drift direction was 15° to the left of the cumulative wind stress vectors. 4.5. Comparison buoy tracks vs infrared imagery The infrared image of 6 August 1995 (Fig. 8) shows a band of upwelled water of various temperatures off the Namibian coast. The most conspicuous thermal front (termed ‘‘offshore front’’ in the figure), is located approximately 200 km offshore. It takes the shape of a convoluted line with filaments of 10—50 km in diameter penetrating the deeper ocean. Inshore of this front there seems to be a second front (termed ‘‘inshore front’’ in the figure) of even lower temperature, also showing offshore protrusions. This latter front seems to start off Lu¨deritz and extend up to Walvis Bay or slightly beyond. Buoy C23 000 seems to have followed the inshore front quite well at the time of the image (when the buoy was at 25—26°S). The two buoys swinging offshore at 25°S (C22 999 and C22 998) were located inside a wider part of the upwelled water, and initially followed an offshore filament of the inshore front during their offshore

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Fig. 8. Temperature enhanced, geometrically corrected infrared image from the NOAA satellite on 6 August 1995. Darker shading corresponds with cooler water. Superimposed are the tracks of the southern satellite-tracked buoys of deployment 6.

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excursion (zonal drift at 25°S, 13°E). Buoy C22 999 was eventually entrained into a narrow upwelling ribbon of the offshore front that is visible on the image (Fig. 8). At its base this filament was only about 10 km wide, causing buoy C22 998, located slightly further north at the time, to be diverted northward and join up with the buoy C23 000. It therefore seems that the buoys were drifting in the direction of 335°T, before being diverted westward by upwelling filaments. 4.6. Comparison buoy tracks vs satellite altimetry Altimetry is mainly a deep-sea tool for studying ocean currents, since only in the deep sea can currents be considered geostrophic. In addition, the tidal correction to the altimetric data becomes inaccurate in shallow inshore regions. Also, the varying termination of the satellite tracks close to the coast influences the accuracy of geoidal computations based on the satellite data in these areas. Since the altimetric geostrophic velocity does not include the wind-driven component, the buoys’ drift can be expected to be a combination of the wind drift current and the altimetric current. The results of the altimetric analysis for the full year on a cycle-by-cycle basis (not shown here) indicate that the current speeds are low (generally below 50 cm s\) and variable. In other altimetric studies executed in southern African waters (e.g. Van Ballegooyen et al., 1994) consistency of the altimetric results in their depiction of the surface topography (and hence the currents) was of major assistance in the interpretation of the results. In areas where the oceanographic signal is low, this consistency becomes less evident. Nevertheless, some features were found to correlate reasonably with the tracks of buoys that drifted well beyond the coastal area, and two examples are presented. Example 1. During deployment 2 all southern buoys drifted in a general NW direction up to about 13°E. Soon thereafter, the buoy tracks showed a conspicuous southwestward offset, centred around 26°S, 12.5°E (Fig. 9). At this time (5 November 1994), the altimetric topography (cycle 79) showed the existence of an eddy centred at about 2°S, 12°E, attached to an even larger eddy further to the southwest (Fig. 9). The clockwise flow around this eddy caused southward flow along its eastern flank, and this flow deflected the drift of all the buoys as they crossed the eddy. The geostrophic speed estimated from the surface topography was approximately 23 cm s\ toward SSW. Such a speed would produce the required offset to the buoy tracks, assuming that the wind drift speed was 12 cm s\ toward the NW (using a speed of 10 m s\, as was recorded at Elizabeth Bay at the time and applying the conversion of Daniault et al., 1985). After the buoys had moved northwestward across the eddy, northeasterly flow was encountered and they were offset to the right. Example 2. All four buoys from the northerly batch of deployment 4 drifted southward to a varying degree, buoy C22 982 drifting furthest before turning westward and northwestward at the beginning of March (Fig. 10). The altimetric topography of 4—14 March 1995 (cycle 91) showed that the area in the vicinity of Walvis Bay was occupied by a cyclonic eddy at this time, while to the south (just north of Lu¨deritz) there was an anticyclonic eddy at the coast.

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Fig. 9. Sea surface topography (cm) from the TOPEX/POSEIDON cycle 79 (5—15 November 1994). Negative (shaded) values indicate clockwise circulation, positive (unshaded) values anticlockwise circulation. Tracks of three buoys from deployment 2 (October—November 1994) are indicated.

The westward turn of the buoy track was well aligned with the transition zone between the two eddies. The NW geostrophic speed derived from the altimetry amounted to about 20 cm s\, which, combined with the wind drift current induced by a wind of 8 m s,\ agreed very well with the daily drift vectors of the buoy at this time (about 30 cm s\). The cyclonic eddy remained visible for about a month before disappearing. The altimeter data suggest that the eddy may be a recurring feature of the area. 4.7. Coastal trapped waves A very good description, with case studies, of the characteristics of coastal trapped waves (CTWs) along the southern African coast has been given by Jury et al. (1990) and Schumann and Brink (1990). Briefly, these CTWs are forced by, and travel with, coastal atmospheric lows. The CTW is confined to about 100 km from the coast, with the sea-level perturbation decreasing exponentially away from the coast. As the coastal low moves down the coast the atmospheric pressure ahead of the low starts decreasing, the wind is southeasterly, but may increase in speed and enhance the slight offshore Ekman (wind-driven) transport and lower the sea level at the coast.

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Fig. 10. Sea surface topography (cm) from the TOPEX/POSEIDON cycle 91 (4—14 March 1995). Negative (shaded) values indicate clockwise circulation, positive (unshaded) values anticlockwise circulation. The track of a buoy from deployment 4 (February—March 1995) is indicated.

Immediately after the passage of the coastal low, the wind direction changes abruptly to northwest. Wind speeds will vary according to the propagation speed and intensity of the low, and may reach gale force. The CTW rapidly causes the sea level to reach a maximum elevation (10—50 cm above the steady state level) and the surface current speed is directed southeastward and onshore (speeds of 40 cm s\ have been reported, Hagen, cited by Jury et al., 1990). This stage may lag the passage of the atmospheric low by about one day. Example 1. Between 5 and 10 October 1994 all the buoys in the southern batch experienced a displacement towards the coast — contrary to the expected northwestward drift direction — shortly after deployment. The first parts of the buoy tracks are individually presented in Fig. 11. At this time, a coastal low was moving southward along the coast. Early on 5 October the weather records at the coastal stations in the vicinity of Lu¨deritz showed a perturbation as the wind swung to northwest and then back to south again on 6 October. The behaviour of all the buoys agreed with the expected circulation during the passage of a CTW. As the CTW approached the position of the buoys, the southeastward and onshore flow carried all the buoys coastward for a day or two (Fig. 11). As

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Fig. 11. Partial tracks of the southerly buoys from deployment 2 (October 1994), showing the passage of two coastal trapped waves (CTW and CTW ).  

this situation abated and the general northwestward flow manifested itself again, all the buoys experienced a degree of offshore motion, carrying them away from the coast again. In the process they performed a circuitous, anticlockwise route, ending up further ‘downstream’ (i.e. northwestward) from their position than before the onset of the event. It is interesting to note that the buoy furthest offshore at the time (C22 967, 33 km) showed the smallest effect and virtually no coastward displacement, suggesting that the CTW had not extended that far offshore. The next buoy (C22 975, 24 km) was displaced onshore at almost right angles to the coastal orientation. Buoy C22 970, second closest to the coast (15 km), was displaced at about 45° to the coast, while buoy C22 965, closest to the coast (4.5 km), was displaced almost parallel to the coast. Example 2. Approximately a week after the occurrence of CTW , a similar event  occurred that was reflected best in the drift of buoy C22 965 during the week of 12—19 October 1994 (Fig. 11). This buoy’s northwestward drift was suddenly reversed on 14 October and the buoy transported southeastward in an anticlockwise route (CTW ). The total detour lasted four days, before the buoy arrived on 17 October at  a point virtually downstream from its position before the excursion. The daily drift rate of the buoy was almost identical immediately before and after the event (33 vs.

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26 cm s\, 321°T vs. 323°T ), indicating the transient and symmetric nature of the perturbation. The buoy track also showed apparent inertial motion (period 26 h). Inspection of the wind records showed that the centre of a coastal low passed Lu¨deritz at about 22 : 00 on 13 October, a few hours before the buoy reversed direction. The response of the other buoys in the batch was more confused (Fig. 11).

5. Summary The median speeds at the weather buoy and the coastal stations in its proximity were virtually identical, while the wind direction at the buoy was 150—160°T or about 22° anticlockwise relative to the coastal stations. Winds further offshore were turned slightly more. Wind speed in the area north of Walvis Bay was lower than at Lu¨deritz. Diurnal variations were mainly reflected in the wind-speed records of Kolmans Kop, with wind-speed fluctuations at times as high as 14 m s\. The diurnal signal at the weather buoy was relatively insignificant, suggesting that the marine boundary layer shielded the weather buoy from this wind system. The maximum batch-by-batch daily drift velocities of the buoys during the six deployments varied between 32 and 72 cm s\, in each case recorded by a buoy from the southern batch. A gridded velocity field, derived from all the daily buoy speeds, compared well with historic observations, although the buoy speeds seemed to be higher and oriented more northwesterly. The northerly buoy tracks especially reflected inertial oscillations, consisting of anticlockwise rotations, with circle diameters of about 5 km. Evidence was found where southern buoys followed the upwelling fronts, and became entrained into upwelling filaments. Altimetry showed that the buoy drifts were combinations of geostrophic speeds and wind drift currents. Nine occasions were found where the buoys were affected by shelf waves induced by coastal lows. These consisted of sudden anticlockwise and coastal-directed displacements, with a typical diameter of 20 km within 100 km of the coast.

Acknowledgements We are grateful to the National Petroleum Corporation of Namibia for funding the investigation. The helpful assistance of many members of the Namibian Ministry of Fisheries and Marine Resources is acknowledged, including the officers and crew of the research vessels ¼elwitchia, Matsuyama and Kuiseb. We also thank Namport, Lu¨deritz Harbour for its assistance, and De Beers Marine (Pty) Ltd, Cape Town for the use of its helicopters for the deployment of the drifter buoys. TOPEX/ POSEIDON data was kindly supplied by the Centre National d’Etude Spatiales, France. Dave Smith supervised the field work, and Louise Watt did all the data processing and plotting.

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References Barange, M., Boyd, A.J., 1992. Life history, circulation and maintenance of Nyctiphanes capensis [Euphausiacea] in the northern Benguela upwelling system. South African Journal of Marine Science 12, 95—106. Boyd, A.J., 1983. Intensive study of the currents, winds and hydrology at a coastal site off central South West Africa, June/July 1978. Sea Fisheries Research Institute. Department of Environmental Affairs Investigational Report, 126, 47 pp. Boyd, A.J., 1987. The oceanography of the Namibian shelf. Ph.D. Thesis, University of Cape Town. 247 pp. Boyd, A.J., Salat, J., Maso´, M., 1987. The seasonal intrusion of relatively saline water on the shelf off northern and central Namibia. In: Payne, A.I.L., Gulland, J.A., Brink, K.H. (Eds.), The Benguela and Comparable Ecosystems. South African Journal of Marine Science 5, 107—120. Daniault, N., Blouch, P., Fusey, F., 1985. The use of free-drifting meteorological buoys to study winds and surface currents. Deep-Sea Research 32, 107—113. Davis, R.E., 1985a. Drifter observations of coastal surface currents during CODE: the method and descriptive review. Journal of Geophysical Research 90, 4741—4755. Davis, R.E., 1985b. Drifter observations of coastal surface currents during CODE: The statistical and dynamical views. Journal of Geophysical Research 90, 4756—4772. Davis, R.E., 1985c. Objective mapping by least squares fitting. Journal of Geophysical Research 90, 4773—4777. Duncombe Rae, C.M., 1991. Agulhas retroflection rings in the southeast Atlantic Ocean: a review. South African Journal of Marine Science 11, 327—344. Gebco, 1994. Gebco (General Bathymetric Chart of the Oceans) Digital Atlas. British Oceanographic Data Centre, Merseyside, UK. Manual & CD ROM. Gill, A.E., 1977. Coastally trapped waves in the atmosphere. Quarterly Journal of the Royal Meteorological Society 103, 431—440. Hagen, E., 1979. Zur Dynamik charakteristischer Variationen mit barotropem Charakter in mesoskalen ozeanologischen Feldverteilungen ku¨stennaher Auftriebsgebiete. Geodatische und Geophysikalisches Vero( ffentlichungen 4(29), 1—71. Hagen, E., Schemainda, R., Michelsen, N., Postel, L., Schultz, S., Below, M., 1981. Zur ku¨stennahen Struktur des Kaltwasserauftriebs vor der Ku¨ste Namibias. Geodatische und Geophysikalisches Vero( ffentlichungen, 4(36), 1—99. Harris, T.F.W., Shannon, L.V., 1979. Satellite-tracked drifter in the Benguela Current System. South African Journal of Science 75(7), 316—317. Hunter, I.T., 1987. Weather of the Agulhas Bank and the Cape south coast. M.Sc. Thesis University of Cape Town, 182 pp. Johnson, M.A., 1989. Southern Ocean surface characteristics from FGGE buoys. Journal of Physical Oceanography 19, 696—705. Jury, M.R., MacArthur, C., Reason, C.J.C., 1990. Observations of trapped waves in the atmosphere and ocean along the coast of southern Africa. South African Geographical Journal 72(1), 33—46. Kamstra, F., 1987. Interannual variability in the spectra of the daily surface pressure at four stations in the southern Hemisphere. Tellus 39A, 509—514. Lutjeharms, J.R.E., Shillington, F.A., Duncombe Rae, C.M., 1991. Observation of extreme upwelling filaments in the southeast Atlantic Ocean. Science 253, 774—776. Lutjeharms, J.R.E., Stockton, P.L., 1987. Kinematics of the upwelling front off southern Africa. In: Payne, A.I.L., Gulland, J.A., Brink, K.H. (Eds.), The Benguela and Comparable Ecosystems, South African Journal of Marine Science 5, 35—49. Lutjeharms, J.R.E., Meeuwis, J.M., 1987. The extent and variability of South-east Atlantic upwelling. In: Payne, A.I.L., Gulland, J.A., Brink, K.H. (Eds.), The Benguela and Comparable Ecosystems, South African Journal of Marine Science 5, 51—62. Poulain, P., Luther, D.S., Patzert, W.C., 1992. Deriving inertial wave characteristic from surface drifter velocities: frequency variability in the tropical Pacific. Journal of Geophysical Research 97 (C11), 17947—17959.

M.L. Gru( ndlingh / Deep-Sea Research II 46 (1999) 453—473

473

Piola, A.R., Figueroa, H.A., Bianchi , A.A., 1987. Some aspects of the surface circulation south of 20°S revealed by First GARP Global Experiment drifters. Journal of Geophysical Research 92(C5), 5101—5114. Preston-Whyte, R.A., Tyson, P.D., 1973. Note on pressure oscillations over South Africa. Monthly Weather Review 101, 650—659. Salat, J., Maso´, M., Boyd, A.J., 1992. Water mass distribution and geostrophic circulation off Namibia during April 1986. Continental Shelf Research 12, 355—366. Schumann, E.H., Brink, K.H., 1990. Coastal-trapped waves off the coast of South Africa: generation, propagation and current structures. Journal of Physical Oceanography 20, 1206—1218. Shannon, L.V., 1985. The Benguela ecosystem. 1. Evolution of the Benguela, physical features and processes. Oceanography and Marine Biology Annual Review 23, 105—182. Shillington, F.A., Peterson, W.T., Hutchings, L., Probyn, T.A., Waldron, H.N., Agenbag, J.J., 1990. A cool upwelling filament off Namibia, southwest Africa: preliminary measurements of physical and biological features. Deep-Sea Research 37(11), 1753—1772. Stockton, P.L., Lutjeharms, J.R.E., 1988. Observations of vortex dipoles on the Benguela upwelling front. South African Geographer 15(1/2), 27—35. Stramma, L., Peterson, R.G., 1989. Geostrophic transport in the Benguela Current region. Journal of Physical Oceanography 19, 1440—1448. Van Ballegooyen, R.C., Gru¨ndlingh, M.L., Lutjeharms, J.R.E., 1994. Eddy fluxes of heat and salt from the southwest Indian Ocean into the southeast Atlantic Ocean: a case study. Journal of Geophysical Research 99, 14053—14070. Van Loon, H., 1972. Pressure in the southern hemisphere. In: Newton, C.W. (Ed.), Meteorology of the Southern Hemisphere. American Meteorological Society. Meteorological Monographs 13, 59—86. Vihma, T., Launiainen, J., 1993. Ice drift in the Weddel Sea in 1990—1991 as tracked by a satellite buoy. Journal of Geophysical Research 98 (C8), 14471—14485. Weller, R.A., Davis, R.E., 1980. A vector measuring current meter. Deep-Sea Research 27A, 565—581.