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Case studies of the response and spatial distribution of wind-driven upwelling off the coast of Africa: 29–34° south
0278~343/88 $3.00 + 0.00 © 1988PergamonPressplc.
ContinentalShelfResearch, Vol. 8, No. 11, pp. 1257-1271, 1988. Printedin GreatBritain.
C a s e s t u d i e s o f t h e r e s p o n s e a n d spatial d i s t r i b u t i o n o f w i n d - d r i v e n u p w e l l i n g o f f the c o a s t o f Africa: 2 9 - 3 4 ° s o u t h MARK R. JURY* (Received 16 December 1987; in revised form 26 April 1988; accepted 16 June 1988) Abstract--The spatial distribution ofwind-driven upwelling along the west coast of Africa (2934°S) in the early summer is described using intermittent and sequential aerial survey observations, supported by surface-based time series. Three semi-permanent upwelling centers, at 2931, 33 and 34°S were examined for response to local topographic and meteorological forcing, both before and after the active phase of upwelling. Differing scales, intensities and response lags in wind-driven coastal upweUing were documented through the analysis of selected aerial survey data sets. The upwelling area centered at 30°S was found to be of greater spatial extent than the upwelling plumes associated with the capes at 33 and 34°S. Variations in the depth of the marine atmospheric boundary layer affected the structure of equatorwards winds and upwelling activity off the capes. When the air temperature inversion was relatively low, wind stress was enhanced seawards of the capes, while equatorwards, a wind shadow reduced offshore Ekman transport and upwelling. The case studies highlight persistent mesoscale features in the wind and sea surface temperature fields which are formed by the relationship between coastal upwelling, wind forcing and local topography.
1. I N T R O D U C T I O N
To a first approximation, it can be said that alongshore winds directed equatorwards with the coast on the right (in the southern hemisphere) may initiate the process of coastal upwelling. If both the coastline and the wind field are uniform, the upwelling will create an inshore decrease in the surface layer sea temperatures. However, it has long been recognized that sea surface temperatures (SST) in all four of the major eastern boundary current regions including the Benguela area (shown in Fig. 1) contain alongshore structure. The analyses of CURRIE (1953) and HART and CURRIE (1960) revealed centers of relatively cool water extending seawards from Namaqualand (29-31°S), Cape Columbine (33°S) and the Cape Peninsula (34°S), but the relationship to local winds remained obscure. It was ANDREWSand CRAM (1969) who first observed the detailed aspects of the SST structures. BANG (1973) identified the capes at 33 and 34°S as small centres of upwelling and BANe and ANDREWS (1974) speculated that the low-level wind field contributes to the observed alongshore variations in coastal upwelling. It remained for subsequent aerial survey research in the late 1970s to demonstrate the spatial relationship between coastal upwelling and wind forcing. Here these findings are presented with emphasis on the horizontal mesoscale structure of SST and the lower atmosphere during the active upwelling season (summer), under the influence of wind events lasting a few days. These results are comparable to similar studies such as OPUS and CODE off * Oceanography Department, University of Cape Town, Rondebosch, 7700, South Africa. 1257
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Fig. 1. Topographic map of the southern Benguela area, showing its position in relation to Africa, bathymetric contours and elevations. The map borders for the case studies are shown (1, 2 and 3). Flight tracks are indicated by the dashed lines. A 30 km distance scale is given.
California, although the c o m m i t m e n t and sophistication of the field resources was m o r e limited. The application of aerial survey techniques over upwelling regions was developed by the Coastal Upwelling Ecosystems Analysis program, during observational studies off the coasts of Oregon, Peru, and northwest Africa in the 1970s. HOLLADAY and O'BRIEN (1975) and STUART (1981), amongst others, have demonstrated that coastal upwelling is variable over short spatial scales. Further refinement of the aerial survey technique off Point Conception, California (BRINK et al., 1984; CALDWELL et al., 1986) have led to a
Spatial distributionof wind-drivenupwelling
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more detailed description of the topographically influenced, spatial structure in low-level winds and SSTs. Geographic features which provide for a unique and enhanced mesoscale structure in the southern Benguela area include the abrupt end to the African continent at 35°S and its elevation at 1200 m. Transient high-pressure cells ridge eastwards along 35-40°S during summer, creating a surface air pressure gradient for equatorwards winds favorable to upwelling. The high-pressure cell is accompanied by a shallow coastal trapped Kelvin wave (GILL, 1977) known as the coastal low. The coastal low initially forms near 23°S and then propagates southwards along the west coast of Africa at a regular rate of between 400 and 600 km day-1, beneath the offshore flow of the ridging high farther south. This coastal trapped disturbance contains a well-defined pressure minimum and is characterized by a low inversion top of order 300-800 m (HEYDENRYCH, 1987). Together with the ridging marine high, the coastal low can increase equatorwards wind stress from a typical value of 0.1 N m-2 to more than 1.0 N m-2 for short periods of time. To adequately quantify this type of weather band forcing, a composite of nine coastal low events was constructed from digitized surface air pressure fields. The first 4 days of the composite sequence are shown in Fig. 2. The coastal low events may be compared with those investigated by DORMAN(1987) in the CODE studies off northern California. Such variability of the surface air pressure gradient reduces the representativeness of intermittent aerial survey data collected on individual days. Further discussion of the macroscale atmospheric forcing mechanisms which drive coastal upwelling along the west coast of Africa (29-34°S) is provided by JuRY (1980), SHANNON et al. (1981), NELSON and HUTCHINGS (1983), and PARRISH et al. (1983). A review paper which covers the background oceanography and variability of the Benguela Current is SHANNON,(1985). The upwelling circulations bordering the southern Benguela area can be considered to be strongly pulsed, as is the case for the CODE area (BEARDSLEYet al., 1987), with wind events repeatedly moving polewards along the coast and slowly evolving under the influence of the upper westerly troughs and local topography. For the case studies analysed here, the time varying wind events may be considered as "noise", out of which the more geographically fixed and persistent mesoscale SST minima and wind maxima emerge. That a spatially coherent and repeatable pattern can emerge from such a noisy environment can be attributed to basic principles of hydraulics, whereby the equatorward-flowing marine atmospheric boundary layer is variably squeezed and released by protruding points and recessed bays. In particular it will be shown that the alongshore variations in coastal upwelling are most affected by the local capes which enhance offshore Ekman transport through acceleration of the three-dimensional marine wind flow. 1.1. Topographic setting The area under investigation is located near the southwest tip of the African continent between latitudes 29-34.5°S and longitudes 16-19.5°E. The shelf bathymetry narrows appreciably in the south where the 500 m contour comes to within 40 km of the capes at 33 and 34°S and two underwater canyons cut diagonally across the shelf in a north-south direction. The land topography is dominated by a complex coastal orientation which runs in a 160-340 ° alignment, acting to compress the equatorward wind field and contain the marine high pressure cell. At 33 and 34°S two prominent capes rise to heights of 250 and 1000 m, respectively, and extend seawards about 40 km from the mean coastal align-
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Fig. 2.
Composite 4 day sequence of air pressures showing the eastwards ridging of a highpressure cell and the poleward migration of a coastal low along the west coast.
Spatial distribution of wind-driven upwelling
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merit. The mountain ridge which reaches the coast at 34°S, 19°E is an extension of the interior plateau. It cuts perpendicularly across the southeast trades and can obstruct shallow flows which accompany the passage of coastal lows. Two river valleys affect the meteorology of the area, one leads into St. Helena Bay just landwards of the cape at 33°S and the other drains a wide basin to the south of 31°S which separates the complex terrain in the south from the uniform and gradually sloped topography to the north. Much of the mesoscale structure in wind-driven coastal upwelling is owed to the terrestial and marine topography and NELSONand HUTCHINGS(1983) highlight the coincidence between shelf slope and upwelling intensity, creating conditions for semi-permanent upwelling at 30, 33 and 34°S. 2. D A T A A N A L Y S I S A N D M E T H O D S
The primary data set for spatial analyses is derived from 600 h of aerial surveys made over three summer seasons, 1978--1981. JuRY (1984) has described the data acquistion and initial analysis techniques, which are reviewed here for completeness. The aircraft used was a Partenavia P68B equipped with a Decca navigator, flight indicator, drift sight, Rosemount air temperature sensor, wet bulb hygrometer and a Barnes PRT-5 infra-red SST sensor. The Decca navigator provided a positional accuracy of about 500 m. A drift sight, together with a comparison of indicated and true (ground) speed, enabled a trigonometric computation of 150 m level wind vectors at regular 10 km intervals. In comparison with local PIBAL, tower and buoy winds, accuracies of 2 m s-1 and 5-10 degrees were achieved by this crude system. Wind stress at the sea surface was estimated by fitting aircraft winds to the mean PIBAL wind speed profile, using the 2 m buoy winds at the lowest level. The 10 m reference wind speed was about 30% of the flight level wind during active upwelling conditions. Shear in the wind direction was limited below the 150 m flight level. Proportionately larger changes in the wind profile were encountered across the air temperature inversion which defined the top of the marine layer, most often found between the levels 300 and 1200 m. SSTs were measured by converting passive infra-red emmitance to a voltage for continuous recording onto a calibrated strip chart, annotated with position. SST accuracies were checked against research ship readings with discrepancies noted only during calm conditions as a result of reduced mixing in the surface layer. Ambient air temperatures were monitored using a Rosemount 101 air temperature sensor, whose voltage output was digitized at 10 km intervals. During vertical profiling, the altimeter was referenced at 30 m (100 r ) intervals for temperature data acquisition from 60 to 900 m above the sea. Aerial survey grids were conducted at the 150 m (500 ft) level perpendicular to the coastline as shown in Fig. 1. Flight grids extended from 20 km inland to 80 km offshore, covering the active coastal upwelling strip and land-sea air temperature boundary just inland. Spatial resolution-was controlled by the alongshore spacing of flight legs, generally set at 30 kin, except near the capes at 33 and 34°S where along-coast grid spacings were reduced to 10 km. All flights were made between the hours of 10h00 and 15h00 local time, when maximum heating and seabreeze influences caused a slight shorewards deflection of gradient wind flow. This diurnal cycle is persistent on the coast, being about 30-40% of the mean V wind component, but is relatively insignificant over the outer shelf. Thus some aliasing of the wind structure may have occurred along the coast. Research flights over the three upwelling centers were repeated at intervals of at
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WIND
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Fig. 3. (a) Progressive wind vectors for the Namaqualand upwelling site (30°S) during spring 1980, (b) sea levels for the same period and site. Case studies are marked by arrows at the bottom.
least weekly and in some cases daily, to describe the evolution of SST structures over the synoptic weather cycle. The consistent method of data acquisition facilitated comparison of mesoscale structures under different macroscale weather settings, but does not make the cases selected here for discussion representative of all possible variations in coastal upwelling. Readers wishing further detail may consult JURY (1984, 1987). Because of the low space/time resolution of data acquisition, some degree of subjectivity enters the analysis. To better understand the temporal response of coastal upwelling to local wind forcing, time series of coastal winds and sea levels were available at each of the upwelling centers. The recorded information included wind, air pressure and temperature within 100 m of the coast at the 5 m level. Figure 3 provides an overview of temporal changes in coastal winds and sea levels near 30°S, applicable to case study sections 3.1 and 3.2. Case study days are identified on the sea level time series by arrows. In case study Section 3°4, the aerial survey data are sequential and time series are not provided. 3. M E S O S C A L E S P A T I A L S T R U C T U R E S
OBSERVED
BY A E R I A L S U R V E Y
Individual maps of SST and wind give insight to the spatial patterns and air-sea responses at the three semi-permanent upwelling sites at 30, 33 and 34°S. The cases use a consistent data analysis scheme and map scale (Fig. 1) for intercomparison. All SST maps were contoured at I°C intervals and appear in the left column. Wind direction streamlines (solid arrows) and isotachs in 2.5 m s-1 intervals (dashed) are shown in the right column. In the cases of the small capes (Sections 3.2, 3.3 and 3.4), the wind observations are supplemented by a calculation of either the estimated surface wind stress curl or the flight level wind vorticity to highlight the gradients forced by the topography. The case study sequences presented below are considered to be representative only for weather band forcing within the active phase of upwelling.
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Fig. 4. Namaqualand aerial survey SSTs (left column) and 150 m winds (right) for the dates 29 October, 10, 24 and 29 November 1980 running down the page. Units are °C and m s I for all aerial surveys. Data for this case were taken at alongshore intervals of 30 km as in Fig. 1.
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3.1. Namaqualand structure 30°S Cases which illustrate the spatial distribution of winds and SST for the Namaqualand upwelling center (30°S) are given in Fig. 4 for the dates 29 October, 10, 24 and 29 November 1980. The four maps indicate a gradual reduction in wind-driven coastal upwelling over the period. The weakening and periodic reversal of equatorward winds (Fig. 3) is attributed to a gradual reduction in the central pressure of the South Atlantic high evident by contrasting the October and December 1980 surface air pressure distributions shown in Fig. 5. Such temporal information provides a suitable background for the interpretation of intermittent data given in Sections 3.1 and 3.2. Viewing the cases individually, the mesoscale structure of SST consisted of a broad upwelli~g center reaching a minimum value of 10°C near 30°S on 29 October and 10 November 1980. The SST minima at the coast were spatially correlated with the marine wind maxima. Coldest water was confined to a narrow coastal strip and isotherms tended to follow the bathymetry, extending offshore north of 30°S. The wind maximum for these two cases of intense upwelling, although displaced seawards some distance, retained a concentric isotach distribution reaching a peak value of 20 m s -1 at 30°S, 16°E. JURY (1985) has linked this acceleration to a particularly strong daytime air temperature contrast between land and sea: 18-32°C. Through a Coriolis deflection of the sea breeze, the air temperature gradient serves to modulate the wind stress over the inner shelf. The last two cases at 30°S were obtained during a reduction in the intensity of upwelling, first during a slackening of equatorward wind flow (24 November) and then in a wind reversal (29 November). The wind history (Fig. 3) indicated that wind stress was directed eastwards onto the shelf from 11 to 20 November 1980. The SST maps show a distinct upwelling plume in its relaxed phase. On 24 November the coastal strip of upwelled water extended offshore along the 30°S latitude, while further south warm water was entrained shorewards. After a few days of polewards wind stress, associated with a cut-off low-pressure cell, SSTs warmed to 14°C on 29 November. The area of offshore plume extension moved southward to 30.5°S and the 16°C isotherm was within 30 km of the entire Namaqualand coast. Such weak SST gradients are rather more typical of coastal upwelling off Point Conception, California (BRINK et al., 1984).
Fig. 5. Monthly averaged surface air pressure distribution for October and December 1980 showing the position of the South Atlantic high. Contours are given in 2 hPa intervals.
Spatial distribution of wind-drivenupwelling
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The wind fields show a breakdown in the previously cohesive speed distribution. The equatorwards wind speed structure which had previously exhibited an offshore maximum, relaxed considerably on 24 November and lost form entirely when wind flow reversed. The wind field exhibited a shorewards and inland increase in speed on 29 November, more typical of a sea breeze circulation. Because of the retreat and relaxation of the South Atlantic high-pressure cell, the mesoscale structure briefly collapsed. 3.2. Cape Columbine structure 33°S The spatial characteristics of wind-driven upwelling off Cape Columbine (33°S) are presented from a similar sequence of aerial survey cases for 1 and 23 November, and 5 and 8 December 1980 in Fig. 6. In addition to the SST and wind maps, vertical profiles of air temperature (at the open triangle in the 1 November SST map) are given on the right. These describe the height of the air temperature (subsidence) inversion and, by definition, marine layer. It can be seen that air temperatures increased by 10°C from the surface up to the 300 m level on 1 November and to the 400 m level on 5 December. In contrast, the inversion was weakened (5°C) and considerably elevated (600 m) on 23 November and non-existent on 8 December. The Cape Columbine headland is elevated about 250 m above sea level and so the inversion-restricted wind flows may have difficulty in passing over the cape. The features which were persistent over the case study include the intense upwelling center and wind maxima on the western side of the cape. The SST minima varied from 10 to 12°C while the wind maxima remained above 15 m s-1 in all cases and thus represent active upwelling. The cold plume adjacent to the cape emanated northwards, in alignment with the bathymetry and wind flow. In leeward St Helena Bay, SSTs remained above 14°C. SST gradients underwent some changes during the sequence, perhaps related to the history of Ekman transport as inferred from local sea level time series in Fig. 3. A point of contrast between the cases is created by the air temperature inversion level, resulting in a variation in upwelling along the coast to the north of the cape. Under shallow flows on 1 November and 5 December scant upwelling activity was present. In contrast, a narrow but intense strip of coastal upwelling (SST less than 12°C), was observed on 23 November and 8 December, when deeper flows were present. It can be inferred that the level of the air temperature inversion controls the magnitude of upwelling-favorable wind stress in the leeward bay. If we distinguish between the shallow (low inversion) and deep (high inversion) wind cases, it can be seen that the wind stress maxima shifted poleward, to the southwest of the cape in the shallow cases and extended more downstream under deeper flow. Quite similar features are reported by ATKINSONet al. (1986) off Point Conception. In all cases a wake effect was set up by the headland such that offshore Ekman transport was reduced in a downstream direction. Topographic friction left the leeward coast protected and wind speeds remained below 10 m s-1. In the shallow cases of 1 November and 5 December a wind speed shear line extended to the north of the cape and the wake or wind shadow was more pronounced. On 1 November wake speeds fell below 2.5 m s-1 and an onshore trajectory of winds was present in the light speed zone. The main impact of the headland was to produce a cyclonic curvature of surface winds over the leeward shelf region. An analysis of wind stress curl for 1 and 23 November indicated an intense cyclonic area of order 10--5 N m -3 situated over the cape, (shaded area on wind maps in Fig. 6). The cyclonic wind stress curl area was restricted to the cape on 1 November.
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Fig. 6. Cape Columbine SSTs and 150 m winds (right) for 1 and 23 November and 5 and 8 December 1980. Temperature profiles at the open triangle in the upper left diagram are shown on the right with height in meters vs °C. Cyclonic wind stress curl areas of -5 x 10-6 N m 3 are shaded. Data were collected at alongshore intervals of 20 km as in Fig. 1.
H o w e v e r , o n 23 N o v e m b e r u n d e r d e e p flow, t h e cyclonic curl a r e a p e n e t r a t e d d o w n s t r e a m to r e a c h t h e l e e w a r d c o a s t , t h e r e b y e n h a n c i n g t h e v e r t i c a l u p w e l l i n g c i r c u l a t i o n t h r o u g h t h e E k m a n p u m p i n g r e l a t i o n s h i p , W = (pw(-f)) -1 (Xcurl), w h e r e t h e v a r i a b l e s a r e d e f i n e d as W v e r t i c a l c u r r e n t c o m p o n e n t , Pw d e n s i t y , f C o r i o l i s , a n d 1~curl = (OT,y/OX -- OT.x/ Oy). C y c l o n i c w i n d stress curl (-'Ccurl) o f o r d e r 10-5 N m -3 (i.e. a c h a n g e in t h e a l o n g s h o r e w i n d c o m p o n e n t o f 10 m S-1 o v e r a 10 k m e a s t - w e s t d i s t a n c e ) will p r o v i d e for an u p w a r d s c u r r e n t c o m p o n e n t o f o r d e r 10-4 m s-1 o r a b o u t 10 m p e r d a y o f a d d i t i o n a l uplift.
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SPatial distribution of wind-driven upwelling
From these cases a spatially anchored wind-driven upwelling center is evident. The wind maxiroa and wake shadow associated with the cape at 33°S, create a sharp alongshore gradient in SSTs. The upwelling plume tends to curve shorewards as a result of a similar curvature in the wind field. Water of 11°C forms the core of the plume which typically extends 20 km to the north of the cape. Further seawards a pronounced SST gradient of 5°C 10 km-1 is observed. 3.3. Cape Peninsula structure 34°S The main focus of wind-driven upwelling research in the southern Benguela has been on this particular upwelling center at 34°S, 18°E partly through the ease with which it could be sampled next to Cape Town. A composite flight map for 29 and 31 January 1981, during steady upwelling conditions, describes the intense spatial structures inherent in the area in Fig. 7. A well-developed plume was connected to the southwestern shore of the cape with a SST minimum of ll°C extending seawards into the warmer 20°C Agulhas-influenced waters, creating a gradient of nearly 10°C 10 km -~. In the lee of the isolated mountains of the Peninsula, wind directions reversed, wind speeds dropped below 2.5 m s-j, a phenomenon similarly found along the northern California coast leewards of Point Arena (BEARDSLEY et al., 1987). Over the shelf break and in the upstream bay, southeast trade wind flow reached 15 m s-z. Small 12°C upwelling centers were observed in the bay to the north of the Cape Peninsula and off the mountainous cape at 34°S, 19°E. Polewards of the cape-forced wind maxima, SST gradients adjusted back to coastal alignment as Ekman transport diminished. Clearly this mesoscale structure is more complex than in the previous cases as a result of the higher mountains. Typically, ridging high-pressure cells impinge upon this section of the coast (as in Fig. 2) on passing eastwards with the supporting planetary wave of the westerly jet stream. It is most instructive to set this analysis into the context of a single wind event. 3.4. A cycle of the Cape Peninsula upweUing center During the period 15-26 November 1979 an equatorward wind event affected the upwelling center off the Cape Peninsula at 34°S. From 16 to 18 November the upwelling center entered a growth phase as shown in Fig. 8. Increasing southeasterly winds and the formation of a distinct 10°C upwelling plume occurred over the 3 day period. The 12°C plume core spread over an area of about 400 km 2, but the offshore SST front remained
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Fig. 7. Cape Peninsula SST and 150 m winds composited for two flights on 29 and 31 January 1981. Wind streamlines and isotachs are shown separately. Data for this case (Figs 8 and 9 also) were collected at alongshore intervals of 10 km as in Fig. 1.
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Fig. 8. Cape Peninsula SST and 150 in wind (right) distributions for a growth phase 16-18 November 1979. Cyclonic vorticity areas o f - 6 × 10~ s ~ are shaded. Data distributions are as in Figs 1 and 7.
static. Wind speeds increased from 15 to 22 m s -1 while directions became more easterly during this growth phase, particularly in the lee of the cape. The response of the upwelling center to wind forcing was quick with a time lag of less than the inertial period (21 h). Using vertical cross sections, JURY (1984) has illustrated the evolution of a wind jet which extended across the Peninsular during this wind event. The wind jet reached speeds of 22 m s-1 at the 150 m level just off the cape (some 40 km seawards from the mean coastal alignment). The core of the wind jet was constrained below the 500 m level by a sharp air temperature inversion which tilted downwards towards the coast. Many characteristics of this wind jet are comparable to those found by ZEMBA and FRIEHE (1987). With the approach of a coastal low towards the end of the wind event, the inversion-constrained marine layer was reduced to a height less than the nearby mountains. Orographic obstruction resulted in the wind jet being moved farther offshore. Considerable shear and curvature was embedded in the wind field. An area of cyclonic wind vorticity (-6 x 10-4 s-1 shaded) formed over the Peninsula and spread offshore, thereby promoting coastal upwelling through the enhancement of vertical upward current components over the shelf.
Spatial distribution of wind-driven upwelling
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Between 24 and 26 November 1979 a reversal of equatorward wind stress coincided with the passage of a coastal low and attendant shelf wave which affected a decay in upwelling. On 24 November significant mesoscale structure was present in the SST and wind field as seen at the top of Fig. 9. A 10°C upwelling center was observed on the western shore of the Cape Peninsula. An alongshore variability in the wind field was noted; 25 m s-1 winds were recorded in the wind jet along the shelf edge, while in the mountain lee, winds of less than 5 m s - 1 occurred. The vorticity field indicated values in excess of--6 × 104 s-1 (shaded) over the southern tip of the Peninsula. By 25 November the equatorward flow and mesoscale structure collapsed, leading to the formation of a weak sea breeze circulation. In the SST map, warm eddies and cold patches appeared in contrast to the previously coherent pattern of upwelling. It is significant for marine biological and fisheries processes that the offshore SST frontal structure became unstable and broke down. With a westerly atmospheric trough approaching on 26 November, wind flow reversed, the sign of the wind vorticity changed to anticyclonic (downwelling) and the SST structure responded rapidly. The cold upwelling center became detached from the coast and the SST gradients relaxed shorewards. Warm 16°C water, possibly of Agulhas origin, penetrated over the shelf to the west of the cape at 34°S. The warm eddy became entrained into a clockwise gyre and was circulated polewards along the coast
'.'~"~ I ~ l ~ . . .
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Fig. 9. Cape Peninsula SST and 150 m wind distributions for a decay phase 24-26 November 1979. Vortieities are shaded as in Fig. 8, data distributions are as in Figs 1, 7 and 8.
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towards the upweUing center. SEND et al. (1987) discuss a similar upwelling relaxation event in the CODE results and also find that warm water approaches the decaying upwelling center at Point Arena along the coast from the equatorwards side where the wind stress minima was previously located. 4. C O N C L U S I O N S
The observations presented here have highlighted an alongshore variability in coastal upwelling along the west coast of Africa, 29-34°S, in response to a wind stress field containing significant spatial structure and temporal variability. Case study sequences of spatial distributions of wind and SST at three points along the coast were discussed in relation to both low- and high-frequency (monthly and weekly) wind events. Over the northern upwelling center at 30°S the wind stress maximum was forced by a locally intensified atmospheric thermal front between warm land and cool sea. The topography of the cape at 33°S accelerated upwelling-favorable winds and consequently initiated cyclonic wind stress curl and more intense upwelling. With the capping inversion lifted above the nearby mountains, deeper wind flow induced a more uniform offshore Ekman transport and upwelling was evident both off the cape and along the coast in its lee. With equatorwards wind of less than 400 m depth, however, a topographic shadow caused a relaxation of upwelling in the lee of the cape. Similar features were noted off the cape at 34°S, where upwelling-favorable winds were seen to be shifted seawards and polewards by the pressure gradient associated with an approaching coastal low (as in Fig. 2). The upwelling center responded by sliding from the northwest to the southwest side of the peninsula. As winds abated and reversed, the upwelling center at 34°S underwent rapid warming, presumably through a number of processes involving vertical diffusion, the horizontal entrainment of warmer offshore water and the input of solar radiation as discussed by SEND et al. (1987) for the case of northern California. The time evolution of coastal upwelling was examined in the last case and a quick response time was indicated. Unfortunately the evolution of the upwelling structures farther to the north could not be adequately assessed due to the infrequent sampling intervals and the inherently instantaneous nature and limited representativeness of the aerial survey technique. Further research is in progress regarding the variability of subsurface shelf circulations (NELSON and POLITO, 1987) and larger scale upwelling filaments which appear to emanate from many of these smaller coastal upwelling sites. Acknowledgements--The aerial survey research was supported by the Sea Fisheries Research Institute. I thank Prof. G. Brnndrit and C. Reason of the Oceanography Department, University of Cape Town for useful advice. REFERENCES ANDREWS W. R. H. and D. L. CRAM (1969) Combined aerial and shipboard upwelling study in the Benguela Current. Nature, 244, 902-904. ATKINSO~ L. P., K. H. BRINK, R. E. DAvis, B. H. JONES, T. PALUSZKmWaCZ and D. W. STUART (1986) Mesoscale hydrographic variability in the vicinity of Points Conception and Arguello during 1983, the OPUS experiment. Journal of Geophysical Research, 91, 12899-12918. BAt~O N. D. (1973) Characteristics of an intense ocean frontal system in the upwelling region west of Cape Town. Tellus, 25, 256-265. BANG N. D. and W. R. H. ANDREWS (1974) Direct current measurements of a shelf-edge frontal jet in the southern Benguela system. Journal of Marine Research, 32, 405-417.
Spatial distribution of wind-driven upwelling
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BEARDSLEY R. C., C. E. DORMAN, C. A. FRIEHE, L. K. ROSENFIELD and C. D. WINANT (1987) Local atmospheric forcing during CODE: The marine atmospheric boundary layer and atmospheric conditions over a northern California upwelling region. Journal of Geophysical Research, 92, 1467-1488. BRINK K., D. W. STUARTand J. C. VAN LEER (1984) Observations of the coastal upwelling region near 34030' N off California: spring 1981. Journal of Physical Oceanography, 14, 378-391. CALDWELL P. C., D. W. STUART and K. H. BRINK (1986) Mesoscale wind variability near Point Conception during spring 1983. Journal of Climate and Applied Meteorology, 25, 1241-1254. CURRIER. (1953) Upwelling in the Southern Benguela Current. Nature, 171,497-500. DORMAN C. E. (1987) Possible role of gravity currents in northern California's coastal summer wind reversals. Journal of Geophysical Research, 92, 1497-1506. GILL A. E. (1977) Coastal trapped waves in the atmosphere. Quarterly Journal of the Royal Meteorological Society, 103, 431-440. HART T. J. and R. CURRIE (1960) The Benguela Current. Discovery Report, 31, 123-298. ~-IEYDENRYCH C. M. (1987) A climatology of the coastal low in the SW Cape. MSc Thesis, Environmental Geographical Science Department, University of Cape Town, 135 pp. HOLLADAY C. G. and J. J. O'BRIEN (1975) Mesoscale variability of sea surface temperatures. Journal of Physical Oceanography, 5,761-772. JURY M. R. (1980) Characteristics of summer wind fields and air-sea interactions over the Cape Peninsula upwelling region. MSc Thesis, University of Cape Town, 131 pp. JURY M. R. (1984) Wind shear and differential upwelling along the SW tip of Africa. PhD Thesis, University of Cape Town, 164 pp. J URY M. R. (1985) Air temperature gradients along the west Cape coast during southerly winds. South African Journal of Science, 81, 17-20. NELSON G. and L. HUTCHINGS (1983) The Benguela upwelling area. Progress in Oceanography, 12, 333-356. NELSON G. and A. POLITO (1987) Information on currents in the Cape Peninsula area, South Africa. South African Journal of Marine Science, 5,287-304. PARRISH R. H., A. BAKUN, D. M. HUSBY and C. S. NELSON (1983) Comparative climatology of selected environmental processes in relation to eastern boundary current pelagic fish resources. Proc. Conslt Change Abundance Species Fish Res., Costa Rica. FAO Fisheries Report, 291,731-777. SEND U., R. C. BEARDSLEY and C. D. WINANT (1987) Relaxation from upwelling in CODE. Journal of Geophysical Research, 92, 1693-1698. SHANNON L. V. (1985) The Benguela Ecosystem I, evolution of the Benguela, physical features and processes. Oceanography and Marine Biology Annual Review, 23, 105-182. SHANNON L. V., G. NELSON and M. R. JURY (1981) Hydrological and meteorological aspects of upwelling in the southern Benguela Current. Coastal Upwelling, AGU, 146-159. STUART D. W. (1981) Sea surface temperatures and winds during Joint II, part 2, temporal variability. Coastal Upwelling, AGU, 32-38. ZEMBA J. and C. A. FRIEHE (1987) The marine atmospheric boundary layer jet in CODE. Journal of Geophysical Research, 92, 1489-1496.
ContinentalShelfResearch, Vol. 8, No. 11, pp. 1257-1271, 1988. Printedin GreatBritain.
C a s e s t u d i e s o f t h e r e s p o n s e a n d spatial d i s t r i b u t i o n o f w i n d - d r i v e n u p w e l l i n g o f f the c o a s t o f Africa: 2 9 - 3 4 ° s o u t h MARK R. JURY* (Received 16 December 1987; in revised form 26 April 1988; accepted 16 June 1988) Abstract--The spatial distribution ofwind-driven upwelling along the west coast of Africa (2934°S) in the early summer is described using intermittent and sequential aerial survey observations, supported by surface-based time series. Three semi-permanent upwelling centers, at 2931, 33 and 34°S were examined for response to local topographic and meteorological forcing, both before and after the active phase of upwelling. Differing scales, intensities and response lags in wind-driven coastal upweUing were documented through the analysis of selected aerial survey data sets. The upwelling area centered at 30°S was found to be of greater spatial extent than the upwelling plumes associated with the capes at 33 and 34°S. Variations in the depth of the marine atmospheric boundary layer affected the structure of equatorwards winds and upwelling activity off the capes. When the air temperature inversion was relatively low, wind stress was enhanced seawards of the capes, while equatorwards, a wind shadow reduced offshore Ekman transport and upwelling. The case studies highlight persistent mesoscale features in the wind and sea surface temperature fields which are formed by the relationship between coastal upwelling, wind forcing and local topography.
1. I N T R O D U C T I O N
To a first approximation, it can be said that alongshore winds directed equatorwards with the coast on the right (in the southern hemisphere) may initiate the process of coastal upwelling. If both the coastline and the wind field are uniform, the upwelling will create an inshore decrease in the surface layer sea temperatures. However, it has long been recognized that sea surface temperatures (SST) in all four of the major eastern boundary current regions including the Benguela area (shown in Fig. 1) contain alongshore structure. The analyses of CURRIE (1953) and HART and CURRIE (1960) revealed centers of relatively cool water extending seawards from Namaqualand (29-31°S), Cape Columbine (33°S) and the Cape Peninsula (34°S), but the relationship to local winds remained obscure. It was ANDREWSand CRAM (1969) who first observed the detailed aspects of the SST structures. BANG (1973) identified the capes at 33 and 34°S as small centres of upwelling and BANe and ANDREWS (1974) speculated that the low-level wind field contributes to the observed alongshore variations in coastal upwelling. It remained for subsequent aerial survey research in the late 1970s to demonstrate the spatial relationship between coastal upwelling and wind forcing. Here these findings are presented with emphasis on the horizontal mesoscale structure of SST and the lower atmosphere during the active upwelling season (summer), under the influence of wind events lasting a few days. These results are comparable to similar studies such as OPUS and CODE off * Oceanography Department, University of Cape Town, Rondebosch, 7700, South Africa. 1257
1258
M . R . JuRY
Fig. 1. Topographic map of the southern Benguela area, showing its position in relation to Africa, bathymetric contours and elevations. The map borders for the case studies are shown (1, 2 and 3). Flight tracks are indicated by the dashed lines. A 30 km distance scale is given.
California, although the c o m m i t m e n t and sophistication of the field resources was m o r e limited. The application of aerial survey techniques over upwelling regions was developed by the Coastal Upwelling Ecosystems Analysis program, during observational studies off the coasts of Oregon, Peru, and northwest Africa in the 1970s. HOLLADAY and O'BRIEN (1975) and STUART (1981), amongst others, have demonstrated that coastal upwelling is variable over short spatial scales. Further refinement of the aerial survey technique off Point Conception, California (BRINK et al., 1984; CALDWELL et al., 1986) have led to a
Spatial distributionof wind-drivenupwelling
1259
more detailed description of the topographically influenced, spatial structure in low-level winds and SSTs. Geographic features which provide for a unique and enhanced mesoscale structure in the southern Benguela area include the abrupt end to the African continent at 35°S and its elevation at 1200 m. Transient high-pressure cells ridge eastwards along 35-40°S during summer, creating a surface air pressure gradient for equatorwards winds favorable to upwelling. The high-pressure cell is accompanied by a shallow coastal trapped Kelvin wave (GILL, 1977) known as the coastal low. The coastal low initially forms near 23°S and then propagates southwards along the west coast of Africa at a regular rate of between 400 and 600 km day-1, beneath the offshore flow of the ridging high farther south. This coastal trapped disturbance contains a well-defined pressure minimum and is characterized by a low inversion top of order 300-800 m (HEYDENRYCH, 1987). Together with the ridging marine high, the coastal low can increase equatorwards wind stress from a typical value of 0.1 N m-2 to more than 1.0 N m-2 for short periods of time. To adequately quantify this type of weather band forcing, a composite of nine coastal low events was constructed from digitized surface air pressure fields. The first 4 days of the composite sequence are shown in Fig. 2. The coastal low events may be compared with those investigated by DORMAN(1987) in the CODE studies off northern California. Such variability of the surface air pressure gradient reduces the representativeness of intermittent aerial survey data collected on individual days. Further discussion of the macroscale atmospheric forcing mechanisms which drive coastal upwelling along the west coast of Africa (29-34°S) is provided by JuRY (1980), SHANNON et al. (1981), NELSON and HUTCHINGS (1983), and PARRISH et al. (1983). A review paper which covers the background oceanography and variability of the Benguela Current is SHANNON,(1985). The upwelling circulations bordering the southern Benguela area can be considered to be strongly pulsed, as is the case for the CODE area (BEARDSLEYet al., 1987), with wind events repeatedly moving polewards along the coast and slowly evolving under the influence of the upper westerly troughs and local topography. For the case studies analysed here, the time varying wind events may be considered as "noise", out of which the more geographically fixed and persistent mesoscale SST minima and wind maxima emerge. That a spatially coherent and repeatable pattern can emerge from such a noisy environment can be attributed to basic principles of hydraulics, whereby the equatorward-flowing marine atmospheric boundary layer is variably squeezed and released by protruding points and recessed bays. In particular it will be shown that the alongshore variations in coastal upwelling are most affected by the local capes which enhance offshore Ekman transport through acceleration of the three-dimensional marine wind flow. 1.1. Topographic setting The area under investigation is located near the southwest tip of the African continent between latitudes 29-34.5°S and longitudes 16-19.5°E. The shelf bathymetry narrows appreciably in the south where the 500 m contour comes to within 40 km of the capes at 33 and 34°S and two underwater canyons cut diagonally across the shelf in a north-south direction. The land topography is dominated by a complex coastal orientation which runs in a 160-340 ° alignment, acting to compress the equatorward wind field and contain the marine high pressure cell. At 33 and 34°S two prominent capes rise to heights of 250 and 1000 m, respectively, and extend seawards about 40 km from the mean coastal align-
1260
M . R . JURY
•
*
!0
" 14 "
28
"
"
"
°
H 24
1
D
18
20
.4
Fig. 2.
Composite 4 day sequence of air pressures showing the eastwards ridging of a highpressure cell and the poleward migration of a coastal low along the west coast.
Spatial distribution of wind-driven upwelling
1261
merit. The mountain ridge which reaches the coast at 34°S, 19°E is an extension of the interior plateau. It cuts perpendicularly across the southeast trades and can obstruct shallow flows which accompany the passage of coastal lows. Two river valleys affect the meteorology of the area, one leads into St. Helena Bay just landwards of the cape at 33°S and the other drains a wide basin to the south of 31°S which separates the complex terrain in the south from the uniform and gradually sloped topography to the north. Much of the mesoscale structure in wind-driven coastal upwelling is owed to the terrestial and marine topography and NELSONand HUTCHINGS(1983) highlight the coincidence between shelf slope and upwelling intensity, creating conditions for semi-permanent upwelling at 30, 33 and 34°S. 2. D A T A A N A L Y S I S A N D M E T H O D S
The primary data set for spatial analyses is derived from 600 h of aerial surveys made over three summer seasons, 1978--1981. JuRY (1984) has described the data acquistion and initial analysis techniques, which are reviewed here for completeness. The aircraft used was a Partenavia P68B equipped with a Decca navigator, flight indicator, drift sight, Rosemount air temperature sensor, wet bulb hygrometer and a Barnes PRT-5 infra-red SST sensor. The Decca navigator provided a positional accuracy of about 500 m. A drift sight, together with a comparison of indicated and true (ground) speed, enabled a trigonometric computation of 150 m level wind vectors at regular 10 km intervals. In comparison with local PIBAL, tower and buoy winds, accuracies of 2 m s-1 and 5-10 degrees were achieved by this crude system. Wind stress at the sea surface was estimated by fitting aircraft winds to the mean PIBAL wind speed profile, using the 2 m buoy winds at the lowest level. The 10 m reference wind speed was about 30% of the flight level wind during active upwelling conditions. Shear in the wind direction was limited below the 150 m flight level. Proportionately larger changes in the wind profile were encountered across the air temperature inversion which defined the top of the marine layer, most often found between the levels 300 and 1200 m. SSTs were measured by converting passive infra-red emmitance to a voltage for continuous recording onto a calibrated strip chart, annotated with position. SST accuracies were checked against research ship readings with discrepancies noted only during calm conditions as a result of reduced mixing in the surface layer. Ambient air temperatures were monitored using a Rosemount 101 air temperature sensor, whose voltage output was digitized at 10 km intervals. During vertical profiling, the altimeter was referenced at 30 m (100 r ) intervals for temperature data acquisition from 60 to 900 m above the sea. Aerial survey grids were conducted at the 150 m (500 ft) level perpendicular to the coastline as shown in Fig. 1. Flight grids extended from 20 km inland to 80 km offshore, covering the active coastal upwelling strip and land-sea air temperature boundary just inland. Spatial resolution-was controlled by the alongshore spacing of flight legs, generally set at 30 kin, except near the capes at 33 and 34°S where along-coast grid spacings were reduced to 10 km. All flights were made between the hours of 10h00 and 15h00 local time, when maximum heating and seabreeze influences caused a slight shorewards deflection of gradient wind flow. This diurnal cycle is persistent on the coast, being about 30-40% of the mean V wind component, but is relatively insignificant over the outer shelf. Thus some aliasing of the wind structure may have occurred along the coast. Research flights over the three upwelling centers were repeated at intervals of at
1262
M . R . JuRY
WIND
N--.~
OCT
NOV
1980
DEC
SEA LEVEL • ~ . . . ~ . . . . ' ~ / ~ . . A ......... ~ ......... .... ~ . . . . . . . . . . . . . . ~.,~..
25
t t
f
tf
t
t
t
Fig. 3. (a) Progressive wind vectors for the Namaqualand upwelling site (30°S) during spring 1980, (b) sea levels for the same period and site. Case studies are marked by arrows at the bottom.
least weekly and in some cases daily, to describe the evolution of SST structures over the synoptic weather cycle. The consistent method of data acquisition facilitated comparison of mesoscale structures under different macroscale weather settings, but does not make the cases selected here for discussion representative of all possible variations in coastal upwelling. Readers wishing further detail may consult JURY (1984, 1987). Because of the low space/time resolution of data acquisition, some degree of subjectivity enters the analysis. To better understand the temporal response of coastal upwelling to local wind forcing, time series of coastal winds and sea levels were available at each of the upwelling centers. The recorded information included wind, air pressure and temperature within 100 m of the coast at the 5 m level. Figure 3 provides an overview of temporal changes in coastal winds and sea levels near 30°S, applicable to case study sections 3.1 and 3.2. Case study days are identified on the sea level time series by arrows. In case study Section 3°4, the aerial survey data are sequential and time series are not provided. 3. M E S O S C A L E S P A T I A L S T R U C T U R E S
OBSERVED
BY A E R I A L S U R V E Y
Individual maps of SST and wind give insight to the spatial patterns and air-sea responses at the three semi-permanent upwelling sites at 30, 33 and 34°S. The cases use a consistent data analysis scheme and map scale (Fig. 1) for intercomparison. All SST maps were contoured at I°C intervals and appear in the left column. Wind direction streamlines (solid arrows) and isotachs in 2.5 m s-1 intervals (dashed) are shown in the right column. In the cases of the small capes (Sections 3.2, 3.3 and 3.4), the wind observations are supplemented by a calculation of either the estimated surface wind stress curl or the flight level wind vorticity to highlight the gradients forced by the topography. The case study sequences presented below are considered to be representative only for weather band forcing within the active phase of upwelling.
I.
10 Nov
|
..
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~\
.
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29 Nov 8 0
Fig. 4. Namaqualand aerial survey SSTs (left column) and 150 m winds (right) for the dates 29 October, 10, 24 and 29 November 1980 running down the page. Units are °C and m s I for all aerial surveys. Data for this case were taken at alongshore intervals of 30 km as in Fig. 1.
1264
M.R. JuRY
3.1. Namaqualand structure 30°S Cases which illustrate the spatial distribution of winds and SST for the Namaqualand upwelling center (30°S) are given in Fig. 4 for the dates 29 October, 10, 24 and 29 November 1980. The four maps indicate a gradual reduction in wind-driven coastal upwelling over the period. The weakening and periodic reversal of equatorward winds (Fig. 3) is attributed to a gradual reduction in the central pressure of the South Atlantic high evident by contrasting the October and December 1980 surface air pressure distributions shown in Fig. 5. Such temporal information provides a suitable background for the interpretation of intermittent data given in Sections 3.1 and 3.2. Viewing the cases individually, the mesoscale structure of SST consisted of a broad upwelli~g center reaching a minimum value of 10°C near 30°S on 29 October and 10 November 1980. The SST minima at the coast were spatially correlated with the marine wind maxima. Coldest water was confined to a narrow coastal strip and isotherms tended to follow the bathymetry, extending offshore north of 30°S. The wind maximum for these two cases of intense upwelling, although displaced seawards some distance, retained a concentric isotach distribution reaching a peak value of 20 m s -1 at 30°S, 16°E. JURY (1985) has linked this acceleration to a particularly strong daytime air temperature contrast between land and sea: 18-32°C. Through a Coriolis deflection of the sea breeze, the air temperature gradient serves to modulate the wind stress over the inner shelf. The last two cases at 30°S were obtained during a reduction in the intensity of upwelling, first during a slackening of equatorward wind flow (24 November) and then in a wind reversal (29 November). The wind history (Fig. 3) indicated that wind stress was directed eastwards onto the shelf from 11 to 20 November 1980. The SST maps show a distinct upwelling plume in its relaxed phase. On 24 November the coastal strip of upwelled water extended offshore along the 30°S latitude, while further south warm water was entrained shorewards. After a few days of polewards wind stress, associated with a cut-off low-pressure cell, SSTs warmed to 14°C on 29 November. The area of offshore plume extension moved southward to 30.5°S and the 16°C isotherm was within 30 km of the entire Namaqualand coast. Such weak SST gradients are rather more typical of coastal upwelling off Point Conception, California (BRINK et al., 1984).
Fig. 5. Monthly averaged surface air pressure distribution for October and December 1980 showing the position of the South Atlantic high. Contours are given in 2 hPa intervals.
Spatial distribution of wind-drivenupwelling
1265
The wind fields show a breakdown in the previously cohesive speed distribution. The equatorwards wind speed structure which had previously exhibited an offshore maximum, relaxed considerably on 24 November and lost form entirely when wind flow reversed. The wind field exhibited a shorewards and inland increase in speed on 29 November, more typical of a sea breeze circulation. Because of the retreat and relaxation of the South Atlantic high-pressure cell, the mesoscale structure briefly collapsed. 3.2. Cape Columbine structure 33°S The spatial characteristics of wind-driven upwelling off Cape Columbine (33°S) are presented from a similar sequence of aerial survey cases for 1 and 23 November, and 5 and 8 December 1980 in Fig. 6. In addition to the SST and wind maps, vertical profiles of air temperature (at the open triangle in the 1 November SST map) are given on the right. These describe the height of the air temperature (subsidence) inversion and, by definition, marine layer. It can be seen that air temperatures increased by 10°C from the surface up to the 300 m level on 1 November and to the 400 m level on 5 December. In contrast, the inversion was weakened (5°C) and considerably elevated (600 m) on 23 November and non-existent on 8 December. The Cape Columbine headland is elevated about 250 m above sea level and so the inversion-restricted wind flows may have difficulty in passing over the cape. The features which were persistent over the case study include the intense upwelling center and wind maxima on the western side of the cape. The SST minima varied from 10 to 12°C while the wind maxima remained above 15 m s-1 in all cases and thus represent active upwelling. The cold plume adjacent to the cape emanated northwards, in alignment with the bathymetry and wind flow. In leeward St Helena Bay, SSTs remained above 14°C. SST gradients underwent some changes during the sequence, perhaps related to the history of Ekman transport as inferred from local sea level time series in Fig. 3. A point of contrast between the cases is created by the air temperature inversion level, resulting in a variation in upwelling along the coast to the north of the cape. Under shallow flows on 1 November and 5 December scant upwelling activity was present. In contrast, a narrow but intense strip of coastal upwelling (SST less than 12°C), was observed on 23 November and 8 December, when deeper flows were present. It can be inferred that the level of the air temperature inversion controls the magnitude of upwelling-favorable wind stress in the leeward bay. If we distinguish between the shallow (low inversion) and deep (high inversion) wind cases, it can be seen that the wind stress maxima shifted poleward, to the southwest of the cape in the shallow cases and extended more downstream under deeper flow. Quite similar features are reported by ATKINSONet al. (1986) off Point Conception. In all cases a wake effect was set up by the headland such that offshore Ekman transport was reduced in a downstream direction. Topographic friction left the leeward coast protected and wind speeds remained below 10 m s-1. In the shallow cases of 1 November and 5 December a wind speed shear line extended to the north of the cape and the wake or wind shadow was more pronounced. On 1 November wake speeds fell below 2.5 m s-1 and an onshore trajectory of winds was present in the light speed zone. The main impact of the headland was to produce a cyclonic curvature of surface winds over the leeward shelf region. An analysis of wind stress curl for 1 and 23 November indicated an intense cyclonic area of order 10--5 N m -3 situated over the cape, (shaded area on wind maps in Fig. 6). The cyclonic wind stress curl area was restricted to the cape on 1 November.
1266
M . R . JURY
32'
zx
I~
Fig. 6. Cape Columbine SSTs and 150 m winds (right) for 1 and 23 November and 5 and 8 December 1980. Temperature profiles at the open triangle in the upper left diagram are shown on the right with height in meters vs °C. Cyclonic wind stress curl areas of -5 x 10-6 N m 3 are shaded. Data were collected at alongshore intervals of 20 km as in Fig. 1.
H o w e v e r , o n 23 N o v e m b e r u n d e r d e e p flow, t h e cyclonic curl a r e a p e n e t r a t e d d o w n s t r e a m to r e a c h t h e l e e w a r d c o a s t , t h e r e b y e n h a n c i n g t h e v e r t i c a l u p w e l l i n g c i r c u l a t i o n t h r o u g h t h e E k m a n p u m p i n g r e l a t i o n s h i p , W = (pw(-f)) -1 (Xcurl), w h e r e t h e v a r i a b l e s a r e d e f i n e d as W v e r t i c a l c u r r e n t c o m p o n e n t , Pw d e n s i t y , f C o r i o l i s , a n d 1~curl = (OT,y/OX -- OT.x/ Oy). C y c l o n i c w i n d stress curl (-'Ccurl) o f o r d e r 10-5 N m -3 (i.e. a c h a n g e in t h e a l o n g s h o r e w i n d c o m p o n e n t o f 10 m S-1 o v e r a 10 k m e a s t - w e s t d i s t a n c e ) will p r o v i d e for an u p w a r d s c u r r e n t c o m p o n e n t o f o r d e r 10-4 m s-1 o r a b o u t 10 m p e r d a y o f a d d i t i o n a l uplift.
1267
SPatial distribution of wind-driven upwelling
From these cases a spatially anchored wind-driven upwelling center is evident. The wind maxiroa and wake shadow associated with the cape at 33°S, create a sharp alongshore gradient in SSTs. The upwelling plume tends to curve shorewards as a result of a similar curvature in the wind field. Water of 11°C forms the core of the plume which typically extends 20 km to the north of the cape. Further seawards a pronounced SST gradient of 5°C 10 km-1 is observed. 3.3. Cape Peninsula structure 34°S The main focus of wind-driven upwelling research in the southern Benguela has been on this particular upwelling center at 34°S, 18°E partly through the ease with which it could be sampled next to Cape Town. A composite flight map for 29 and 31 January 1981, during steady upwelling conditions, describes the intense spatial structures inherent in the area in Fig. 7. A well-developed plume was connected to the southwestern shore of the cape with a SST minimum of ll°C extending seawards into the warmer 20°C Agulhas-influenced waters, creating a gradient of nearly 10°C 10 km -~. In the lee of the isolated mountains of the Peninsula, wind directions reversed, wind speeds dropped below 2.5 m s-j, a phenomenon similarly found along the northern California coast leewards of Point Arena (BEARDSLEY et al., 1987). Over the shelf break and in the upstream bay, southeast trade wind flow reached 15 m s-z. Small 12°C upwelling centers were observed in the bay to the north of the Cape Peninsula and off the mountainous cape at 34°S, 19°E. Polewards of the cape-forced wind maxima, SST gradients adjusted back to coastal alignment as Ekman transport diminished. Clearly this mesoscale structure is more complex than in the previous cases as a result of the higher mountains. Typically, ridging high-pressure cells impinge upon this section of the coast (as in Fig. 2) on passing eastwards with the supporting planetary wave of the westerly jet stream. It is most instructive to set this analysis into the context of a single wind event. 3.4. A cycle of the Cape Peninsula upweUing center During the period 15-26 November 1979 an equatorward wind event affected the upwelling center off the Cape Peninsula at 34°S. From 16 to 18 November the upwelling center entered a growth phase as shown in Fig. 8. Increasing southeasterly winds and the formation of a distinct 10°C upwelling plume occurred over the 3 day period. The 12°C plume core spread over an area of about 400 km 2, but the offshore SST front remained
-
::": ."
ssT
DIR
34--
ss
~ t ~
Fig. 7. Cape Peninsula SST and 150 m winds composited for two flights on 29 and 31 January 1981. Wind streamlines and isotachs are shown separately. Data for this case (Figs 8 and 9 also) were collected at alongshore intervals of 10 km as in Fig. 1.
";',
1268
M.R. JURY ,
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t6Nov 79
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Fig. 8. Cape Peninsula SST and 150 in wind (right) distributions for a growth phase 16-18 November 1979. Cyclonic vorticity areas o f - 6 × 10~ s ~ are shaded. Data distributions are as in Figs 1 and 7.
static. Wind speeds increased from 15 to 22 m s -1 while directions became more easterly during this growth phase, particularly in the lee of the cape. The response of the upwelling center to wind forcing was quick with a time lag of less than the inertial period (21 h). Using vertical cross sections, JURY (1984) has illustrated the evolution of a wind jet which extended across the Peninsular during this wind event. The wind jet reached speeds of 22 m s-1 at the 150 m level just off the cape (some 40 km seawards from the mean coastal alignment). The core of the wind jet was constrained below the 500 m level by a sharp air temperature inversion which tilted downwards towards the coast. Many characteristics of this wind jet are comparable to those found by ZEMBA and FRIEHE (1987). With the approach of a coastal low towards the end of the wind event, the inversion-constrained marine layer was reduced to a height less than the nearby mountains. Orographic obstruction resulted in the wind jet being moved farther offshore. Considerable shear and curvature was embedded in the wind field. An area of cyclonic wind vorticity (-6 x 10-4 s-1 shaded) formed over the Peninsula and spread offshore, thereby promoting coastal upwelling through the enhancement of vertical upward current components over the shelf.
Spatial distribution of wind-driven upwelling
1269
Between 24 and 26 November 1979 a reversal of equatorward wind stress coincided with the passage of a coastal low and attendant shelf wave which affected a decay in upwelling. On 24 November significant mesoscale structure was present in the SST and wind field as seen at the top of Fig. 9. A 10°C upwelling center was observed on the western shore of the Cape Peninsula. An alongshore variability in the wind field was noted; 25 m s-1 winds were recorded in the wind jet along the shelf edge, while in the mountain lee, winds of less than 5 m s - 1 occurred. The vorticity field indicated values in excess of--6 × 104 s-1 (shaded) over the southern tip of the Peninsula. By 25 November the equatorward flow and mesoscale structure collapsed, leading to the formation of a weak sea breeze circulation. In the SST map, warm eddies and cold patches appeared in contrast to the previously coherent pattern of upwelling. It is significant for marine biological and fisheries processes that the offshore SST frontal structure became unstable and broke down. With a westerly atmospheric trough approaching on 26 November, wind flow reversed, the sign of the wind vorticity changed to anticyclonic (downwelling) and the SST structure responded rapidly. The cold upwelling center became detached from the coast and the SST gradients relaxed shorewards. Warm 16°C water, possibly of Agulhas origin, penetrated over the shelf to the west of the cape at 34°S. The warm eddy became entrained into a clockwise gyre and was circulated polewards along the coast
'.'~"~ I ~ l ~ . . .
%
~
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,,
~
x
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. . . .
.,
Fig. 9. Cape Peninsula SST and 150 m wind distributions for a decay phase 24-26 November 1979. Vortieities are shaded as in Fig. 8, data distributions are as in Figs 1, 7 and 8.
1270
M . R . JURY
towards the upweUing center. SEND et al. (1987) discuss a similar upwelling relaxation event in the CODE results and also find that warm water approaches the decaying upwelling center at Point Arena along the coast from the equatorwards side where the wind stress minima was previously located. 4. C O N C L U S I O N S
The observations presented here have highlighted an alongshore variability in coastal upwelling along the west coast of Africa, 29-34°S, in response to a wind stress field containing significant spatial structure and temporal variability. Case study sequences of spatial distributions of wind and SST at three points along the coast were discussed in relation to both low- and high-frequency (monthly and weekly) wind events. Over the northern upwelling center at 30°S the wind stress maximum was forced by a locally intensified atmospheric thermal front between warm land and cool sea. The topography of the cape at 33°S accelerated upwelling-favorable winds and consequently initiated cyclonic wind stress curl and more intense upwelling. With the capping inversion lifted above the nearby mountains, deeper wind flow induced a more uniform offshore Ekman transport and upwelling was evident both off the cape and along the coast in its lee. With equatorwards wind of less than 400 m depth, however, a topographic shadow caused a relaxation of upwelling in the lee of the cape. Similar features were noted off the cape at 34°S, where upwelling-favorable winds were seen to be shifted seawards and polewards by the pressure gradient associated with an approaching coastal low (as in Fig. 2). The upwelling center responded by sliding from the northwest to the southwest side of the peninsula. As winds abated and reversed, the upwelling center at 34°S underwent rapid warming, presumably through a number of processes involving vertical diffusion, the horizontal entrainment of warmer offshore water and the input of solar radiation as discussed by SEND et al. (1987) for the case of northern California. The time evolution of coastal upwelling was examined in the last case and a quick response time was indicated. Unfortunately the evolution of the upwelling structures farther to the north could not be adequately assessed due to the infrequent sampling intervals and the inherently instantaneous nature and limited representativeness of the aerial survey technique. Further research is in progress regarding the variability of subsurface shelf circulations (NELSON and POLITO, 1987) and larger scale upwelling filaments which appear to emanate from many of these smaller coastal upwelling sites. Acknowledgements--The aerial survey research was supported by the Sea Fisheries Research Institute. I thank Prof. G. Brnndrit and C. Reason of the Oceanography Department, University of Cape Town for useful advice. REFERENCES ANDREWS W. R. H. and D. L. CRAM (1969) Combined aerial and shipboard upwelling study in the Benguela Current. Nature, 244, 902-904. ATKINSO~ L. P., K. H. BRINK, R. E. DAvis, B. H. JONES, T. PALUSZKmWaCZ and D. W. STUART (1986) Mesoscale hydrographic variability in the vicinity of Points Conception and Arguello during 1983, the OPUS experiment. Journal of Geophysical Research, 91, 12899-12918. BAt~O N. D. (1973) Characteristics of an intense ocean frontal system in the upwelling region west of Cape Town. Tellus, 25, 256-265. BANG N. D. and W. R. H. ANDREWS (1974) Direct current measurements of a shelf-edge frontal jet in the southern Benguela system. Journal of Marine Research, 32, 405-417.
Spatial distribution of wind-driven upwelling
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