Rapp. P.-v. Réun. Cons. int. Explor. Mer, 180: 50-57. 1982. Large-scale circulation along the coast of Northwest Africa

Ekkehard Mittelstaedt

Deutsches Hydrographisches Institut Postfach 220, 2000 Hamburg 4, Bundesrepublik Deutschland

The meridional large-scale pressure gradient in the eastern North Atlantic tends to produce northward flow along the Northwest African continent. On the shelf this tendency is masked as long as the trades are strong enough to maintain the equator- ward flow, which is additionally enhanced by the coastal jet. In deeper layers the pressure gradient maintains the undercurrent concentrating all along the Northwest African continental slope. There is strong evidence of a near-surface countercurrent to the north, offshore, flowing against the winds and against the wind-driven flow on the shelf during the upwelling season. The boundary between the north-flowing countercurrent offshore and the south- flowing currents inshore tends to be a convergence with descending motions along either side. Current measurements along the continental slope frequently show mean north­ ward flow from the near-surface layer down to depths of 500 to 1000 metres. From the measurements the near-surface countercurrent and the undercurrent seem to be one and the same current system with one and the same dynamic cause. The exist­ ence of the countercurrent provides a mechanism by which upwelling water may recirculate.

Seasonal variations

The large-scale phenomenon of coastal upwelling has a Because of the well-developed trades farther north the distinct seasonal signal everywhere. But there are coas­ tropical water cannot advance beyond these latitudes tal areas with favourable upwelling conditions through­ for an extended period of time. The countercurrent out the year, e.g. along the Northwest African coast now forms the inshore limb of a large cyclonic gyre between 20°N and 25°N. There are other areas, for whose diffuse offshore limb is the south-flowing instance along the Northwest African coast south of Canary Current. Weak offshore upwelling can be 20°N or off California and Oregon, where coastal expected along the meridional axis of this gyre. O ther­ upwelling does occur only during a season. These fluc­ wise, coastal upwelling has ceased south of 20°N and is tuations in coastal upwelling are predominantly cou­ most intense north of 25°N. The area around 20°N now pled with the large-scale and seasonal variations of the forms a pronounced hydro-climatological transition winds. zone between the cool upwelling waters in the north Along the Northwest African coast, north of 25°N, and the warm coastal waters of tropical origin in the most intense upwelling can be expected in summer and south. autumn (cf. Wooster et al., 1976). South of 20°N In late autumn the north-flowing coastal currents are upwelling occurs essentially during winter and spring gradually replaced again by south-flowing currents when the trade belt has its southernmost extension associated with upwelling due to the increasing influ­ (down to 5°-10°N). In late spring and summer the trade ence of the trades south of 20°N. belt moves northward. During this time northward- In late winter, there are two areas of distinct density flowing currents prevail on the shelf. The counterflow decreases at the surface towards the south along the advects warm and low-saline tropical water towards coast (Fig. 34). The northern broad frontal zone at higher latitudes and causes sinking along the coast. In latitudes between 19°N and 23°N marks the boundary late summer the southern boundary of the trade belt between the warmer and saltier northern waters, lies at latitudes around 18°N to 20°N. South of these appreciably influenced by upwelled North Atlantic latitudes northerly winds are still frequent during this Central W ater and the cooler and less saline southern season. But they are usually weak and are replaced, at waters, appreciably influenced by upwelled South times, by a moderate southerly monsoon prevailing Atlantic Central Water. south of about 15°N. At this time the coastal north- The southern tropical frontal zone at about 10°N flowing current reaches up to about Cape Blanc. separates the very warm and very low saline waters in

50 south in the surface layer and a barotropic component associated with the large-scale circulation. Both baro­ 26 clinic and barotropic components are persistent fea­ tures throughout the year (with seasonal variations). 25 The baroclinic component is basically limited to the 24 coastal areas and the surface layer. The light surface water in the south tends to flow northward along the Figure 34. Density at the surface off the Northwest African coast. The barotropic component is a more general coast during winter. phenomenon and reaches down into deeper layers. In the open eastern Atlantic it forces a flow towards the east at low latitudes (North Equatorial Countercur­ rent). Along the meridional west coast and in deeper the south from the cool and more saline waters of the layers along the continental slope the higher pressure upwelling area north of it. in the south should cause northward motions. When A typical feature of subsurface layers along the con­ the trades are well developed, however, the tendency tinental slope is the rise of the isohalines and isotherms towards a northward flow is usually covered on the towards the south at latitudes between 19°N and 23°N. shelf by southward-directed wind-driven currents. Contrary to this, the density does not exhibit analogous behaviour in this area (Fig. 35). The figure shows the conditions during late winter 1973. The data have been The undercurrent taken from Schemainda et al. (1975) and Huber et al. (1977). The undercurrent, flowing poleward along the conti­ During winter the mean air pressure difference along nental slope at subsurface depths with mean speeds of 5 the African coast between 5°N and 30°N is about 10 to to 20 cm/s, is presumably maintained by the large-scale 15 millibars. This corresponds to a rise of the sea sur­ pressure gradient. This flow is a general feature of the face towards the equator of approximately 10 to 15 cm eastern boundary circulation and occurs throughout between 30°N and 5°N. Supposing hydrostatic adjust­ the year. It has been observed along the continental ment, an assumed subsurface isopycnal accordingly slope of Southwest Africa and Northwest Africa as well would be about 100 to 150 m deeper at 5°N than at as off the west coasts of South America and North 30°N. In addition, the dynamically generated pressure America. gradient in the ocean due to the Equatorial Counter- Off Northwest Africa the undercurrent tends to be a current (and the Guinea Current) in the south and the rather narrow flow (30 to 60 km wide), nestling against Canary Current in the north enhances (doubles) the the continental slope. Geostrophical computations effect of a purely hydrostatically induced sea-surface (Defant, 1941) and isentropic analysis (Montgomery, slope (Defant, 1941). The observed inclination of the 1938) clearly indicate the existence of the undercurrent isopycnal a, of 26-8 between 30°N and 10°N (Fig. 35) indicates the total pressure conditions. It appears, how­ ever, that the atmospheric large-scale pressure gradient over the eastern North Atlantic contributes appreci­ 0 ably to the alongshore variations of the pressure in the m ocean along the continental slope. The pronounced subsurface horizontal gradients of 100 temperature and salinity are persistent throughout the year (with seasonal variations) and reflect the bound­ ary between North Atlantic Central Water in the 200 north, and South Atlantic Central Water in the south. Uniform upwelling conditions along the coast, thus, would result in different temperatures and salinities at 300 the surface between 15°N and 25°N. It should be emphasized here, that the conditions described repre­ sent mean large-scale features. Deviations from the 400 mean climatological conditions may occur during any specific year and on a smaller scale. Besides the monsoon, the large-scale meridional 500 pressure gradient in the ocean along the eastern bound­ ary favours northward motions along the coast. This Figure 35. Depth variations along the continental slope of the pressure gradient is composed of a baroclinic compo­ 15°C isotherm, the 35-6%c isohaline, and the 26-8 isopycnal as nent due to a pronounced density decrease towards the observed in winter 1973.

4 * 51 > 1 0

200-300 m C.Blanc

21° 4 0 'N

C.Verde

>10

DISTANCE FROM SHORE

C. Palmas

20° W 15° 10° 5° 0°

Figure 36. The undercurrent. Small arrows offshore represent the location where a poleward-flowing subsurface current has been measured. North of 10°N the core of the undercurrent sinks into deeper layers (. .. 200-300 m, 300-400 m ...). The section on the right shows the alongshore flow during the upwelling season at 21°40'N (JOINT-I). The section on the left indicates the associated onshore component.

between the latitudes of about 8°N and 20°N. From the nounced salinity maximum at about 1250 m depth, analysis of salinity profiles, Tomczak (1973) and caused by the outflow of Mediterranean Water. Hughes and Barton (1974) traced the undercurrent According to the dynamic computations for 100 m towards the north up to Cape Bojador (26°N). South of depth by Defant (1941), the undercurrent seems basi­ Cape Blanc the undercurrent appears to concentrate at cally to originate from two different regions. One sub­ depths between 100 and 200 m. North of Cape Blanc surface current branch flows along the continental the core of the undercurrent sinks into deeper layers. slope, coming from the Gulf of Guinea. From the Hydrographic data collected aboard RV “Meteor” dur­ analysis of the vertical thermal structure, Defant (1936) ing the CINECA Multi-Ship Programme in January assumed a westward-flowing undercurrent below the 1973, suggest a faint tendency towards a poleward Guinea Current along the east-west continental slope undercurrent (in the density sections) at latitudes be­ in the Gulf of Guinea. He suggested that this undercur­ tween 30°N and 34°N within the layer between 500 and rent presents a compensatory flow of the eastward- 1000 m (cf. Huber et al., 1977). Below this layer, flowing Guinea Current in the upper layer. southward motions occur, associated with a pro­ The other subsurface current branch originates from

52 N

*

CtL

N r-g -

.■>'Uiii//JM,i.ii»//,,,va\\)iiMln\\uniiiiiüuiiiilliiinm\\\uiiiiin\\\\\\\\\u\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\mm\\\\\^^\\\\\\\\\x\s'^^

?3 ?9 30 31 1 ? 3 i» 5 6 ? 8 9 10 11 12 13 1V 15 16 1 ? IS 19 ? ? ? ? Figure 37. Vector time series of low-passed winds (top) and currents at the depths of 50 m, 200 m, 400 m, and 1000 m as observed at Station 1 (see Fig. 38) during the upwelling season.

the central tropical Atlantic. This flow branches off the gests that the undercurrent presents an important North Equatorial Countercurrent and joins the under­ source of water, upwelling inshore. In a frictional bot­ current along the continental slope between 10°N and tom layer a weak offshore component of a few cen­ 15°N. timetres per second is observed at depths. This off­ The locations where the undercurrent has been shore flow implies subsurface near-bottom downwell- observed by means of current measurements are indi­ ing along the continental slope. cated in Figure 36. Most of the measurements were The current data available reveal that the poleward- carried out with moored current meter arrays and flowing undercurrent has a vertical thickness of more lasted for several weeks. References are given in the than 500 m close to the shelf edge. The thickness of the Appendix. Every arrow in Figure 36 denotes substan­ undercurrent seems to increase with increasing water tial mean poleward flow as observed at the respective depth. Measurements from an offshore location at a locations at subsurface depths along the continental water depth of 2000 m around 23°N (Stat. BCM 3; slope. Broken arrows indicate only a temporary occur­ Brockmann et al., 1977) showed a northward flow dur­ rence of the undercurrent. In these cases (at about 7°N ing a period of 30 days between depths of about 200 m and 23 °N) the location of the current meter array may and 800 m, with mean speeds of 5 to 10 cm/s at 800 m. have been unfavourable with respect to the undercur­ Above 200 m the currents were more variable and usu­ rent, for this flow is rather narrow and may fluctuate ally flowed in a southerly direction. laterally because of topographic irregularities. From the speeds at 800 m the undercurrent may be The section to the right in Figure 36 depicts the aver­ assumed to reach considerably deeper than 800 m. At age alongshore flow in February/March 1974 at 21°40'N 17°N, northward flow has been observed offshore, during the upwelling experiment JOINT-I. The section from the near-surface layer to a depth of 1000 m, dur­ of the associated onshore component to the left sug­ ing a three-week period in February 1977 (Fig. 37).

53 Near-surface countercurrents

Observational data from the waters along the coast of C.Bojador Mauritania provide strong evidence of a quasi-persis- 25" tent near-surface countercurrent towards the north off­ shore from the shelf edge during the upwelling season. Geostrophic countercurrents in the surface layer off­ shore are known off southern California during the upwelling season in spring, while the inshore flow is 20" southward (Sverdrup and Fleming, 1941; Wyllie, 1966). Wyrtki (1963) deduced an offshore near-surface countercurrent off Peru, and Hart and Currie (1960) also found some evidence of eddy-like motions against the winds and opposite to the current direction inshore C.Verde of Southwest Africa. No near-surface countercurrents have been observed at higher latitudes along the coast of Oregon in summertime when upwelling prevails. The countercurrents offshore seem to be the inshore limbs of large eddies whose offshore parts form the great oceanic currents flowing towards the equator. Density sections normal to the coast often show a downwarping of the isopycnals within the upper layer , C . Po I mas, towards the continental slope. Approaching the shelf, they rise again owing to upwelling (and geostrophical " 5° 0“ adjustment). The downwarping of the isopycnals can 20’ W 15" 10 be interpreted as near-surface geostrophic northward Figure 38. Offshore locations where a near-surface counter- motions offshore. Dynamic computations (Fedoseev, current to the north has been measured (see Table 3). 1970; Kirichek, 1971) suggest northward flow at the surface as part of large-scale quasi-stationary cyclonic eddies, which seem to reflect the response of the Canary Current to irregular topography and frictional Near-surface current measurements off Northwest effects along the eastern boundary. Hughes and Barton Africa between 17°N and 26°N support the idea of a (1974) also found some evidence that the near-surface countercurrent offshore during the upwelling season. geostrophic flow offshore tended to set northward The current meter arrays were moored over periods of against the winds between Cape Verde and Cape Blanc 3 to 5 weeks during various years at different locations during April/May 1969. (Table 3; Figure 38).

Table 3. Current measurements offshore the Northwest African shelf edge (see Fig. 38) during the upwelling season

Station Water Observation Length of Days with Experiment Station number depth depth of time series northward identification (m) top current (days) flow meter (m) (%)

1 2025 50 21 100 A uftrieb’77 a Komet

2 ...... 500 28 18 55 A u ftrieb ’72 b 34

3 ...... 500 60 33 73 Auftrieb '7 5 c CCM2

4...... 1200 12 21 62 JOINT-I d Foxglove

5...... 500 60 29 52 Auftrieb ’75 c BCM2

6 ...... 3026 99 33 24 Auftrieb ’75 c ACM3

a See Figure 37. b Shaffer (1976); Mittelstaedt (1976). c Brockmann et al. (1977). d Mittelstaedt et al. (1975); Barton et al. (1976).

54 The percentage of days with northward flow has This indicates that no alongshore advection took place. nothing to do with the variability of the assumed coun­ The reason for the sudden onset of the northern flow tercurrent, but merely reveals something about the va­ on the shelf is unknown, so far. Perhaps it was caused riability at the respective locations. In most cases the by a lateral shifting towards the coast of the northern northward flow occurs during a continuous time in­ flow offshore, during weak winds. Another éxplana- terval. tion of these ‘counterflow events’ on the shelf might be During all experiments, listed in Table 3, current that the local wind stress (at wind forces less than 4 measurements were carried out offshore from the shelf Beaufort) no longer balances the large-scale meridio­ edge as well as on the shelf. According to the observa­ nal pressure gradient and a ‘down-slope’ flow to the tions there is, in general, a wind-driven southward flow north starts to predominate. at all depths on the shelf, whereas a near-surface coun­ tercurrent frequently occurs seaward of the shelf edge. As the observed mean vector speed of the countercur­ Discussion rent may be as high as 10 to 20 cm/s for a week or longer, these motions are assumed to reach the surface The countercurrent borders the coastal upwelling sys­ or very close to the surface, similar to the observations tem seaward. The boundary between the northward- off Peru (cf. Smith, 1978). The speeds near or at the flowing countercurrent and the southward flow inshore surface are probably reduced since the winds blow in tends theoretically to be a (two-sided) convergence, opposite directions. implying descending motions along this boundary Most of the time the near-surface countercurrent within the upper 50 to 100 m. Descending motions are apparently occurs in the same region along the conti­ assumed to be of the same magnitude and as variable as nental slope as the undercurrent at greater depths. Dis­ ascending motions close to the coast. The convergence tinguishing between these currents would be difficult in favours the formation of frontal zones with pronounced terms of current data. With regard to such data, the density gradients and accumulations of phytoplankton. undercurrent, which may obviously extend below 1000 The frontal zones are of highly variable structure, and m at times, and the near-surface countercurrent could usually occur in the vicinity of the shelf edge (cf. be one and the same current system with one and the Hagen, 1974; Keunecke and Tomczak, 1976). In these same dynamic cause. Smith et al. (1971) interpreted cases the countercurrent may be expected just offshore similar observations of a countercurrent during the upwelling season off Peru as a manifestation of the Peru-Chile Undercurrent near the surface. On the shelf, currents are usually southward-flowing at all depths as long as the trades are well developed. In contrast to this, there exist subsurface currents moving poleward opposite to the surface flow and the wind on the shelf off Oregon (see e.g., Huyer et al., 1975) and off Peru (Smith, 1978). There is some evidence that a northward counter­ flow at all depths opposite to the winds may occur on C.Blonc the African shelf when the trades are exceptionally weak. From density sections and current measurements the countercurrent seems to have a width of 50 to 150 km. During strong trades the countercurrent apparently stays offshore at distances of more than 50 km from the shelf edge. When local winds are decreasing, north­ ward motions in the near-surface layer seem to advance - D IV. shorewards and may prevail on the outer shelf or even CONV. on the entire shelf. The latter has been observed during the upwelling experiment Auftrieb ’77 by means of cur­ rent meter arrays on the shelf between 17°N and 19°N along the coast of Mauritania in February 1977. The event occurred during weaker winds (5 m/s from NE) C. Palmas, than usual and lasted about six days. In this case the upwelling system shrank temporarily to a narrow coas­ tal strip within which upwelling and downwelling were 20° W 10’ 5” 0" only a few kilometres apart. Mid-shelf the northern Figure 39. Hypothetical near-surface circulation during up­ flow started only 12 hours earlier at 17°N than at 19°N. welling.

55 from the shelf edge. Surface water of the countercur­ TROUGH RIDGE rent may be involved in the upwelling circulation on the shelf by downwelling along the convergence. If strong trades prevail for a sufficiently long time (five to ten days), ‘classical' upwelling fronts with descending motions inshore of the front, and shallow weak ascending motions offshore may develop as described theoretically by Suginohara (1977). The basic trends of the near-surface circulation dur­ ing the upwelling season off Northwest Africa between 15°N and 25°N are shown in Figure 39. The real struc­ ture of the flow pattern will deviate considerably from this idealized sketch. The countercurrent may be con­ tinuous or may break up in large eddies. The boundary between the different regimes may be locally sharp or wide, at times (see also Shaffer, 1976). Fraga’s (1974) water-mass analysis suggests that the Figure 40. Cross-circulation within the upper layer (below) countercurrent is essentially a mixture of upwelling and elevation of the sea surface Z0, (above). Thick lines indi­ cate a typical course of isopycnals. water and oceanic water from farther offshore, and some water advected from the south. The cross circulation in a vertical plane normal to the coast associated with Figure 39 is represented in Figure 40 together with some imaginary isopycnals, whose course, in principle, is intended to reflect features often References observed. Besides the probability of sinking along the Barton, E. D., Huyer, A., and Smith, R. L. 1977. Temporal convergence, there is indicated a weak and shallow variation observed in the hydrographic regime near Cabo upwelling offshore along the divergence between the Corveiro in the northwest African upwelling region, Feb. to April 1974. Deep-Sea Res., 24: 7-23. countercurrent and the Canary Current. Barton. E. D., Pillsbury, R. D.. and Smith. R. L. 1975. A The existence of an occasionally closed upwelling cir­ compendium of physical observations from JOINT-I. Ref. culation cell inshore provides the possibility that some 75-17, School of Oceanography, Oregon State University, of the upwelled water recirculates. Assuming the up­ Corvallis, Oregon, 60 pp. Brockmann, C., Hughes, P., and Tomczak, M. 1977. Cur­ welling cell on the shelf to have an offshore horizontal rents, winds and stratification in the NW African upwelling diameter of about 40 km, it would take the upwelled region during early 1975. Data Report. Ber. Inst. Meer- water approximately two to three weeks to recirculate esk., 32, Kiel, 26 pp. on the shelf. At the same time the water would flow Defant, A. 1936. Die Troposphäre des Atlantischen Ozeans. Wiss. Ergebn. dt. atlant. Exped. “Meteor” 1925-27, VI, 1: about 350 km towards the south1. The total residence 289-411. time of the water on the shelf would then be about a Defant, A. 1941. Die absolute Topographie des physikal­ month, starting from the time when water enters the ischen Meeresniveaus und der Druckflächen im Atlant­ shelf in subsurface layers and ending at the shelf edge ischen Ozean. Wiss. Ergebn. dt. atlant. Exped. “Meteor" 1925-27, VI, 2: 191-260. in the surface layer after having passed two upwelling Fahrbach. E. 1976. Einige Beobachtungen zur Erzeugung und cycles. The total southward displacement would be Ausbreitung interner Gezeitenwellen am Kontinental­ about 600 km. abhang vor Sierra Leone. “Meteor’’-Forsch.-Ergebn., A, If water upwelled inshore should sink down along 18: 64-77. Fedoseev, A. 1970. Geostrophic circulation. Rapp. P.-v. the convergence, offshore it could become trapped by Réun. Cons. int. Explor. Mer, 159: 32-37. the northward-flowing subsurface current and eventu­ Fraga, R. 1974. Distribution des masses d'eau dans l'upwell- ally upwell again inshore. This would reduce the esti­ ing de Maurétanie. Téthys, 6: 5-10. mated alongshore scale of recycling processes. Prob­ Gostan, J., and Guibout, P. 1974. Sur quelques mesures de courant effectuées dans la zone d'upwelling de Maurétanie, ably more intensive and effective than vertical recycl­ en voisinage et à l'intérieur d'un canon. Téthys, 6: 349-361. ing by ascending and descending motions are horizon­ Hagen, E. 1974. Ein einfaches Schema der Entwicklung tal recycling processes. One possible kind of horizontal von Kaltwasserauftriebszellen vor der nordwest-afrikanis- recycling is an anticyclonic recirculation with south­ chen Küste. Beitr. Meeresk., 33: 115-125. Hart, T. H., and Currie, R. J. 1960. The Benguela Current. ward transport inshore and northward transport of the Discovery Rep., XXXI: 127-297. same water, or some of it, within the near-surface Houghton, R. N. 1976. Circulation and hydrographic struc­ countercurrent offshore. ture over the Ghana continental shelf during the 1974 upwelling. J. phys. Oceanography, 6: 909-924. Huber, K., Mittelstaedt, E., and Weichart, G. 1977. Zur 1 Assuming in the upper layer: v = -25 cm/s, u = - 1 0 cm/s; Hydrographie der Gewässer vor Marokko. Meeresk. Beob. in the lower layer: v = -1 5 cm/s, u = + 5 cm/s: Ergehn., 46, Deutsches Hydrographisches Institut, Ham­ and w = ± 1Ü"2 cm/s. burg, 131 pp.

56 Hughes, P., and Barton, E. D. 1974. Stratification and water sea, vol. 2, pp. 513-535. Ed. by J. D. Costlow. Gordon and mass structure in the upwelling area off NW Africa in April/ Breach, New York. May 1969. Deep-Sea Res., 24: 611-628. Suginohara, N. 1977. Upwelling front and two-cell circula­ Huyer, A., Pillsbury, R. D., and Smith, R. L. 1975. Seasonal tion. J. oceanogr. Soc. Jap., 33: 115-130. variation of the alongshore velocity field over the continen­ Sverdrup, H. U., and Fleming, R. H. 1941. The waters off the tal shelf off Oregon. Limnol. Oceanogr., 20 (1): 90-95. coast of Southern California, March to July 1937. La Jolla Johnson, P. R., Barton, E. D., Hughes, P., and Mooers, Bull., Tech. Ser. 4, 10: 261-378. C. N. K. 1975. Circulation in the Canary Current upwelling Tomczak, M., Jr. 1973. An investigation into the occurrence region off Cabo Bojador in August 1972. Deep-Sea Res and development of cold water patches in the upwelling 22: 547-557. region of NW Africa. “Meteor”-Forsch.-Ergebn., A, 13: 1- Keunecke, K. H., and Tomczak, M., Jr. 1976. Evidence of 42. increased turbulent mixing in the coastal jet of the NW Wooster, W. S., Bakun, A. and McLain, D. R. 1976. The African upwelling region. "Meteor’'-Forsch.-Ergebn., A seasonal upwelling cycle along the eastern boundary of the 17: 88-98. North Atlantic. J. mar. Res., 34: 131-141. Kirichek, A. D. 1971. Water circulation in the northeastern Wyllie, J. G. 1966. Geostrophic flow of the California Current part of the tropical Atlantic. ICES CM 1971/C7, 8 pp. at the surface and at 200 metres. State of Calif. Mar. Res. (mimeo). Comm., Calif. Coop, oceanic Fish., Invest. Atlas No. 4: 1- Meincke, J., Mittelstaedt, E., Huber, K., and Koltermann, 288. K. P. 1975. Strömung und Schichtung im Auftriebsgebiet Wyrtki, K. 1963. The horizontal and vertical field of motion in vor Nordwest-Afrika. Meeresk. Beob. Ergebn., 41, the Peru Current. Bull. Scripps Inst. Oceanogr., 8: 313— Deutsches Hydrographisches Institut, Hamburg, 117 pp. 346. Mittelstaedt, E., 1976. On the currents along the Northwest African coast south of 22°North. Dt. hydrogr. Z. 29 (3V 97-117, Mittelstaedt, E., and Koltermann, K. P. 1973. On the cur­ rents over the shelf off Cap Blanc in the Northwest African upwelling area. Dt. hydrogr. Z., 26: 193-215. Appendix Mittelstaedt, E., Pillsbury, R. D., and Smith, R. L. 1975. Flow patterns in the Northwest African upwelling area. Dt. References on observations of the undercurrent by means of hydrogr. Z., 28: 145-167. current meters Montgomery, R. B. 1938. Circulation in the upper layers of the Southern North Atlantic deduced with use of isentropic Author Date Latitude of obser­ analysis. Pap. phys. Oceanogr. Met., Woods Hole vation (approx.) Oceanogr. Inst., 6(2): 5-55. Pillsbury, R. D., Bottero, J. S., Still, R. E., and Mittelstaedt, Houghton 1976 5-5°N E. 1974. Wind, currents and temperature off Northwest Fahrbach 1976 7°N Africa along 21°40'N during JOINT-I, February-April Gostan and Guibout1 1974 18-5°N 1974. Data Report. Oregon State Univ., 143 pp. Meincke et al. 1975 19°-20°N Schemainda, R., Nehring. D., and Schulz, S. 1975. Shaffer 1976 19°-20°N Ozeanologische Untersuchungen zum Produktionspotential Mittelstaedt and der nordwestafrikanischen Wasserauftriebsregion 1970- Koltermann 1973 21°N 1973. Geod. Geoph. Veröff., R. IV, H. 16, 88 pp. Mittelstaedt et al. 1975 21-7°N Shaffer, G.. 1976. A mesoscale study of coastal upwelling Mittelstaedt 1976 20-7°N variability off NW Africa. "Meteor”-Forsch.-Ergebn. A Mittelstaedt (unpublished) 17°-18°N 17: 21-72. Hughes and Barton1 1974 21°N Smith, R. L. 1978. Physical oceanography of coastal upwelling Barton et al. 1975 21-7°N regions. A comparison: Northwest Africa, Oregon and Pillsbury et al. 1974 21-7°N Peru. Pap. no. 40 I, Symposium on the Canary Current: Brockmann et al. 1977 21°-26°N Upwelling and Living Resources, Las Palmas. Johnson et al.1 1975 26°N Smith, R. L., Mooers, C. N. K., and Enfield, P. B. 1971. Mesoscale studies of the physical oceanography in two coas­ tal upwelling regions: Oregon and Peru. In Fertility of the 1 Short measurement of a few days only.

57