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.