May 1969

Deep-Sea P___meerch,1974, Vol. 21, pp. 611 to 628. Pergamon Preu. Printed in Great Britain.

Stratification and water m a s s structure in the upwelling area off northwest Africa in A p r i l / M a y 1 9 6 9 P. HUGHES* and E. D. BARTON,"

Abstraet--A study of the temperature and salinity in the upwelling region between the Canary Islands and Cap Vert was made on Discovery Cruise 26 (April/May, 1969). Relative to offshore conditions cold surface water was present near the coast throughout most of the area surveyed. Upwelling appeared to be most intense off Cap Blanc and Cabo Bojador. A water mass analysis showed that near Cap Blanc there was a fairly rapid transition in the upper layer from N o r t h Atlantic Central Water to water having the lower salinity characteristics of South Atlantic Central Water. Surface observations showed a similar division, confirming that longshore gradients of the water properties in the area of the cape are not negligible, and that there is a separate southern circulation. A further division of the upper layer water masses was preseat at depths shallower than 300 m at several inshore stations on the three most southerly sections and seems to indicate that there was a northward transport near the coast originating south of Cap Vert. The available evidence further seems to indicate that a fairly narrow northward subsurface flow, centred at approximately 250 m on the potential density surface o0 ffi 26-8 is present over the ccmtinental slope from Cap Vert to Cabo Bojador and is nowhere wider than 60 nautical miles (1 nautical mile ---- 1"852 k_m). South of Cap Blanc the undercurrent is part of the general northward transport in the layers above 300 m, but north of Cap Blanc it is confined to a subsurface layer below the prevailing southwesterly drift.

INTRODUCTION

THE AREASof upweiling in the world's oceans are receiving increasing attention by physical oceanographers, in attempts to understand the complicated physical processes which produce upward vertical currents near to the coasts and hence renew the concentrations of nutrients so vital to the maintenance of high productivity. International co-operation in such ventures as CINECA (Co-operative Investigations of the Northern Part of the Eastern Central Atlantic) sponsored by ICES, IOC and SCOR, and CUEA (Coastal Upwelling Ecosystems Analysis), a component of the U.S. contribution to the International Decade of Ocean Exploration (IDOE), is providing the much-needed synoptic approach to the problems of upwelling on a scale not possible for national groups. Most European nations at one time or another have contributed to the present state of knowledge of upwelling conditions off northwest Africa. Many surveys have been mainly of a biological nature directly related to the assessment of fish stocks, but few cruises have been specifically orientated towards marine physics. Discovery Cruise 26, which took place in April/May, 1969, was organized by the Department of Oceanography, The University of Liverpool, to carry out investigations (in two phases), into physical and chemical conditions in an upwelling area. The first *Department of Oceanography, University of Liverpool, P.O. Box 147, Liverpool L69 3BX, El~land. t ~ h o o l of Oc~mosraphy, Oregon State Uniwrsity, Corvallis, Oregon 97331, U.S.A.

611

612

P. HUGHES and E. D. BARTON

phase of the cruise was a general survey which was made over the area from the coast to about 100 nautical miles offshore between the Canaries (Lat. 28°N), and the Cape Verde Islands (Lat. 15°N). The preliminary results of this survey were utilized immediately to identify areas of active upwelling where concentrated measurements in particular localities could be made in the second part of the cruise. The intensive measurements were directed towards an assessment of the variability of the physical and chemical parameters, to assist in the study of specific processes important to upwelling. The purpose of the present paper is not to provide an exhaustive account of the cruise data but to concentrate on certain aspects of the stratification, water mass structure and transport. In particular, we have observed a division of the water masses in the vicinity of Cap Blanc indicating the existence of separate circulations north and south of about 21°N, and we have also found evidence of a poleward undercurrent over the continental slope at about 300-m depth extending throughout the area surveyed from Cap Vert in the south to Cabo Bojador in the north. OBSERVATIONS

The data consist of 59 hydrographic stations spaced along the cruise track between the Canaries and Cap Vert as shown in Fig. I. The survey consisted of nine lines of stations running diagonally towards and away from the coast. This particular pattern was adopted in order to achieve reasonable sampling coverage in the short time allocated to the first part of the cruise. It is c o m m o n oceanographic practice to sample on lines of stations normal to the coast, but our procedure proved to be most satisfactory because of the spreading of the data points over the area as a whole. The outermost station was about 100 nautical miles offshore and the spacing between the deep-water stations was 30-35 miles. This distance was reduced as the continental shelf was approached to give adequate coverage over the continental slope and shelf. At the five outer stations (Nos. 6896, 6905, 6917, 6930 and 6947) S.T.D. records were obtained to 1500 or 2000 m, but at the rest S.T.D. dips were restricted to 500 or 750 m except in shallower water where the instrument was lowered until it was close to the bottom. In the second part of the cruise the sampling programmes were concentrated in the areas off Cap Blanc and Cabo Bojador. At a position (20°50'N, 17°45'W) off Cap Blanc on the continental slope in a depth of 746 m, repeated S.T.D. lowerings to 300 m were made every half-hour for 2 days and to supplement this time series, three hydrographic sections perpendicular to the coast were completed. Further north off Cabo Bojador (26°15'N, 14°43'W) the same time series procedures were repeated on the continental shelf in a depth of 140 m. It should be noted that only those portions of the cruise data that are directly related to the processes to be described are presented here. The time series data is being analysed by Dr. C. N. K. Mooers, University of Miami, in a study of the variability in a coastal upwelling area, and will not be discussed in this paper. However, the mean profiles of T, S and et down to 300 m as determined from 100 S.T.D. dips off Cap Blanc will be included as they help to establish the picture of average oceanographic conditions. In the survey area there is a dearth of meteorological data, the only wind observations available for us to use were those carried out on board Discovery on a routine

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basis. These are presented on the track chart (see Fig. 1) and indicate that the northeast trade winds prevailed over the whole area during the time of the survey but decreased in strength from north to south. From the Canaries to Cap Blanc they blew from the northeast at 20-30 knots,* the speed being greatest during the night and early morning and decreasing in the afternoon. From Cap Blanc to Cap Vert the wind speed decreased gradually from 20 to 10 knots or less, the direction remaining between north-east and north. The limitations of this wind data are obvious, but even now the measurement of meteorological conditions in this area is quite inadequate for the detailed study of upwelling which depends so much on air-sea interaction processes. STRATIFICATION AND WATER MASS STRUCTURE

A discussion of the stratification and the water mass structure must emphasize the repeatable features of the vertical distributions of temperature and salinity and the related distribution of density. In a sense they may be regarded as the overall background of quasi-stationary conditions. It is worth mentioning, in passing, that on the continuous profiles obtained with the S.T.D. system there is evidence of a wealth of variability due to advection and the effects of internal waves. The advective variation may be the result of either the movement of isolated cold water patches travelling past the point of observation as described by TOMCZAK(1973) or, where there exist horizontal gradients of the water properties, they may be produced by horizontal turbulence of various time scales. Velodty com*1 knot ffi 0"51 m s -1.

614

P. HUGHESand E. D. BARTON

ponents perpendicular to the coastline can generate fairly large variations at a single geographical location particularly in the inclined frontal zone where the pycnocline approaches the sea surface. Internal modes of oscillation are present at all levels but are most noticeable where the vertical density gradients are large. A number of investigators have reported the presence of large amplitude organized oscillations which can be interpreted as internal waves (MITTELSTAEDT,1972). They occur mainly in the vicinity of the shelf break and very often exhibit predominantly tidal or inertial periods. Accounts of similar fluctuations of higher frequency have also been given (JONES, 1 9 7 2 ; MITTELSTAEDT, 1972). It is possible that these internal waves may transfer dissipative energy to the water column in the shelf area and thus enhance upwelling by increasing the intensity of the mixing processes.

Mean vertical profiles off Cap Blanc Returning to the main theme of average hydrographic conditions, selected profiles are presented to show the main features of the stratification, whilst horizontal changes will be discussed later. To illustrate conditions from the surface to 300 m depth, the profiles of the arithmetic mean temperature T, salinity S and density ~t with respect to time, obtained from 100 S.T.D. lowerings to 300 m carried out at half-hourly intervals for 2 days (24.IV.69 to 26.IV.69) are shown in Fig. 2. During this time series station, the ship maintained her position near a Dahn buoy on the continental slope in a depth of 746 m off Cap Blanc (Lat. 25°51 'N, Long. 17°45.4'W). The area in the vicinity of Cap Blanc is where much of the oceanographic research carried out by

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Stratification and water mass structure

615

various European nations has been concentrated, principally because it enjoys high biological productivity. The water column appears to be fairly well mixed in the surface layer which extends to about 40 m. The rates of decrease of temperature and salinity are then more rapid in the upper part of the pycnocline. Below 110 m the density increases at a rate characteristic of the upper layer water masses. Variation in the series may be judged by a maximum standard deviation in temperature of 0.67°C at 85 m and in salinity of 0.147%o at 80 m, these depths being, as might be expected in the pycnocline. The average distributions seem to convey correctly the main structure of the individual S.T.D. profiles except at the base of the mixed layer where changes of layer thickness with time produce in the mean a smoother transition than is in fact the case. The salinity variations with depth below the halocline exhibit two inconspicuous turning values which deserve some comment. At 170 m there is a small maximum of salinity which may possibly be related to the high salinity water formed due to excess evaporation over the shallow area of the Banc d'Arguin situated just southeast of Cap Blanc. The water so formed is dense enough to sink below the surface layer and produce an extensive high salinity tongue as described by TOMCZAK(1973). The second notable feature is to be found at 295 m where a salinity minimum of value 35.39~ is associated with a density ~, = 26.82. Low salinity water moving northwards along the continental slope is probably responsible for this minimum, as will be revealed in a more detailed treatment later on in the paper. The associated vertical variation of the stability parameter E, has been computed from the formula E=

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given by HESSELBERGand SVERDRUP(1915), where c is the speed of sound and g is the acceleration due to gravity. The stability is zero in the surface mixed layer, it rises to a maximum value in the pycnocline and then decreases with increasing depth to quite low values in deep water further offshore. Apart from conditions near the sea surface at hight when convectional overturning can occur due to surface cooling, there is no evidence of persistent instabilities. The magnitude of E depends on the density gradient and defines the buoyancy or Brunt-V~is/il~ frequency, N by the relation N 2

=

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The buoyancy frequency which is a fundamental parameter in the study of internal waves is a function of depth, the density fluctuations at a particular level being suitably averaged out, and it is not surprising therefore that the depth profile of E computed from the mean density distribution of 100 S.T.D. lowerings is different from the results of computations on an individual density profile (see Fig. 3). The average stability as a function of depth (dashed line in Fig. 3), is smoother and has a much lower maximum in the pycuocline region. This is the value most appropriate to internal wave computations. Nevertheless, the stability profile obtained for a single S.T.D. lowering gives an estimate of the instantaneous structure and is the most common representation due to the practical limitations of oceanographic sampling.

616

P. HUGHES and E. D. BARTON

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In the discussion of the horizontal distribution of water properties in the next section, the topography of the pycnocline stability maximum is derived from station data in which time variability is not taken into account. In contrast to the eastern Pacific where salinity increases with depth in the upper layer, off northwest Africa, the salinity decreases with depth but the rate of decrease of temperature is large enough to ensure that the water column remains statically stable. Before leaving the time series observations it is interesting to note that from S.T.D. lowerings the water column appears to be stable when viewed over vertical distances of about 10 m or more. There are apparent inversions in the S.T.D. output, but these are instrumental effects depending as is well known on the characteristics of the temperature compensating circuits. Several numerical techniques have been used in attempts to reduce these effects, but the accuracy of salinity measurements is considerably reduced whenever there are strong temperature gradients such as are found in the thermocline. The speed of lowering the probe is also important and improvements in the quality of the measurements were obtained in a later series off Cabo Bojador by reducing the lowering speed at depths where rapid temperature changes were expected.

Surface temperature, salinity and density The distributions of temperature, salinity and density at the sea surface are shown in Fig. 4 in an attempt to represent the overall patterns of these variables to the exclusion of small-scale anomalies which are common features over the continental shelf and slope. Figure 4a shows the isotherms at intervals of I°C. The most notable hydrographic feature associated with coastal upwelling is a decrease of temperature towards the coast, and this is well illustrated by the isotherm chart which reveals changes of as much as 4°C within 100 nautical miles. The temperature change is a consequence of the upward movement of cold water from below the. surface near the shelf area and its subsequent movement offshore by wind-induced advection and diffusion. The oceanic surface temperatures are about 21°C whilst the lowest temperatures of less than 17°C were recorded off Cap Blanc and Cabo Bojador indicating that these were areas of more intense upwelling.

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The isohalines also tend to be aligned parallel to the coast with salinity generally decreasing in the onshore direction (Fig. 4b), but there is a notable decrease of salinity north to south of about 1%0. The major change of salinity occurs near Cap Blanc where the isohalines sweep out to sea in a southwesterly direction. Thus the salinity south of 21°N is significantly less than to the north and there is a definite indication of a physical division of the surface circulation at this latitude. Evidence to show that the division extends below the surface layer into the upper layer will be given later on. The sea surface isopycnals (Fig. 4c) show an increase of density towards the coast over nearly all the survey area, thus indicating the presence of upward vertical transports of dense water over the continental slope and shelf. The highest surface density recorded was ~t = 26.6 off Cabo Bojador in the north. From this area southwards to Cap Blanc the isopycnals are well aligned with the shelf edge, but thereafter longshore density gradients are of significant magnitude and are evidence of the more complicated hydrographic conditions off Mauritania. The density decreases to 25.2 in the south of the region.

The depth and intensity of maximum stability If we define the position of the pycnocline in terms of the depth of maximum static stability, a chart can be constructed showing the depth of the pycnocline throughout the area surveyed (Fig. 5a). It provides a clear picture of the position of the inclined frontal zone in the upweUing area. The pycnocline rises towards the surface as the coast is approached, the gradient being steepest in the northern half of the region. As a whole, the offshore depth of the pycnocline decreases from north to south consistent with the approach to tropical conditions where there is often only a shallow mixed layer and the pycnoeline is consequently nearer to the surface. The numerical magnitudes of maximum static stability based on computations carried out on individual S.T.D. profiles show a general increase of stability well offshore from 20 x 10-s m -1 in the north to 50 x 10-6 m -1 in the south (see Fig. 5b). The increased stability is again associated with the tropical regime. Towards the coast, the stability weakens, showing a minimum value of 10 x 10-6 m -x in the pycnocline off Cabo Bojador. Both diagrams help to support the view that Cap Blanc and Cabo Bojador are areas of active upwelling. Water mass division in the upper layers off Cap Blanc Potential temperature--salinity (0-S) curves for the five outside stations of the survey (6896, 6905, 6917, 6930, 6947) are shown in Fig. 6. They illustrate the general water mass structure down to about 2000 m and its relation to the classification of water masses according to ALLArN (1970). Three of the stations, viz. 6896, 6905 and 6917 are situated north of Cap Blanc and share common properties in the upper layer, which extends downwards from 150 to about 800 m. The 0-S characteristics resemble North Atlantic Central Water (NACW) whereas the two Stas. 6930 and 6947 between Cap Blanc and Cap Vert exhibit the lower salinities associated with South Atlantic Central Water (SACW). On a still finer scale, 0-S diagrams of the stations on sections 5 and 6 show quite clearly the water mass division in the upper layer just to the north of Cap Blanc. We have seen already that the sea surface isohalines turn towards the southwest off Cap Blanc and it is now apparent that between Stas. 6918 and 6919 the water characteristics change significantly (See Fig. 7). For example, at 300 m there

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is a decrease of salinity of 0"2%0 and a corresponding temperature decrease of 0.4°C with no density change between these two stations. All stations to the south maintain low salinities. It can be stated therefore that in the upper layer of the upweUing region the 0-S characteristics are transitional between NACW and SACW, but the offshore waters to the north of Cap Blanc are closely related to NACW whereas in the southern part of the region south of 21°N, there is an affinity to SACW. Referring again to Fig. 6 showing 0-S relations for the outside stations, it is seen that below the central water masses the salinity reaches a minimum and then increases with increasing depth to a maximum at about 1200 m. Whether or not some traces of Antarctic Intermediate Water (AIW) are responsible for the minimum at 100 m is problematical, but certainly the intermediate layer is influenced by high salinity water of Mediterranean origin. Mediterranean water (MW) cause the inversion centred at about 1200 In, and its intensity is a function of position, becoming less marked south of Cap Blanc. Below the intermediate layer all stations show a decrease of potential temperature and salinity with increasing depth, the characteristics approaching those of North Atlantic Deep Water (NADW). At these depths, unlike conditions near the surface where there is a large range of water properties, the variation throughout the region is quite small. Water mass structure south o f Cap Blanc

One particularly interesting feature of the upper layer water mass structure is clearly seen from the temperature and salinity data on sections 7, 8 and 9. Except for the stations closest to the offshore ends of the sections there is evidence of a distinct water mass above 300 m which is characterized by its low salinity. The 0-S diagrams for sections 5, 6, 7, 8 and 9 are given in Fig. 7 to demonstrate the uniform distribution

620

P. HUOHES and E. D. BARTON

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of SACW over the area below 300 m and to show how it is restricted to the outside end of the sections above that level while further inshore water of lower salinity is present. The effect is most obvious on the section furthest south off Cap Vert but decreases towards the north. There is some evidence in measurements taken on Crawford Stas. 154 and 155 during I.G.Y. (FUGLISTER,1960) and earlier Meteor observations, for example, Stas. 267 (18.11.27. 13.7°N, 19.8°W), 218 (28.X.26. 9.0°N, 17.7°W) and 219 (29.X.26. 9.6°N, 16.4°W) (W~3ST, 1938) to suggest that low salinity water closely related to the type we have observed is present south of Dakar and moves northwards into the area between Cap Vert and Cap Blanc. In an attempt to supplement the analysis of T, S data we have produced in Fig. 8 a chart of the dynamic topography of the sea surface relative to 500 db. Several difficulties make the interpretation of dynamic topography charts uncertain. Of these probably the sparse station coverage in the south is the most important, but there is also the question of the extension of isopleths into shallow water and finally the adverse effects of short-term variability on geostrophic balance, particularly in the near-shore region. We have not attempted to extend these lines across the 500 m contour. Nevertheless, there is some evidence to support the conclusion that the fresher water mass at less than 300 m, which was detected at certain stations on the three southerly sections, actually originates from south of Dakar. THE POLEWARD UNDERCURRENT The surface flow in coastal upwelling regions is predominantly equatorward, but the presence of a poleward undercurrent adjacent to the continental slope at a depth of about 300 m has been reported as an important feature in most major upwelling

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areas (WOOSTER and REID, 1963). The Peru-Chile Undercurrent was mentioned by GtrNTrmR (1936) as a current providing a compensating flow in the upwelling zone, and later WOOSTER and GILMARTIN (1961) confirmed its existence by parachute drogue measurements, dynamic calculations based on the distribution of density, and the high temperature, high salinity and low oxygen content tracer characteristics of the south-going water. An analogous flow pattern off California was reported by WOOSTER and JONES (1970), who found a northward subsurface flow beneath the prevailing equatorward flow at the surface. In the Pacific, the poleward flow was readily detected by the presence of a high salinity tongue of water at about 300 m in which salinity decreased with increasing latitude due to the effects of mixing with surrounding waters of lower salinity. Unlike the eastern Pacific, the eastern tropical Atlantic does not possess a supply of high salinity water, so that an analysis of temperature and salinity data is unlikely to be as conclusive. Despite this difficulty, HART and CtmRIE (1960), in their investigations in the Benguela Current in 1950, deduced the presence of a southward current on the continental slope off the southwest African coast from the downwarping of the mid-depth isopycnal surfaces as they approached the continental slope. There has been little published evidence, however, of the presence of an undercurrent off northwest Africa except by TOMCZAK(1973). His discussion of the existing historical data in the area and inference of the existence of such a current from salinity data led us to examine the relevant observations from Discovery Cruise 26. We have found evidence of a northward flow in the subsurface layers extending from Cap Vert to Cabo Bojador, a distance of more than 800 nautical miles over the complete length of our survey.

622

P. HUGHESand E. D. BARTON

Distributions of salinity and potential temperature on surfaces of equal potential density The study of the distribution of one variable on the iso-surface of another variable is called isentropic analysis and its application to oceanography was first discussed by MONTC,OMERY (1938). For example, the isolines of salinity on a surface of equal potential density can provide information on the flow pattern, at least as regards direction. In the oceans there is a tendency towards horizontal flow, or more specifically, flow along levels of equal potential density, because minimum energy is required; the current therefore tends to follow the direction of the isolines. This technique is virtually equivalent to the 'core method' of tracing water movements because the core usually follows a density surface. Survey data from Discovery Cruise 26 were used to construct charts of the distribution of salinity on several surfaces of equal potential density off northwest Africa. The most interesting distribution is shown in Fig. 9b and depicts the isohalines on the % = 26.8 surface. The depth of this density surface is illustrated in Fig. 9a. In the north of the region it slopes steeply upwards towards the sea surface in an onshore direction indicating that strong upwelling was occurring at the time of the cruise. To the south of Cap Blanc the contour lines of the surface are not simply parallel to the coastline, the depth of the density surface being least at some distance away from the coast, and greater to both the seaward and shoreward sides of the ridge thus formed. In this area, upwelling is apparent only in the very shallow layers, and is weaker than in the north. The depth of the density surface is nowhere greater than 300 m. The distribution of salinity on this surface shows that throughout the region a tongue of lower salinity water was situated parallel to the coast, orientated in a northward direction. The width of the tongue everywhere was less than 60 nautical miles. In the south the salinity minimum lies to the shoreward side of the ridge in the depth contours of the density surface. The tongue of less salty water appears to lie at a depth of between 200 and 250 m to the north of Cap Blanc (21°N) and also at about the same depth to the south, becoming shallower only where it crosses the area off the Bane D'Arguin. Potential temperature on the same surface of potential density (Fig. 9c) indicates a tongue of cooler water roughly parallel to the coast and directed towards the north, although this is not so well defined as in salinity. The distributions of salt and heat on the surface ~0 ----26.8 are clearly indicative of a fairly narrow stream of cooler, less salty water flowing towards the north at a depth of 200-300 m parallel to the continental shelf edge. The vertical extent of the influences of this flow is apparently quite limited as there were no similar indications of a poleward movement in the distributions on the density surfaces % ----26.7 and % -----26.9.

The meridional salinity distribution A vertical section of salinity was drawn up for those hydrographic stations situated in the centre of the tongue of low salinity water (Fig. 10). At all stations south of 24°N a minimum value of salinity was found near the depth of the density surface % = 26.8. The maximum depth interval affected by the salinity anomaly was nowhere greater than 150 m; its intensity showed an overall decrease from south to north, being greatest at Sta. 6952 in the south. At the northernmost end of the meridional section it could not be identified at all, The extent of the anomaly is shown

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624

P. HUGHF.S and E. D. BARTON

by the shaded area in Fig. 10. The anomaly was present at Sta. 6968, situated in the centre of the less saline tongue at latitude 20°50'N off Cap Blanc, which was surveyed some time later than the other stations in the section. At this station the anomaly was found on the same density surface, although at a slightly greater depth than previously. For this reason the station was omitted from the section, but was included in the 0-S diagram discussed below. The evidence of the salinity section again indicates a northward flow of water at the potential density surface ~0 = 26-8 (dashed line in Fig. 10). If the range of depth affected by the flow is defined by the limits of the salinity minimum, then it appears that the undercurrent is confined to a layer less than 150 m in thickness. However, this definition may be seen to be somewhat inadequate because the vertical anomaly cannot be traced as far north as the horizontal salinity anomaly illustrated in Fig. 9b.

Temperature-salinity analysis The relevant portions of the 0-S curves for all the stations situated in the centre of the low salinity tongue together with lines of equal potential density % are presented on one 0-S diagram (Fig. 11). The wide spread in the upper parts of the curves shows the equatorward decrease in the depths of the isohalines and hence in surface salinity values. Below the depth of the salinity minimum the 0-S curves converge to the characteristics of NACW. The curve % = 26-8 passes through the salinity minima of all stations plotted, except for those northernmost ones where there is no minimum. At these Stas. (6898, 6903, 6906), however, there is a well-defined inflexion in the 0-S surves at that isopycnal surface. It is clearly visible that a well-defined salinity minimum at the southernmost stations decreases in size as the stations are progressively situated further north until only an inflexion can be observed. This may be thought suggestive of a weakening of the flow in the north. It should be noted that the 0-S curve for Sta. 6968 fits well into the set of curves although the station was surveyed at a later time in the cruise than the rest of the stations. At this position, the depth of the density surface at which the salinity miniSt(]. 6903

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Stratification and water mass structure

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m u m is situated was found at a different level, but it is seen that this does not materially affect the 0 - S characteristics. Direct current measurements The only relevant current measurements made on Discovery Cruise 26 were two relatively short series of parachute drogue observations at Sta. 6970 off Cap Blanc. The depth at this station situated on the continental slope was 746 m and was a position where the undercurrent might be expected. Separate parachute drogues at 50 and 300 m were released and subsequently tracked by radar while the ship maintained her position relative to an anchored buoy. Figure 12 shows the progress of the surface markers for 12 h in the case of the 50-m parachute and for 2 days for the drogue at 300 m. The dashed line covers the period between 2100, 24.IV,69 and 1900, 25.IV.69, when contact with the 300 m drogue was lost. The average drift was generally northwards at both levels and thus in the opposite direction to the surface wind. At 50 m the displacement over 12 h resulted from a northward drift of 24 cm s -1, whereas at 300 m the northward speed over the measurement period of 2 days was 3 cm s -1. The short duration of these observations makes it impossible to arrive at any definite conclusion about the longer term average currents, but it was encouraging to record a northward flow at 300 m. It seems likely that the current at 50 m was associated with the southern circulation whose northern limit is in the vicinity of Cap Blanc. Discussion The evidence seems to suggest that a northward subsurface flow parallel to the coast is present at a depth of 250-300 m throughout the region, although south of

626

P. Hu(3ri~ and E. D. B.~TON

1300 26.4.69

0000 26.4.69

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Fig. 12. Parachute drogue tracks off Cap Blanc, showing northerly flow at 50 and 300 m.

Cap Blanc the undercurrent is part of a general northward transport in the layers above 300 m, as already discussed in a previous section. The distribution of salinity on the surface of potential density *0 = 26,8 does not indicate any lessening of the undercurrent towards the north of the region, but both the meridional section and the T-S diagrams show a marked decrease in the size of the vertical anomaly in the north. However, TOMCZ,~K (1973) found a salinity maximum between the ** surfaces 26.7 and 27.0 at depths of 200-300 m throughout the region from Cabo Bojador to Cap Blanc in January, 1970. Tomczak concludes that above the maximum is a salinity minimum which may be explained by a northward undercurrent. It may well be that the intensity of the current varies appreciably with time of year and season, and was strong at the time at which Tomczak's data were acquired and weak at the time of the Discovery cruise. Tomczak also found evidence of a salinity maximum in a survey of data obtained from NODC. His survey of the historical data included measuremeats taken at all times of year, but did not take into account temporal variations in the salinity distributions, because of the relatively small number of stations possessing a salinity maximum. ALL,~rN (1970) has produced a T-S diagram for stations off Cap Blanc surveyed in December, 1962. The curves for his stations show a minimum centred approximately at the surface ** = 26.9, which Allain interprets as evidence of a northward flow in the top 300 m and a southward flow in the lower levels. It seems probable that the salinity minimum found by Allain and the one discussed above represent the same feature. However, our investigations show that north of Cap Blanc the polvward flow is confined to a subsurface layer beneath the prevailing southwesterly drift.

Permanence of the undercurrent The undercurrent seems to be a major feature of upwelling regimes in general and its existence on theoretical grounds has been postulated by YosnmA (1967). Furthermore, its appearance in recent coastal upwelling models has been discussed in an

/

Stratification and water mass structure

627

important review by O'BRIEN (1973). Time-dependent aspects of the phenomeon are unknown, however, but it seevas likely that in common with most features of the ocean circulation it will be the subject of considerable variation, and may at times not be present at all. Similar remarks apply to its spatial distribution normal to the coast. Tomczak comments on the possibility of missing the characteristic anomaly of salinity if standard hydrographic techniques with water sampling bottles are used instead of the now familiar S.T.D. measurements. Similarly, of course, the spacing of hydrographic stations could result in the feature leaking through the observational network by virtue of its narrow horizontal extent. CONCLUSIONS

The foregoing study leads us to the following conclusions: 1. There was evidence of upwelling over the whole coastal region from the Canaries to Cap Vert during Diacovery Cnfisv 26. 2. Upwelling was most intense off Cabo Bojador and Cap Blanc. 3. There was a distinct division of water properties in the vicinity of Cap Blanc, probably as the result of a boundary between the waters from the north which at this latitude are advectcd southwest and a separate circulation in the southern part of the region off the coast of Mauritania. 4. The maximum stability in the pycnocline varies from 10-5 m -1 near the coast in the north to 5 × 10-5 m -1 offshore in the south, although the maximum value of the timeaveraged stability is less. The depth of the pycnocline as defined by the depth of maximum stability rises on occasions from 100m offshore to the surface in the nearshore regions. 5. The presence of a northward movement of fresher water along the Mauritanian shelf region is inferred from the study of 0-S diagrams. This water seems to originate south of Cap Vert. 6. Analysis of potential temperature, salinity and potential density data has indicated the presence of a poleward flowing undercurrent off the coast of northwest Africa. Evidence of the current, which appeared to be centred at a depth of about 250 m approximately on the potential density surface % = 26.8, was found from Cap Vert to Cabo Bojador, remaining parallel to and close to the continental shelf edge (within 60 nautical miles) throughout the region. Acknowledgements--We would like to thank C~OT~dN R. H. A. DAVIES,his officers and crew on the R.R..S. Discovery and our own colleagues from the Department of Oceanography for their assistance with the experimental work on Discovery Cruise 26. E. D. Barton is supported financially by a N,E.R.C. studentship (GT4/70/OF/13). REFERENCES

ALLAINC. (1970) Observations hydrologiques sur le talus du Bane d'Arguin on ~ b r e 1962 ( C a m p a g n e do la Thalassa du 2 N o v e m b r e au 21 ~ b r e 1962). Rapport et proc~s-verbaux des rdunions. Conseil permanent international pour l'exploration de la mer,

1~, 86-89. FumzsT~ F. C. (1960) Atlantic ocean atlas of temperature and salinity. Profiles and data from the I.G.Y. of 1957-1958. Woods Hole Oceanographic Institution Atlas Series, 1, 209 pp. GTY~CrH~E. R. (1936) A report on oceanographical investigationsin the Peru coastal current. Discovery Report, 13, 109-276.

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P. Huot-ms and E. D. B,~Tbr~

HART T. J. and R. I. CURRIE (1960) The Bcnguela Current. Discovery Report, 31, 123-298. HEaSELBER~ TH. and H. U. SVERDRUP(1915) Die Stabilitfitsverh~ltnisse des Secwassers bei vertikalen Verschiebungen. Bergens museums Aarbok, 1914-15, No. 15, 16 pp. JONES P. G. W. (1972) The variability of oceanographic observations off the coast of northwest Africa. Deep-Sea Research, 19, 405-431. M1TTELSTAEDT E. (1972) Der hydrographische Aufbau und die zeitliche Variabilit~it der Schichtung und Strfmung im nordwestafrikanischen Auftriebsgebiet im Frtihjahr 1968. 'Meteor' Forschungsergebnisse, A, 11, 1-57. MONTOaMERY R. B. (1938) Circulation in upper layers of southern North Atlantic deduced with use of isentropic analysis. Papers in Physical Oceanography and Meteorology, 6, 2, 55 pp. O'BRIEN J. J. (in press) Models of coastal upwelling. Journal of Physical Oceanography. TOMCZAK M., JR. (1973) An investigation into the occurrence and development of cold water patches in the upwelling regions of N.W. Africa ('Meteor'---cruise 19). 'Meteor" Forschungsergebnisse, A, 13, 1-42. WOOSTER W. S. and M. GILMARTIN(1961) The Peru-Chile undercurrent. Journal of Marine Research, 19, 97-122. WOOSTER W. S. and J. H. JONES (1970) Californian Undercurrent off northern Baja California. Journal of Marine Research, 28, 235-250. WOOSTER W. S. and J. L. REID (1963) Eastern boundary currents. In The sea, M. N. HILL, editor, Interscience, 2, pp. 253-280. W/2ST G. (1938) Dynamische Werte. WissenschaftlicheErgebnisse der Deutschen ,4tlantischen Expedition auf dem Vermessungs- und Forschungsschiff'Meteor', 1925-27, 6, 2, pp. 99-181. YosmDA K. (1967) Circulation in the eastern tropical oceans with special reference to upwelling and undercurrents. Japan Journal of Geophysics 4(2), 1-75.