By Y. Gouriou
The conditions of the oceanic environment encountered by tuna, temperature, food, oxygen, currents, etc. …, broadly govern the abundance of stocks as well as their migrations and the possibilities for fishermen to capture them. It is therefore essential to describe the oceanic environment of the study zone. Spatio-temporal variationns of the climate largely govern the oceanic environment and will be studied first (section 3.2). The surface and subsurface oceanic circulation in the region will be described next (section 3.3). The upwelling mechanisms of cold water, rich in nutrients that enrich superficial waters, will be described and their origin discussed (section 3.4). Finally, enrichment mechanisms and formation of phytoplankton will be described (section 3.5).
The dynamics of the upper layer of the oceans is intimately tied to the lower layers of the atmosphere; the wind notably plays a driving role in the genesis and evolution of superficial and subsurface oceanic currents. It is therefore indispensable to have good knowledge of the atmospheric parameters that influence the oceanic region studied. The changes and the variations of these parameters can only be truly included if one limits the study zone, the atmospheric conditions on this region being tied to the dynamics of the atmosphere of the whole Atlantic. In this chapter, we will present a general view of the climate permiting us to begin the study of oceanic dynamics in the study zone.
The air circulation in the lower layers of the atmosphere of the tropical Atlantic is organized around two permanent circulation centers:
the Azores high pressure center in the North Atlantic,
the Saint Helena high pressure center in the South Atlantic.
These two high pressure centers determine anticyclonic systems in which the flow dominates the marine regime. On the continent, the circulation is subject to the influence of the Egypto-Lybian anticyclone and that of the Mascarene (Indian Ocean) anticyclone. These anticyclonic belts are created by subsident movements, toward the latitudes 30° N and 30° S, coupled to subtropical and polar jet streams at altitude. These movements are of dynamic origin, and entirely independent of surface thermal factors.
The excess of energy absorbed in the intertropical belt creates a zone of low pressure of thermal origin separating the centers of high pressure of each hemisphere. In each hemisphere, a flow of air between the tropical high pressures and the intertropical low pressures is established. This flow is diverted toward the west by the rotation of the earth: these permanent and stable winds are called trade winds. The limit between the flows of each hemisphere is called the meteorologic equator or the intertropical convergence zone (ITCZ).
Figure 3.1 Surface atmospheric circulation pattern. The circulation above the ocean is determined by the Azores anticyclone in the northern hemisphere and by that of Saint Helens in the southern hemisphere. The trade winds convergence zone (ITCZ) moves according to season. In July, it is in its most northern position.
This simple schema of circulation in the lower layers permits a satisfactory description of the wind field observed in the eastern area (to the east of 20° W) of the tropical Atlantic.
In the eastern part, the asymmetry observed in the distribution of the continental mass of Africa from one side of the equator to the other significantly disrupts the preceding schema. The the thermal intertia of the land being lower than that of the ocean (the land heats or cools more rapidly than the sea) causes an important seasonal movements of continental intertropical low pressures, coupled with the zenithal movement of the sun. The seasonal latitudinal translation of these low pressures is much more important in the region of the tropical Atlantic subject to the continental influence than in the oceanic domain.
The heating of the eastern region of the continent during the boreal summer drives the formation of a trans-equatorial pressure gradient above the Gulf of Guinea. This pressure gradient introduces a deviation of trade winds toward the east, reinforced by the change of sign of the Coriolis force crossing the equator (the rotation of the earth is the origin of this force that forces movements toward the right in the northern hemisphere and toward the left in the southern hemisphere). The trade winds thus transform to western monsoons.
The action centers cited in section 3.2.2 affect the origin of air flow in the lower layers on the intertropical Atlantic. In the maritime regime, three flows of different origins are joined (figure 3.1; Wauthy, 1983):
- Flow originating in the South Atlantic
In the southern hemisphere the importance of the continents is slight; oceanic influence is predominant. The flow is organized around the St. Helena circulation cell. This anticyclone, dominating all of the Gulf of Guinea, is centered on 28° S and 10°W. Its intensity is maximum during the boreal summer (July, August, September).
On the eastern side, the wind blows from the south toward the north along the African coast; this maritime trade wind is permanent to 16° S and extends to Cape Lopez in boreal summer.
On the equatorial side, the trade wind is from the south-east, drawn by the continental low pressures and is displaced toward its right to the north of the equator, amplified by the change of sign of the Coriolis Force crossing the equator. This trade wind changes to a monsoon, a warm and humid wind, on the African continent. The penetration of this monsoon flow is controlled by the movement of intertropical continental low pressures.
On the western side, the trade wind heads toward the west until it meets with the trade wind of the northern hemisphere along the meteorological equator (ITCZ). During this long oceanic voyage this tradewind warms, humidifies, and becomes unstable.
- Flow originating in the North Atlantic
In the northern hemisphere, the continental mass is predominant and the reheating during the boreal summer prevents the establishment of anticyclones in Africa. Only the ocean permits the installation of permanent high pressure centers. The circulation cell which is organized around the anticyclonic center of the Azores controls the flow of maritime tradewinds from the northern sector on the western boundary of the North African continent. During the boreal summer, these tradewinds can be drawn by the continental low pressures and penetrate the coastal boundary of Senegal (wind from WNW).
- Flow originating from the Egypto - Lybian anticyclone
This anticyclone controls a dry, warm continental flow of trade winds known as harmattan. This flow of air concerns all of the continent situated to the north of the meteorological equator; its variability important. It is not rare for it to be felt all the way to the equatorial boundary of the African continent (Ghana, Ivory Coast…).
The variations in intensity and position of the anticyclonic centers determine the seasonal change in surface wind.
In boreal winter, the cooling of the northern hemisphere permits the establishment of high pressures on the African continent that prolong the Azores anticyclone. This reaches its maximum intensity and most meridional position in March. Along the Mauritania-Sengal coast, the tradewinds blow from the north to north-east sector from November to February. It is a period of continental tradewinds and harmattan. The speeds are in order of 4 to 5 m/s. From March to May the winds are from the north, the easterly component becomes dominant in proportion to their progression toward the south. It is the period of the maritime tradewinds where the winds reach their maximum intensity: 5 to 6 m/s along the coasts, 7 m/s to the west of 18°W. In the Gulf of Guinea, the southern hemisphere trade winds reaches the equator with a low horizontal speed (3 m/s) and cross it between 20°W and the Gabonese coast. The monsoon flow is then weak and is of importance only on the coastal boundary of Africa in the Gulf of Guinea. The winds, with a strong zonal component, are maximum in the western region of the basin (10°N, 40°W) where they reach speeds equal or superior to 7 m/s.
In boreal summer (July, August, September), the Saint Helena anticyclone is reinforced and migrates toward the north. The meteorological equator reaches its most northern position (10°N) in July-August. The heating of the African continent to the north of the equator permits the establishment of low pressure zones. Along the Mauritania-Senegal coast the wind from the north-east sector is minimum in September (2–3 m/s) and may be drawn by the continental low pressures, it blows into Senegal to the south of Cape Verde and takes a strong westerly component. When the ITCZ reaches the latitude of Cape Verde, the winds become unstable. The tradewinds resulting from the Saint Helena anticyclone cover all the intertropical Atlantic to 10°N. The wind intensity reaches its maximum in August (5 to 7 m/s at 40°W), the monsoon flow, diverted across the equator by the Coriolis force and by the continental low pressures, sweeping through the continent to Tibesti in the north and the Ethiopian heights in the east.
The study of the variability of wind pressure on the intertropical Atlantic done by Servain et al. (1985) shows that the regions for which the seasonal variability is maximum are situated around the meteorological equator. The extreme variations occur along the mean position of the ITCZ. The variability of the wind pressure in the Gulf of Guinea is 3 to 4 times less than in the west of the basin.
The wind is the principal driving force of the surface oceanic circulation. It entrains a more or less thick layer of the ocean surface by friction; thus an anticyclonic circulation is found in each hemisphere associated with the Azores and of Saint Helena anticyclones. The asymmetry of their geographic position in relation to the equator is reflected in the superficial oceanic circulation. The study zone will be subjected to the influence of the circulation of the southern hemisphere.
- In the northern hemisphere, running along the North Atlantic Drift (NAD) we find: (figure 3.2; Wauthy, 1983).
The Canary Islands Current. Running along the Moroccan and Mauritanian Coast, it leaves the coast around 20° N and heads toward the south-west.
On the equatorial side of the North Atlantic Drift, the North Equatorial Current (NEC) is the extension of the Canary Islands Current. It flows toward the west with a meridional component oriented toward the south in the east, and toward the north in the west. It's average speed exceeds 10 cm/s, and is weaker on the eastern edge than the western edge. This current presents weak seasonal variations and weakens to the east between June and September.
Between 4° N and 8° N, the north equatorial counter current (NECC) flows to the west. It is permanent to the east of 20° W and is extended into the Gulf of Guinea by the Guinea Current. It is subjected to strong seasonal variations. From May-June, the NECC extends to the west and the north; it reaches its maximum extension around September where it occupies all of the basin to the east of 50° W between 4° and 10° N. Its speed is on the order of 40 cm/s. From November to January the NECC progressively disappears in the west and in March exists only to the east of 20° W. From March to June, the current flows to the west, to the west of 20° W (Richardson, 1984).
The NECC extension, or the Guinea Current, extends along the African coast (5° N-2° N) to the end of the Bay of Biafra. It intensifies to the east of Cape Palmas to reach speeds on the order of 30cm/s. Between 4° E and 8° W, two maximum speeds are observed, one in July-August (60cm/s), and the other in February (40cm/s). The permanence of this current causes an accumulation of water at the far end of the Bay of Biafra which is drained by the northern branch of the south equatorial current.
- In the southern hemisphere, bordering the South Atlantic Drift (SAD) we find:
The Benguela Current that extends along the Namibia coast toward the north, shifts to the west at the level of Cape Frio (17° S).
On the equatorial side of the SAD, the south equatorial current (SEC), that flows toward the west is much more developed than its counterpart, the north equatorial current, since it reaches 3° N.
Figure 3.2 Surface temperature and circulation in January. The surface currents are driven by the winds and the anticyclonic movements of the northern and southern hemisphere atmosphere are reflected in this circulation. The current system moves towards the north from January to July Upper panel January; lower panel July. (Wauthy, 1983).
Figure 3.3 Trajectories of drifting buoys. Tracking buoys shows the complexity of the surface circulation in the inner Gulf of Guinea. Only buoy E1475, released south of the haline front caused by the Congo River, was taken in the south equatorial current and out of the Gulf of Guinea (Piton and Kartavtseff, 1986).
In the central Atlantic the SEC divides in two branches, around 2° N and 4° S, and flows toward the west. Their average speed is 35 cm/s.
Before reaching the South American continent, the southern branch of the SEC divides into two currents: the Brazil Current (15cm/s) that flows toward the south, and the Guiana Current (60 cm/s) toward the north. The northern branch of the SEC merges with the latter near 4° N - 50° W. The northern branch of the SEC presents an annual cycle marked with maximum speeds in June and December. The southern branch presents weaker monthly fluctuations dominated by an annual period. From September to February, the two branches are of equal speeds, from June to August the northern branch is more rapid, while the situation from March to May is the inverse.
In the west of the basin (35° to 45° W), the two branch structure of the SEC is only present from August to November. In the east of the basin (10° to 20° W), the northern branch is at its maximum speed (66 cm/s) in June near 2° N. The minimum speeds are generated in October and February. The southern branch has a maximum speed of 50 cm/s from May to July near 4° S. During several months, from December to February, the north and south branches merge to form a unique current (Richardson and McKee, 1984). The low relative speed of the South-East equatorial current is induced by the equatorial upwelling which slows the current near 1° S.
It is interesting to note that two buoys (E1475 and E1476) released less than 24 hours apart in December 1984 at the mouth of the river Congo followed totally different trajectories. The buoy E1476 placed in brackish water of the river, flowed into the Bay of Biafra, while the buoy E1475 placed at the south of the haline front headed toward the west; it is the only buoy taken in the SEC that did not stay trapped in the Bay of Biafra (figure 3.3).
Figure 3.4 Sections along 4° W : temperature, salinity, zonal component of the current. Left, cold season CIPREA I (August 1978); right, warm season CIPREA 2 (April 1979) (Voituriez, 1983).
In the intertropical Atlantic there is a system of three subsurface counter-currents (Hisard et al., 1976) flowing toward the east. The best known is the equatorial under-current (EUC) or the Lomonosov Current that crosses the entire Atlantic along the equator; from one end to the other, near the latitudes 5° N and 5° S, the north subsuperficial counter-current (NSCC) and the south subsuperficial counter current (SSCC) flow.
The equatorial under current flows to the east against the dominant winds. These induce an accumulation of water on the western edge of the ocean where the thermocline disappears; to the east this is close to the surface. The east-west slope of the thermocline generates a pressure gradient toward the east that is the cause of the equatorial under current.
The Lomonosov under current is a permanent and stable current that is born off the coast of Brazil and terminates at the end of the Gulf of Guinea. It is centered on the equator, has a width of around 200 km, a thickness of 150 m, and its core of maximum speed lies between 50 and 125 m in depth, in the upper region of the thermocline. The speed of this current is on the order of 60 to 130 cm/s (figure 3.4; Voituriez, 1983). The core of maximum speed is associated with a maximum of salinity the value of which diminishes from west to east. This maximum is not always a permanent property in the Gulf of Guinea where it can disappear in the boreal summer.
The equatorial under current presents latitudinal oscillations, 30 to 40 miles in amplitude, with a 15 day period. Measurements of seasonal transport during the first GARP (Global Atmospheric Research Program) global experiment between August 1978 and March 1980 showed that the average transport of this current was 21,106 m3/s (Katz et al., 1981).
The termination of the SEC is not precisely known. At the end of the Gulf of Guinea, the under current separates in two branches, one heading toward the north (Bay of Biafra), the second toward the south along the coast of Gabon (Hisard et al., 1973).
The two north and south subsurface counter currents (NSCC and SSCC), symmetrical at the equator (5° N - 5° S), are associated with a downward slope of the isotherms (9 – 10° C to 12 – 13 C) toward the equator between 150 and 400 m in depth. They are independent of the north and south equatorial counter currents that flow on the surface; they curve toward the poles as they approach the African continent. Only the SSCC persists in the Gulf of Guinea. Their total average output has been estimated between 30 and 40 106 m3/s.
Finally, one notes the presence of an under current flowing to the west at the equator, associated with the second thermocline around 300 – 400 m in depth.
Furthermore, in the Gulf of Guinea, under the Guinea Current (between 20 and 50 m in depth) there is, hugging the coast, a flow running to the west: the Côte d'Ivoire under-current. Its average speed is around 30 to 40 cm/s and the central vein reaches the surface when the Guinea Current disappears. (Lemasson and Rebert, 1973).
Finally, further south (between 100 and 300 m in depth) there is a current flowing to the west that Lemasson and Rebert (1973) have called the Guinea Counter-current.
The surface and subsurface ocean currents transport waters of very different origins. The junction of these water masses create frontal zones that prove to be important to the living environment. The Cape Lopez frontal zone, at the far end of the Gulf of Guinea, has been studied in detail (Hisard et al., 1975). It forms in June at the beginning of the cold season, and separates the warm, less saline, Guinean waters, to the north, from cold, more saline, waters to the south. The latter stem from the equatorial under current:
This frontal zone is temporary, and from July or August the Guinean water penetrates in the cold waters. These cells have been known to be very productive zones where tuna temporarily concentrate.
The wind regime in the equatorial Atlantic induces an accumulation of warm water in the western region of the basin; the thermocline disappears, contrary to the Gulf of Guinea where it meets the surface. In consequence the seasonal vertical movements of the thermocline to the east (in the boreal summer) carry significant variations of the ocean's surface temperature (5° to 7° C compared to 1 to 2° C in the west. This upward rising of the thermocline (upwelling) allows cold water, rich in nutrient, to reach the layers illuminated by the sun, an essential factor for photosynthesis. The principal zones of upwelling of the intertropical Atlantic are: the Senegalese Coast (principally to the south of the Cape Verde peninsula), the “north” coast of the Gulf of Guinea (Côte d'Ivoire, Ghana …), the “south” coast of the Gulf of Guinea (Gabon, Congo …) and the equatorial band.
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Figure 3.5 Surface temperature off the coast of Senegal. In summer, the surface temperature is uniform throughout. In winter, the cold waters from upwelling are pressed to the coast north of Dakar; to the south a tongue of cold water is detached from the coast (Rebert, 1973).
The mechanisms of these upwellings, although they are well known for the coast of Senegal (Ekman Drift), have been the object of recent theoretical developments in the Gulf of Guinea. A specific structure of the thermocline, the “dome” will be the object of section 3.4.4.
The annual sea-surface temperature cycle along the coasts of the Gulf of Guinea can be subdivided:
The short cold season has, until present, attracted less interest than the major cold season because surface thermal variations are weaker (from 1 to 3° C) and limited to the coast. Nevertheless these present strong interannual variations resulting in its complete disappearance; the small cold season does not appear at the equator and thus seems to be a purely coastal phenomenon. Roy (1982) has shown that the minimum sea surface temperature of the minor cold season propagates along the coast of Ghana and Côte d'Ivoire from the east toward the west counter to the Guinea Current, but has not been able to clarify the mechanisms allowing the explanation of this phenomenon.
Figure 3.6 Various enrichment zones in the tropical eastern Atlantic. These are due principally to the upwelling of deep water between July and September. They are localized along the coast and along the Equator (Herbland et al., 1983).
On the west African coast, the wind constitutes the principal driver of upwelling, even if the bottom topography and the coastal form have an influence on its intensity.
At the end of the warm season (October), the surface layer is warm and desalinated and the water column is strongly stratified at the thermocline. The tradewinds provoke upward movement of the thermocline toward the surface. As the winds are weak and irregular, the stratification persists until December before being progressively destroyed by turbulent diffusion. From February until May, the wind direction stabilizes to the north-east and its intensity increases (5 – 7 m/s). Along the coast, two upwellings zones develop, permanently separated by a convergence zone to the north of Dakar (near Kayar) where the winds blow practically perpendicular to the coast. To the north of this zone, the upwelling moves away from the coast because:
- Coastal upwelling
In the gulf of Guinea there are two coastal upwelling zones (figure 3.6; Herbland et al., 1983):
along the coast north of the equator between 2° N and 8° W.
along the coast south of the equator; between the equator and 20° S, the upwelling is seasonal; south of 20° S it is permanent.
The upwelling lasts during July, August and September but one notes differences in the establishment date and the duration following the zones: the minimum temperature, for example, on average appears 30 days earlier at Point Noire than at Abidjan.
The local actions of wind and currents do not entirely explain these upwellings.
* Ekman Divergence
Along the north coast (Côte d'Ivoire, Ghana …) the monsoon blows practically parallel to the coast, but the weak seasonal wind variations in the Gulf of Guinea make it impossible to correlate this with the upwelling (Bakun, 1978). On the coast south of the equator the correlation is weak between 2°S and 13°S; it becomes strong between 13°S and 20° S.
* Advection by the currents
Along the north coast, a counter current runs below the surface directed toward the west that is too weak to explain the upwelling by advection of cold water from the Bay of Biafra.
Along the south coast the transport of cold water by the Benguela Current does not explain the upwelling to the north of 13°S because this current heads toward the west from 18°S.
* Upwelling induced by the currents
Ingham (1970) proposed that the rise of the isotherms, induced by reinforcement of the Guinea current in geostrophic balance, could be the cause of cooling observed along the Ivory-Ghanan littoral; numerical models have shown however that the cooling induced would be minimal.
- Equatorial upwelling (figure 3.7 and 3.8)
Equatorial upwelling is present to the east of 20° W between July and September. Here again the mechanisms, calling on the local actions of winds and currents, do not permit full explanation of the upwelling.
* Ekman Divergence
A zonal wind produces a divergence centered on the equator, but in the Gulf of Guinea the wind is principally meridional and must create a divergence at the south of the equator according to the classic scheme of Cromwell (1983). This divergence is effectively observed near 2°30' S. However these winds can not explain the rise in thermocline as it is observed at the equator.
* Advection by the currents
The equatorial upwelling could be fed by the cold waters of the south coast upwelling, advected by the Benguela Current. Satellite observations (Citeau et al., 1980) and the CIPREA oceanographic expeditions have shown however the equatorial phenomenon to be independent of coastal upwelling.
* Vertical mixing
Vertical mixing is particularly intense in the boundary zone between the equatorial under current at the subsurface and the south equatorial current at the surface. It was thought that the increase of this mixing in summer was responsible for the cooling of the surface, but it seems that the seasonal variations in vertical mixing is weak, at least to the east of 4° W. Furthermore Voituriez (1983) has shown that this cooling was associated with a rise in the hydrological structure across the equatorial under current. The mechanism of this rise is now better understood thanks to Remote Forcing theory.
Figure 3.7 Mean thermal sections from 0 to 300 m between Abidjan and 6° S to 4° W. The upward move of the isotherms in July at the coast (5° N) and at the equator is clearly visible. The surface temperature is minimum between 2° S and 3° S because of divergence due to the winds.
Figure 3.8 Changes in surface temperature along 4° W, between 5° N and 6° S during an average year. The warm season, from January to May, is characterized by temperatures above 27° C; from July to September, the temperature is below 25° C, it is lowest at the south equatorial divergence (2° S) and along the coast.
Moore et al. (1978) have brought forward the hypothesis according to which the upwelling in the Gulf of Guinea would be the consequence of the augmentation of wind intensities off the coast of Brazil. The intensification of zonal winds provokes an accumulation of water on the west shore. To the east of the circulation center, a wave propagates toward the east. The equator, because of the nullification of the Coriolis Force, plays the role of a wave guide; this property causes the generated wave, called a Kelvin Wave, to be propagated along the equator. On arrival on the African coasts, this wave separates in two coastal Kelvin waves propagating along the african coast toward the north and south. This wave in the course of its propagation along the equator and the African coast lifts the thermal structure permitting the thermocline to reach the surface. Relying on this scheme:
Picaut (1983) has shown that the surface temperature signal during the upwelling period propagates:
from the equator, toward the south along the south coast.
along the north coast toward the west from Togo-Benin to Cape Palmas.
The explanation of Moore et al. based on a sudden augmentation of the wind off Brazil is schematic. In reality the spatial and temporal wind structure is complex, and wind intensification and relaxation take place in a continuous manner.
Numerical models have permitted the clarification of ocean response to wind action (Cane, 1984; Busalacchi and Picaut, 1983; Du Penhoat and Treguier, 1985). The continued variations of the wind generate multiple waves that propagate and reflect on the east shores and west of the basin. The superposition (reflection + interference) of all of these waves contributes to the state of equilibrium of the basin. It is therefore difficult to distinguish the propagation of an individual wave. Schematically the ocean reacts at the equator to the wind tension integrated on all of the basin and not only to the pressure off Brazil.
Figure 3.9 Thermoclimatic domes in the Atlantic. They are characterized by an upward movement of the thermocline that never reaches the surface. The Guinea dome (12° N to 22° W) appears in summer from July to September. The existence of an Angola dome, near 10° S – 10° E, is uncertain; observations have not confirmed the statistical estimations made by Mazeika (Mazeika, 1967).
The term “dome” designates a rising of the thermocline that shows at the surface without ever reaching it. In the intertropical Atlantic, domes are associated with the termination of north and south equatorial subsuperficial counter currents; these are permanent and head toward the poles when they reach the African continent. This rotation of currents provokes a cyclonic circulation that is created under the thermocline of permanent subsuperface thermal domes. When the wind and pressure conditions are favorable to the rising of the under currents to the surface, these deep domes emerge at the level of the thermocline: it is this specific structure that takes the name thermal dome (Voituriez, 1981).
In the east Atlantic, two domes have been identified:
- the Guinea dome, centered approximately 12° N – 22° W and joined with the termination of the north subsuperficial counter current (figure 3.9; Mazeika, 1967), appears at the thermocline in summer (July to September). It is associated with the intertropical convergence zone, therefore with weak and unstable winds and low atmospheric pressures, favorable conditions for the appearance of the north equatorial under current on the surface (Voituriez, 1981).
Figure 3.10 Superimposition of temperature, oxygen, nitrate and nitrite and chlorophyll a sections established by Costes. This structure is characteristic of the Typical Tropical Structure. This is a two layer structure: at the surface, a homogeneous warm layer poor in nutrient salts and chlorophyll pigments; below, a cold layer rich in nutrients (Coste, 1977).
Otherwise the wind rotation, positive in the region at this time of year (corresponding to a cyclonic circulation) provokes an exit of water toward the right of direction of wind rotation (Coriolis Force). The deficit of water at the center of this circulation is replenished by a rising of subsurface water that accentuates that engendered by the currents.
- The Angola dome is centered near 10° S – 9° E and is associated with the termination of the south subsuperficial counter current. However its existence is uncertain and the statistical estimations made by Mazeika (1967) near 10° S – 9° E has not been realistically confirmed by subsequent observation. A subthermocline ridge associated with the south equatorial under current is observed, but not at the level of the thermocline which would permit it to be called a dome. The permanence and intensity of the wind in this region opposes the formation of a thermoclinal dome, except perhaps in winter (Voituriez, 1981).
The zones of high production identified in the equatorial Atlantic are represented in figure 3.6 Voituriez and Herbland, 1982); these are:
coastal upwellings off the coasts north of the equator (Maritania, Senegal, Côte d'Ivoire, Togo, Benin) and those south of the equator (Gabon, Congo, Angola).
thermal domes of Guinea and Angola.
equatorial divergence.
These zones are not entirely independent as they are supplied with central south Atlantic water, rich in oxygen, by the system of three subsuperficial counter currents.
The enrichment processes are different according to the subsurface thermal structure:
The domes correspond to thermal ridges; it is thus a two layer system, in which the upper mixed layer is poor in nitrates.
The upwellings are zones where the entire thermal structure is elevated. The thermocline having dissipated at the surface, the nitrate poor mixed layer has disappeared.
One of the important characteristics of the euphotic zone of the intertropical Atlantic is the permanence of the vertical structure in the physical and chemical parameters. Herbland et al. (1983), have named it “typical tropical structure” or TTS (fig. 3.10; Costes, 1977). It is a two layer structure: at the surface, a warm layer, poor in nutrient and chlorophyll pigments; below, a cold layer rich in nutrient. The nitracline, always coinciding the oxycline and maximum chlorophyll, is situated statistically below the thermocline when its depth does not exceed 50 – 60 m and above when it exceeds 50 – 60 m. Statistically the temperature gradient in the thermocline diminishes as it deepens; this does not therefore act as a barrier for the nitracline. Observations made at different times and places in the East Atlantic (Guinea and Angola dome, equatorial zone) shows that one always finds the TTS even when the depth of the thermocline varies; the stratification of physio-biological parameters is remarkably stable. The same type of structure has in fact been observed in the Costa Rica Dome in the Pacific. The typical tropical structure ceases to exist when nitrates appear at the surface, that is to say in boreal summer in the upwelling zones and the equatorial divergence. In these regions, the maximum primary production takes place at the surface where nutritiients and necessary light for production are found in combination. The domes have a permanent typical tropical structure where the thermocline is close to the surface; in these zones the maximum production appears at the top of the nitracline, rich in nutritive salts.
The very close relation that exists between nitrate and oxygen has permitted the creation of maps of primary production from more numerous and probably more reliable oxygen measurements.
In February – March (figure 3.11; Equalant, 1963; Herbland et al., 1983), the maximum value (70 – 80 mgC/m2) is situated at the south equatorial ridge near 3°S, along the Senegalese-Mauritanian Coast, and the minimum values along the north equatorial convergence. One must note that 1963 was an exceptionally warm year and that the primary production values were probably low. In the cold season, the principal zones of production are the Guinea Dome and the equatorial divergence, whereas the values are minimal along the north equatorial convergence.
At the equator, the primary production is high all year, which ssignifiesignifies that the appearance of nutrient salts at the surface during the upwellings periods is not synonymous with an increase in primary production: it is the paradox of the equatorial zone. This is explained by the existence of a positive linear relation between phytoplankton and zooplankton. The grazing of phytoplankton by zooplankton limits the growth of this biomass, and consequently the consumption of nutritient which permits the presence of high concentrations of nitrates in the East Equatorial Atlantic (Herbland et al., 1983).
For a long time tropical waters have been considered as biological deserts, with the exception of enrichment zones as upwellings, thermal domes …The utilization of increasingly finer filters has shown the presence of very small organisms (1 micron), unsuspected until now, which contribute a significant fraction to photosynthetic activity in tropical waters. A re-evaluation of the fertility of these waters is necessary.
Figures 3.12 and 3.13 (Merle, 1978) show the principal characteristics of oxygen distribution in the course of an average year at 50 and 200 m. The concentration is weaker to the east of 20° W (4 ml.l-1 at 50 m and 2 ml.l-1 at 200 m.) than to the west (4 ml.l-1 at 50 m and 3 ml.l-1 at 200 m), it presents however a relative maximum all along the equator, superior to 4 ml.l-1 at 50 m and 2.8 ml.l-1 at 100 m. One notes the presence of two minima off the coasts of Senegal and Guinea, and off Angola. At 50 m the concentrations are less than at 3 ml.l-1 off the coast of Angola.
Zonal and Meridional distribution The section at 4° W between 5° N and 24° S (figure 3.14; Oudot, 1983) describes the meridional distribution of oxygen in the east equatorial Atlantic. The surface concentration increases progressively toward the south as a result of the cooling of waters which increases the solubility of oxygen. This cooling permits the oxygenated homogeneous layer to thicken (it reaches 150 m near 20° S). At the equator, the oxycline (maximum vertical gradient layer) approaches the surface and the oxygen content shows a minimum (4.6 ml.l-1); the upwelling of deep water (equatorial divergence), less rich in oxygen, depletes the surface layer. Below the surface, the distribution of oxygen is strongly influenced by the by the zonal circulation. At the equator, the equatorial current transports waters rich in oxygen. A second maximum (3 ml.l-1) is associated with the southern subsurface counter current near 4° south – 5° south. On both sides, these maxima appear among the oxygen depleted zones from Côte d'Ivoire to near 12° south.
Figure 3.11 Map of primary production based on measurements of oxygen (depth of undersaturation) during the Equalant 1 and 2 cruises. The year 1983 in which these cruises occurred were exceptionally warm; the values of primary production were thus probably lower than normal (Herbland, et al., 1983).
Figure 3.15 (Oudot, 1983) gives an example of zonal distribution in the Gulf of Guinea between Saint Helena and Luanda. It is characterized by a maximum (5 ml.l-1) located above the oxycline that disappears near the coast; this maximum is the result of photosynthetic production.
Seasonal variations (figure 3.16; Oudot, 1983)
The cooling of the surface layer between April and August augments the oxygen concentration at the surface. The difference is greater in the south than in the north because the variations in temperature between the two seasons is greater. In the hot season (April), an oxygen maximum appears above the oxycline near 20 m at 5° north and 50 m at 10° south. This maximum is interrupted at the equator; it does not appear in the cool season (August). The equatorial current is richer in oxygen in April than in August. A vertical maximum is not observed. Along Côte d'Ivoire, the thickness of the oxygenated homogeneous layer (4 ml.l-1) is reduced during the upwelling period (July – October) and may even disappear during particularly intense upwelling.
Figure 3.12 Annual distribution of oxygen (ml.l-1) at a depth of 50 m. The concentrations are lower to the east of 20° W. The presence of a relative maximum all along the equator is noticeable. The minimum values are located off the coasts of Senegal, Guinea and Angola (Merle, 1978).
Figure 3.13 Annual distribution of oxygen (ml.l-1) at a depth of 200 m (after Merle, 1978). Same comments as figure 3.12.
Figure 3.14 Vertical distribution of dissolved oxygen (ml.l-1) along the 4° W meridian in November 1971. The surface distribution increases progressively towards the south. The surface minimum at the equator is visible. Below the surface at the equator the relative maximum associated with the equatorial undercurrent is noticeable and depletes the waters along the coast (Oudot, 1983).
Figure 3.15 Vertical distribution of dissolved oxygen (ml.l-1) between Saint Helens and Luanda in January 1975. This distribution is characterized by a subsurface maximum situated above the oxycline. This maximum nears the surface as it moves towards the east (Oudot, 1983).
Figure 3.16 Vertical distribution of dissolved oxygen (ml.l-1) along the 4° W meridian in August 1978 and April 1979. The cooling of the surface layer in August allows the surface oxygen tension to increase. The SCE is richer in oxygen in April (Oudot, 1983).
Assessment of current knowledge of the environment in the study zone demonstrates the extent of current scientific knowledge of this domain. Nevertheless many question marks remain concerning certain zones that remain very poorly studied (Angola dome or the Liberian fisheries zone) as well as the general mechanism of enrichment of tropical oceanic waters. This last problem will be the object of a more detailed examination in chapter 7.
