The seasonal upwellings in the Gulf of Guinea

Verstraete, Jean-Marc

doi:10.1016/0079-6611(92)90002-h

Cited by 76 publications

(61 citation statements)

References 44 publications

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“…It is deeper in fall and winter, and shallower in late spring and summer. The hydrological structure moves seasonally upward and downward, and strongly outcrops during late spring and summer [Voituriez, 1983;Verstraete, 1992]. Consequently, the thermocline moves under or above the EUC velocity core, not only leading to significant variations in the vertical distribution of the EUC transport according to isopycnal layers, but also to the strong seasonal erosion of the upper EUC highsalinity core within the mixed layer.…”

Section: Discussionmentioning

confidence: 99%

Seasonal variability of the Equatorial Undercurrent at 10°W as inferred from recent in situ observations

Kolodziejczyk¹,

Bourlès²,

Marin³

et al. 2009

J. Geophys. Res.

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[1] Eighteen cross-equatorial shipboard current profiling sections along 10°W with conductivity-temperature-depth measurements taken between 1997 and 2007 are used to analyze the mean meridional structure and the seasonal variability of the Equatorial Undercurrent (EUC) at 10°W. Our analysis suggests a seasonal cycle for the EUC transport at 10°W, with a well-defined annual harmonic and some indication of a semiannual component, with a first maximum in January and a second stronger maximum from June to September. The mean EUC transport at 10°W is estimated to be 12.1 Sv and, compared to earlier estimates farther in the west, at 35°W (20.9 Sv) and 26°W (13.8 Sv). The eastward flow transport exhibits a strong variability at 10°W (with a total range of transports from 7.1 Sv to more than 31.7 Sv). The seasonal amplitude of the eastward flow variability is ±8.9 Sv, from a minimum of 8.2 Sv in November to 25.9 Sv in August. The eastward flows within the thermocline are divided in two parts: a permanent part within the s q = 24.5-26.5 isopycnal layer, with a semiannual cycle, known as the EUC, and a nonpermanent part in the deep thermocline (beneath s q = 26.5 isopycnal), associated with a strong eastward transport (up to 15 Sv) during the boreal summer, that is not observed during the rest of the year. The current at the equator in the deep thermocline is even westward during boreal fall. The disappearance of the salinity core of the EUC during the boreal summer, associated with the upwelling of the hydrological structure at 10°W, reveals that the saline subtropical waters carried by the EUC within the thermocline no longer flow into the Gulf of Guinea during the boreal summer. Our data also show the presence of the South Equatorial Undercurrent (SEUC) at 10°W in the Gulf of Guinea with a strong latitudinal and depth variability throughout the year. The mean SEUC at 10°W is centered near 5°S and is farther south than observed at 35°W and 26°W, suggesting its poleward shift from west to east.

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Section: Discussionmentioning

confidence: 99%

Seasonal variability of the Equatorial Undercurrent at 10°W as inferred from recent in situ observations

Kolodziejczyk¹,

Bourlès²,

Marin³

et al. 2009

J. Geophys. Res.

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“…An analogous process occurs annually in the Atlantic when the ITCZ moves north in the boreal spring and summer and the increase in wind stress over the western Atlantic propagates Kelvin waves into the eastern Atlantic (Hastenrath and Merle, 1987;Peterson and Stramma, 1991). The gravity wave is reflected off the African continent and moves westward across the Gulf of Guinea producing regional upwelling along the coast (Peterson and Stramma, 1991;Verstraete, 1992). Thus, when the ITCZ moves northward, strengthening the countercurrent system, cool waters return to the Gulf of Guinea.…”

Section: Hydrography and Relationship To The Surface Wind Fieldmentioning

confidence: 99%

“…Warm surface water flows to the west in the South Equatorial Current system, which piles warm water against Brazil and removes it from the West African Bight, allowing cold subsurface waters to reach the surface in the eastern Atlantic. The Gulf of Guinea is particularly sensitive to changes in the wind field over the western tropical Atlantic since the thermocline and surface-water temperatures are directly controlled by changes in the countercurrents and undercurrents produced by changes in wind stress over the western basins (Hay and Brock, 1992;Hisard and Merle, 1980;Peterson and Stramma, 1991;Verstraete, 1992). These same current systems also modulate rainfall over equatorial Africa and the strength of the African monsoon (Peterson and Stramma, 1991).…”

Section: Introductionmentioning

confidence: 99%

“…The main eastward flow is concentrated in the Equatorial Undercurrent (EUC), part of which is deflected northward into the Gulf of Guinea (Peterson and Stramma, 1991;Siedler et al, 1992;Verstraete, 1992). The EUC carries cool, saline water from the South Atlantic and produces a particularly intense thermocline as it flows under the warm, relatively low salinity Guinea Current.…”

Section: Hydrography and Relationship To The Surface Wind Fieldmentioning

confidence: 99%

“…When the ITCZ moves south in the late winter and spring, and the strength of the trade winds drops, the EUC also weakens ( Fig. 7; Peterson and Stramma, 1991;Verstraete, 1992).…”

Section: Hydrography and Relationship To The Surface Wind Fieldmentioning

confidence: 99%

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MiocenePliocene surface-water hydrography of the eastern equatorial Atlantic

Norris¹

1998

Proceedings of the Ocean Drilling Program, 159 Scientific Results

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Stable isotope analyses of four species of planktonic foraminifers and the benthic foraminifer, Cibicides wuellerstorfi, from Ocean Drilling Program Site 959 show that the Gulf of Guinea had a strong, shallow thermocline during the latest Miocene and early Pliocene prior to the first appearance of the Guinea Current at about 4.9 Ma. Gradients of δ 18 O between the surface-water species, Globigerinoides sacculifer, and the thermocline species, Neogloboquadrina dutertrei and Globorotalia margaritae, were about 2‰ in the late Miocene and early Pliocene, but decreased to less than 0.5‰ in several large swings beginning about 4.9 Ma. A decreased oxygen isotope gradient in the upper ocean after 4.9 Ma is consistent with the initial establishment of the Guinea Current although several large swings in the oxygen isotope gradient shortly after this suggest that the Guinea Current did not become a permanent feature until about 4.3 Ma. The initiation of the Guinea Current in the Gulf of Guinea suggests that the Intertropical Convergence Zone (ITCZ) migrated south to near its present location at ~4.9 Ma and is in agreement with evidence for a southward shift in the ITCZ over the Pacific between 4−5 Ma. Modern hydrographic studies suggest that the position of the ITCZ modulates the strength of the North Equatorial Counter Current whose eastern extension forms the Guinea Current in the Gulf of Guinea. Notably, the interval ~4.7 to 4.3 Ma is associated with the return of a strong, shallow thermocline in the eastern Atlantic, which suggests a brief, northward drift of the ITCZ at this time and the disappearance of the Guinea Current. The Guinea Current returned to the Gulf after 4.3 Ma.The highest sea-surface temperature (SST) occurred between ~4.4 and 5.5 Ma in the Gulf of Guinea. Warm surface waters coupled with a shallow thermocline during much of this period may have contributed to a strong West African monsoon and relatively moist conditions over sub-Saharan Africa. In contrast, the large oscillations in SST and upwelling after ~4.5 Ma may have contributed to greater variability in the monsoon and the widespread increase in terrigenous fluxes observed in deep-sea cores within the eastern Atlantic about this time.

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New insights on the upper layer circulation north of the Gulf of Guinea

et al. 2016

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The oceanic circulation in the upper layers of the north of the Gulf of Guinea is analyzed, as inferred from in situ observations and numerical simulations. This particular region, in spite of the presence of a coastal upwelling and its impact on resources and regional climate, is still poorly documented. Cruises carried out in the framework of different international programs (e.g., EGEE/AMMA, PIRATA) allowed to show the existence of an eastward flowing undercurrent, found under the Guinea Current, named the Guinea UnderCurrent (GUC). Numerical results from high resolution simulation allowed the description of the seasonal variability of this current. It appears that the GUC is stronger in spring and reverses westward in August–September. We also depict the fate and the sources of the GUC based on selected trajectories from numerical particle tracking. The simulated trajectories reveal: (i) a preferred route of the GUC along 4°N from Cape Palmas to Cape Three Points and following the coast east of Cape Three Points; (ii) strong recirculations in the most eastern part of the Gulf of Guinea and off Cape Palmas including warm and salty waters of the South Equatorial Current; (iii) a weak inflow from northern latitudes through a subsurface current flowing southward along the West African coast. In addition, Lagrangian experiments show that the GUC is not an extension of the North Equatorial UnderCurrent and confirm that this current does not penetrate into the Gulf of Guinea.

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The seasonal upwellings in the Gulf of Guinea

Cited by 76 publications

References 44 publications

Seasonal variability of the Equatorial Undercurrent at 10°W as inferred from recent in situ observations

Seasonal variability of the Equatorial Undercurrent at 10°W as inferred from recent in situ observations

MiocenePliocene surface-water hydrography of the eastern equatorial Atlantic

New insights on the upper layer circulation north of the Gulf of Guinea

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The seasonal upwellings in the Gulf of Guinea

Cited by 76 publications

References 44 publications

Seasonal variability of the Equatorial Undercurrent at 10°W as inferred from recent in situ observations

Seasonal variability of the Equatorial Undercurrent at 10°W as inferred from recent in situ observations

MiocenePliocene surface-water hydrography of the eastern equatorial Atlantic

New insights on the upper layer circulation north of the Gulf of Guinea

Contact Info

Product

Resources

About

MiocenePliocene surface-water hydrography of the eastern equatorial Atlantic