Seasonal changes in the tropical Atlantic circulation: Observation and simulation of the Guinea Dome

Siedler, Gerold; Zangenberg, Norbert; Onken, Reiner; Morlière, Alain

doi:10.1029/91jc02501

Cited by 108 publications

(87 citation statements)

References 31 publications

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“…The wind systems of the northern and southern hemispheres converge in the ITCZ, producing heavy rain shower activity and an associated Tropical Salinity Minimum (TSM). The position of the ITCZ varies seasonally; it is close to the equator during northern winter and moves northward in northern summer with an abrupt shift occurring in late June (Siedler et al, 1992;Sultan and Janicot, 2000). During the cruise, rain shower activity typical of the ITCZ was located between 5.5 N and 8.3 N and the TSM was observed between 1 N and 13 N. South of the TSM, the Equatorial Upwelling (EqUp, 1 N-4 S) was indicated by a lowered surfacewater temperature and a small increase in surface-water salinity.…”

Section: Air Masses and Hydrographymentioning

confidence: 99%

Atmospheric iron deposition and sea-surface dissolved iron concentrations in the eastern Atlantic Ocean

Sarthou¹,

Baker²,

Blain³

et al. 2003

Deep Sea Research Part I: Oceanographic Research Papers

183

145

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Atmospheric iron and underway sea-surface dissolved (o0.2 mm) iron (DFe) concentrations were investigated along a north-south transect in the eastern Atlantic Ocean (27 N/16 W-19 S/5 E). Fe concentrations in aerosols and dry deposition fluxes of soluble Fe were at least two orders of magnitude higher in the Saharan dust plume than at the equator or at the extreme south of the transect. A weaker source of atmospheric Fe was also observed in the South Atlantic, possibly originating in southern Africa via the north-easterly outflow of the Angolan plume. Estimations of total atmospheric deposition fluxes (dry plus wet) of soluble Fe suggested that wet deposition dominated in the intertropical convergence zone, due to the very high amount of precipitation and to the fact that a substantial part of Fe was delivered in dissolved form. On the other hand, dry deposition dominated in the other regions of the transect (73-97%), where rainfall rates were much lower. Underway sea-surface DFe concentrations ranged 0.02-1.1 nM. Such low values (0.02 nM) are reported for the first time in the Atlantic Ocean and may be (co)-limiting for primary production. A significant correlation (Spearman's rho=0.862, po0:01) was observed between mean DFe concentrations and total atmospheric deposition fluxes, confirming the importance of atmospheric deposition on the iron cycle in the Atlantic. Residence time of DFe in the surface waters relative to atmospheric inputs were estimated in the northern part of our study area (1778 to 28716 d). These values confirmed the rapid removal of Fe from the surface waters, possibly by colloidal aggregation. r

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Section: Air Masses and Hydrographymentioning

confidence: 99%

Atmospheric iron deposition and sea-surface dissolved iron concentrations in the eastern Atlantic Ocean

Sarthou¹,

Baker²,

Blain³

et al. 2003

Deep Sea Research Part I: Oceanographic Research Papers

183

145

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“…Within the tropical Atlantic Ocean off northwestern Africa (TANWA; 12 to 22 • N and 26 to 15 • W), the large-scale surface circulation responds to the seasonal variability of the trade winds and the north-south migration of the Intertropical Convergence Zone (ITCZ) (e.g., Stramma and Isemer, 1988;Siedler et al, 1992;Stramma and Schott, 1999). The seasonal wind pattern results in a strong seasonality of the flow field along the northwestern African coast and in coastal upwelling of different intensity.…”

Section: Introductionmentioning

confidence: 99%

Occurrence and characteristics of mesoscale eddies in the tropical northeastern Atlantic Ocean

2016

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Abstract. Coherent mesoscale features (referred to here as eddies) in the tropical northeastern Atlantic Ocean (between 12-22 • N and 15-26 • W) are examined and characterized. The eddies' surface signatures are investigated using 19 years of satellite-derived sea level anomaly (SLA) data. Two automated detection methods are applied, the geometrical method based on closed streamlines around eddy cores, and the Okubo-Weiß method based on the relation between vorticity and strain. Both methods give similar results. Mean eddy surface signatures of SLA, sea surface temperature (SST) and sea surface salinity (SSS) anomalies are obtained from composites of all snapshots around identified eddy cores. Anticyclones/cyclones are identified by an elevation/depression of SLA and enhanced/reduced SST and SSS in their cores. However, about 20 % of all anticyclonically rotating eddies show reduced SST and reduced SSS instead. These kind of eddies are classified as anticyclonic mode-water eddies (ACMEs). About 146 ± 4 eddies per year with a minimum lifetime of 7 days are identified (52 % cyclones, 39 % anticyclones, 9 % ACMEs) with rather similar mean radii of about 56 ± 12 km. Based on concurrent in situ temperature and salinity profiles (from Argo float, shipboard, and mooring data) taken inside of eddies, distinct mean vertical structures of the three eddy types are determined. Most eddies are generated preferentially in boreal summer and along the West African coast at three distinct coastal headland regions and carry South Atlantic Central Water supplied by the northward flow within the Mauretanian coastal current system. Westward eddy propagation (on average about 3.00 ± 2.15 km d −1 ) is confined to distinct zonal corridors with a small meridional deflection dependent on the eddy type (anticyclones -equatorward, cyclones -poleward, ACMEs -no deflection). Heat and salt fluxes out of the coastal region and across the Cape Verde Frontal Zone, which separates the shadow zone from the ventilated subtropical gyre, are calculated.

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“…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%

“…The NECC weakens during boreal winter and spring when the ITCZ shifts to the south and westward-flowing surface winds oppose the eastward-flowing current (Fig. 7), whereas the Guinea Current flows eastward throughout the year as it is always located south of the ITCZ in an area of dominantly eastward-moving surface winds (Siedler et al, 1992).…”

Section: Hydrography and Relationship To The Surface Wind Fieldmentioning

confidence: 99%

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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Seasonal changes in the tropical Atlantic circulation: Observation and simulation of the Guinea Dome

Cited by 108 publications

References 31 publications

Atmospheric iron deposition and sea-surface dissolved iron concentrations in the eastern Atlantic Ocean

Atmospheric iron deposition and sea-surface dissolved iron concentrations in the eastern Atlantic Ocean

Occurrence and characteristics of mesoscale eddies in the tropical northeastern Atlantic Ocean

MiocenePliocene surface-water hydrography of the eastern equatorial Atlantic

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Seasonal changes in the tropical Atlantic circulation: Observation and simulation of the Guinea Dome

Cited by 108 publications

References 31 publications

Atmospheric iron deposition and sea-surface dissolved iron concentrations in the eastern Atlantic Ocean

Atmospheric iron deposition and sea-surface dissolved iron concentrations in the eastern Atlantic Ocean

Occurrence and characteristics of mesoscale eddies in the tropical northeastern Atlantic Ocean

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

Contact Info

Product

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MiocenePliocene surface-water hydrography of the eastern equatorial Atlantic