Zonal intermediate currents in the equatorial Atlantic Ocean

Ollitrault, Michel; Lankhorst, Matthias; Fratantoni, David M.; Richardson, Philip L.; Zenk, Walter

doi:10.1029/2005gl025368

Cited by 46 publications

(62 citation statements)

References 21 publications

(31 reference statements)

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“…Unlike the EDJs, the EICs are quasi-steady zonal currents that have been observed consistently from direct velocity measurements (Firing 1987;Firing et al 1998;Gouriou et al 2001;Schott et al 1998Schott et al , 2003Brandt et al 2008) and from the displacements of floats (Ollitrault et al 2006;Ascani et al 2010;Cravatte et al 2012;Ollitrault and Colin de Verdière 2014). Most prominently, they include ( Fig.…”

Section: Observationsmentioning

confidence: 99%

“…1;Firing 1987;Schott et al 1995Schott et al , 2003Firing et al 1998;Gouriou et al 1999Gouriou et al , 2001Bourlès et al 2002Bourlès et al , 2003Send et al 2002;Ollitrault et al 2006;Eden and Dengler 2008;Ascani et al 2010;Brandt et al 2011Brandt et al , 2012Cravatte et al 2012;Youngs and Johnson 2014). This deep equatorial circulation (DEC; see a list of abbreviations in Tab.…”

Section: Overviewmentioning

confidence: 99%

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The Deep Equatorial Ocean Circulation in Wind-Forced Numerical Solutions

Ascani

Firing

McCreary

et al. 2015

Journal of Physical Oceanography

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propagating mostly downward, consistent with observations. We demonstrate that both types of currents arise from the rectification of DEIV, consistent with previous theories. We also find that the EDJs contribute to maintaining the EICs, suggesting that the nonlinear energy transfer is more complex than previously thought. In Solution 2, the DEC is unrealistically weak and less spatially coherent than in the first simulation probably because of its weaker DEIV. Using intermediate solutions, we find that the main reason for this weaker DEIV is the use of realistic coastlines in Solution 2. It remains to be determined, what needs to be modified or included to obtain a realistic DEC in the more realistic configuration.

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Section: Observationsmentioning

confidence: 99%

Section: Overviewmentioning

confidence: 99%

The Deep Equatorial Ocean Circulation in Wind-Forced Numerical Solutions

Ascani

Firing

McCreary

et al. 2015

Journal of Physical Oceanography

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“…, with a mean transport of 5-10 Sv (Firing 1987;Schott et al 2003;Ollitrault et al 2006;Brandt et al 2008). They have been observed in synoptic sections across the Pacific and Atlantic Oceans (Firing et al 1998;Gouriou et al 2001).…”

mentioning

confidence: 99%

“…1a;Firing 1987;Firing et al 1998;Schott et al 1995;Gouriou et al 1999Gouriou et al , 2001Bourlè s et al 2002Bourlè s et al , 2003Schott et al 2003;Ollitrault et al 2006). The currents may be divided into two sets.…”

Section: Introductionmentioning

confidence: 99%

Deep Equatorial Ocean Circulation Induced by a Forced–Dissipated Yanai Beam*

Ascani

Firing

Dutrieux

et al. 2010

Journal of Physical Oceanography

100

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A complex pattern of zonal currents below the thermocline has been observed in the equatorial Pacific and Atlantic Oceans. The currents have typical speeds from 10 to 15 cm s 21 and extend as deep as 2500 m. Their structure can be divided into two overlapping parts: the equatorial deep jets (EDJs), centered on the equator and alternating in the vertical with a wavelength of several hundred meters, and the Equatorial Intermediate Current system (EICS), composed of currents with large vertical scale and alternating with latitude over several degrees on either side of the equator. The strongest EICS current is a westward flow on the equator flanked by eastward currents at 28N and 28S.In the present study, the authors use idealized numerical simulations and analytical solutions to demonstrate that the EICS currents within 2.58 from the equator could result from the self-advection with dissipation of a downward-propagating beam of monthly periodic Yanai (Rossby gravity) waves. The zonally restricted beam is generated in the eastern part of the basin by instabilities of the swift near-surface equatorial currents. For a weak Yanai wave amplitude and no dissipation, mean Eulerian currents resembling the three strongest EICS currents are obtained but only within the beam; in this case, the Eulerian flow is balanced by the waveinduced Stokes drift, yielding a zero-mean Lagrangian flow, and the water parcels conserve their potential vorticity (PV) and are stationary over a wave cycle. For larger amplitudes, the Yanai waves break, losing their energy to small vertical scales where it is dissipated. This dissipation changes the mean (wave averaged) PV of a water parcel within the beam, allowing the parcel to have a persistent equatorward drift across PV contours. This can be viewed as a wave-induced Sverdrup transport; by continuity and by virtue of the westward group velocity of long Rossby waves, this Lagrangian-mean meridional flow requires a Lagrangian-mean zonal flow within and to the west of the beam, with a meridional structure consistent with the three strongest EICS currents. This mechanism of EICS formation is active in some ocean general circulation models; its importance in the ocean remains to be evaluated.

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“…Further poleward alternating zonal jets are present at intermediate depths including the latitude range of the OMZs. Their strengths have been quantified using subsurface drift trajectories from floats (Maximenko et al, 2005;Ollitrault et al, 2006) and repeated shipboard sections . Such currents have been reproduced by idealised process modelling (Mé-nesguen et al, 2009;Ascani et al, 2010;Qiu et al, 2013),…”

Section: Introductionmentioning

confidence: 99%

On the role of circulation and mixing in the ventilation of oxygen minimum zones with a focus on the eastern tropical North Atlantic

et al. 2015

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Abstract. Ocean observations are analysed in the framework of Collaborative Research Center 754 (SFB 754) "ClimateBiogeochemistry Interactions in the Tropical Ocean" to study (1) the structure of tropical oxygen minimum zones (OMZs), (2) the processes that contribute to the oxygen budget, and (3) long-term changes in the oxygen distribution. The OMZ of the eastern tropical North Atlantic (ETNA), located between the well-ventilated subtropical gyre and the equatorial oxygen maximum, is composed of a deep OMZ at about 400 m in depth with its core region centred at about 20 • W, 10 • N and a shallow OMZ at about 100 m in depth, with the lowest oxygen concentrations in proximity to the coastal upwelling region off Mauritania and Senegal. The oxygen budget of the deep OMZ is given by oxygen consumption mainly balanced by the oxygen supply due to meridional eddy fluxes (about 60 %) and vertical mixing (about 20 %, locally up to 30 %). Advection by zonal jets is crucial for the establishment of the equatorial oxygen maximum. In the latitude range of the deep OMZ, it dominates the oxygen supply in the upper 300 to 400 m and generates the intermediate oxygen maximum between deep and shallow OMZs. Water mass ages from transient tracers indicate substantially older water masses in the core of the deep OMZ (about 120-180 years) compared to regions north and south of it. The deoxygenation of the ETNA OMZ during recent decades suggests a substantial imbalance in the oxygen budget: about 10 % of the oxygen consumption during that period was not balanced by ventilation. Long-term oxygen observations show variability on interannual, decadal and multidecadal timescales that can partly be attributed to circulation changes. In comparison to the ETNA OMZ, the eastern tropical South Pacific OMZ shows a similar structure, including an equatorial oxygen maximum driven by zonal advection but overall much lower oxygen concentrations approaching zero in extended regions. As the shape of the OMZs is set by ocean circulation, the widespread misrepresentation of the intermediate circulation in ocean circulation models substantially contributes to their oxygen bias, which might have significant impacts on predictions of future oxygen levels.

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Zonal intermediate currents in the equatorial Atlantic Ocean

Cited by 46 publications

References 21 publications

The Deep Equatorial Ocean Circulation in Wind-Forced Numerical Solutions

The Deep Equatorial Ocean Circulation in Wind-Forced Numerical Solutions

Deep Equatorial Ocean Circulation Induced by a Forced–Dissipated Yanai Beam*

On the role of circulation and mixing in the ventilation of oxygen minimum zones with a focus on the eastern tropical North Atlantic

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