Mean Circulation and Variability of the Tropical Atlantic during 1952–2001 in the GECCO Assimilation Fields

Rabe, Benjamin; Schott, Friedrich; Köhl, Armin

doi:10.1175/2007jpo3541.1

Cited by 33 publications

(50 citation statements)

References 42 publications

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“…This agrees with other studies on the role of the NBC in the AMOC upper branch crossing the equatorial Atlantic (e.g. Rabe et al, 2008;Sebille et al, 2011;. Figure 4g shows the southward maximum of the east-west accumulated transports between 15 • S and the Equator.…”

Section: Time Mean Transportssupporting

confidence: 91%

South Atlantic meridional transports from NEMO-based simulations and reanalyses

2018

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Abstract. The meridional heat transport (MHT) of the South Atlantic plays a key role in the global heat budget: it is the only equatorward basin-scale ocean heat transport and it sets the northward direction of the global cross-equatorial transport. Its strength and variability, however, are not well known. The South Atlantic transports are evaluated for four state-of-the-art global ocean reanalyses (ORAs) and two free-running models (FRMs) in the period 1997-2010. All products employ the Nucleus for European Modelling of the Oceans (NEMO) model, and the ORAs share very similar configurations. Very few previous works have looked at ocean circulation patterns in reanalysis products, but here we show that the ORA basin interior transports are consistently improved by the assimilated in situ and satellite observations relative to the FRMs, especially in the Argo period. The ORAs also exhibit systematically higher meridional transports than the FRMs, which is in closer agreement with observational estimates at 35 and 11 • S. However, the data assimilation impact on the meridional transports still greatly varies among the ORAs, leading to differences up to ∼ 8 Sv and 0.4 PW in the South Atlantic Meridional Overturning Circulation and the MHTs, respectively. We narrow this down to large inter-product discrepancies in the western boundary currents (WBCs) at both upper and deep levels explaining up to ∼ 85 % of the inter-product differences in MHT. We show that meridional velocity differences, rather than temperature differences, in the WBCs drive ∼ 83 % of this MHT spread. These findings show that the present ocean observation network and data assimilation schemes can be used to consistently constrain the South Atlantic interior circulation but not the overturning component, which is dominated by the narrow western boundary currents. This will likely limit the effectiveness of ORA products for climate or decadal prediction studies.

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Section: Time Mean Transportssupporting

confidence: 91%

South Atlantic meridional transports from NEMO-based simulations and reanalyses

2018

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“…However, multidecadal changes in the strength of Atlantic STCs were detected in assimilation model runs. These changes include a minimum STC-layer (about 50-300 m) convergence in the early 1970s and a maximum in the early 1990s (Rabe et al, 2008), which would affect the supply of newly subducted oxygen-rich water masses from the subtropics to the tropics.…”

Section: Long-term Variability In Etna Omzmentioning

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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“…In simple linear vorticity theory, the Ekman convergence in subtropics drives an equatorward Sverdrup transport that explains many aspects of the wind-driven gyre circulation, such as the subtropical cells (STC). Schott et al (2004) calculated the Ekman divergence (21-24 Sv, 1 Sv = 10 6 m 3 s −1 ) between 10 • N and 10 • S in the tropical Atlantic using climatological wind to infer the strength of the STC; Rabe et al (2008) further analysed the variability of the STC using the same sections based on assimilation products, and found that on timescales longer than 5 years to decadal, the variability of poleward Ekman divergence leads the variability of geostrophic convergence in the thermocline.…”

Section: Introductionmentioning

confidence: 99%

On the meridional ageostrophic transport in the tropical Atlantic

Karstensen

Brandt

2017

Ocean Sci.

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Abstract. The meridional Ekman volume, heat, and salt transport across two trans-Atlantic sections near 14.5 • N and 11 • S were estimated using in situ observations, wind products, and model data. A meridional ageostrophic velocity was obtained as the difference between the directly measured total velocity and the geostrophic velocity derived from observations. Interpreting the section mean ageostrophy to be the result of an Ekman balance, the meridional Ekman transport of 6.2±2.3 Sv northward at 14.5 • N and 11.7±2.1 Sv southward at 11 • S is estimated. The integration uses the top of the pycnocline as an approximation for the Ekman depth, which is on average about 20 m deeper than the mixed layer depth. The Ekman transport estimated based on the velocity observations agrees well with the predictions from in situ wind stress data of 6.7 ± 3.5 Sv at 14.5 • N and 13.6 ± 3.3 Sv at 11 • S. The meridional Ekman heat and salt fluxes calculated from sea surface temperature and salinity data or from highresolution temperature and salinity profile data differ only marginally. The errors in the Ekman heat and salt flux calculation were dominated by the uncertainty of the Ekman volume transport estimates.

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Mean Circulation and Variability of the Tropical Atlantic during 1952–2001 in the GECCO Assimilation Fields

Cited by 33 publications

References 42 publications

South Atlantic meridional transports from NEMO-based simulations and reanalyses

South Atlantic meridional transports from NEMO-based simulations and reanalyses

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

On the meridional ageostrophic transport in the tropical Atlantic

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