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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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Brandt, P., Bange, H. W., Banyte, D., Dengler, M., Didwischus, S.-H., Fischer, T., Greatbatch, R. J., Hahn, J. +8 more·2015·Biogeosciences
DOI 10.5194/bg-12-489-2015

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Abstract

Abstract. Ocean observations are analysed in the framework of Collaborative Research Center 754 (SFB 754) "Climate-Biogeochemistry 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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  • About 30% of the oxygen consumed in the OMZs of the eastern tropical Atlantic is replenished by interior ocean mixing processes sustained through internal wave-wave interaction.

    “Nevertheless, about 30% of the oxygen consumed in the OMZs of the eastern tropical Atlantic is replenished by interior ocean mixing processes sustained through internal wave-wave interaction (Brandt et al., 2015).”

    From The tropical atlantic observing system (2019) · cites this paper as Brandt et al., 2015

    The Abstract and §9 (Summary and discussion; page not provided) say vertical mixing supplies about 20% of the oxygen needed to balance consumption in the deep eastern tropical North Atlantic OMZ, reaching about 30% only locally. The claim generalizes that local figure to the region’s OMZs. The paper does not identify internal wave–wave interaction as the process sustaining that interior mixing; §6.3 instead discusses internal waves enhanced by tide–topography interaction at the continental margin. (LLM, conf=0.90)

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  • Mechanisms responsible for the other half of the multi-decadal oxygen decline are unclear and are likely related to changes in ventilation and circulation, though changes in biological activity cannot be ruled out.

    “Mechanisms that are responsible for the other half are unclear and are likely related to changes in ventilation and circulation (Brandt et al., 2015;Oschlies et al., 2018), though changes in biological activity cannot be ruled out.”

    From The tropical atlantic observing system (2019) · cites this paper as Brandt et al., 2015

    Section 7 (Long-term variability in ETNA OMZ) says the driving mechanisms of observed deoxygenation remain uncertain and discusses circulation, mixing, and biogeochemical oxygen consumption. Section 9 (Summary and discussion) identifies circulation changes as a likely contributor to the regional decline. The paper does not attribute a specific ‘other half’ of the multidecadal decline to these mechanisms; its quantitative statement instead concerns an imbalance equal to about 10% of oxygen consumption. (LLM, conf=0.85)

    Experimental — accuracy not yet measured

  • Based on oxygen observations from repeat ship sections in the OMZ of the eastern tropical North Atlantic for the recent decade (2006-2015), Hahn et al. (2017) suggested the existence of strong oxygen variations over this decade superimposed on the multi-decadal deoxygenation pattern.

    “Based on oxygen observations from repeat ship sections in the OMZ of the eastern tropical North Atlantic for the recent decade (2006)(2007)(2008)(2009)(2010)(2011)(2012)(2013)(2014)(2015), Hahn et al. (2017) suggested the existence of strong oxygen variations over this decade that are superimposed on the multi-decadal deoxygenation pattern (Stramma et al., 2009;Brandt et al., 2015;Santos et al., 2”

    From The tropical atlantic observing system (2019) · cites this paper as Brandt et al., 2015

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Research works printed but not linked: 1

  1. Robinson, A., Brink, K. H., Wiley, New York, 213-252, 1998.

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Authors
Brandt, P., Bange, H. W., Banyte, D., Dengler, M., Didwischus, S.-H., Fischer, T., Greatbatch, R. J., Hahn, J., Kanzow, T., Karstensen, J., Körtzinger, A., Krahmann, G., Schmidtko, S., Stramma, L., Tanhua, T., & Visbeck, M.
Affiliations
  • GEOMAR Helmholtz Centre for Ocean Research Kiel (DE)
  • Alfred-Wegener-Institut Helmholtz-Zentrum für Polar- und Meeresforschung (DE)
Corresponding author
Brandt, P.
ORCID
Venue
Biogeosciences
Publisher
Copernicus GmbH
Published
2015-01-27
Volume
12
Issue
2
Pages
489-512
ISSN
  • 1726-4170 (ISSN-L)
  • 1726-4189
DOI
10.5194/bg-12-489-2015
OpenAlex
W2110455993
Cited by
180
References
115
Citations by year
2026: 6 · 2025: 15 · 2024: 20 · 2023: 19 · 2022: 14 · 2021: 31 · 2020: 13 · 2019: 17 · 2018: 15 · 2017: 7 · 2016: 15 · 2015: 8
FWCI
12.77
Type
journal article
Language
en
Open access
Licence
Locations
  • Biogeosciences (publishedVersion, cc-by)
  • Helmholtz Centre for Ocean Research Kiel (GEOMAR) (acceptedVersion, other-oa)
  • DOAJ (DOAJ: Directory of Open Access Journals) (submittedVersion, cc-by-sa)
  • Helmholtz-Zentrum für Polar-und Meeresforschung (Alfred-Wegener-Institut) (submittedVersion)
Funders
  • Deutsche Forschungsgemeinschaft (SFB 754)
  • Bundesministerium für Bildung und Forschung (03F0443B, 03F0651B, 03F0662A, 01DG12073E)
Topics
  • Physical Sciences › Earth and Planetary Sciences › Oceanography › Marine and coastal ecosystems
  • Physical Sciences › Earth and Planetary Sciences › Oceanography › Oceanographic and Atmospheric Processes
  • Physical Sciences › Earth and Planetary Sciences › Paleontology › Paleontology and Stratigraphy of Fossils
Keywords
oxygen minimum zone, eastern tropical North Atlantic, ocean ventilation, ocean circulation, vertical mixing, zonal advection, oxygen budget, deoxygenation, water mass age, transient tracers, Eastern Tropical South Pacific

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