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Atlantic Equatorial Undercurrent intensification counteracts warming-induced deoxygenation

Nature Geoscience volume 14, pages 278–282 (2021)Cite this article

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Abstract

The tropical Atlantic upper-ocean circulation experiences multiannual to decadal changes associated with different climate modes and is simultaneously adjusting to climate warming. The most energetic current in the tropical Atlantic is the Equatorial Undercurrent (EUC), which flows eastwards along the Equator. On the basis of long-term moored observations, we show that the EUC strengthened by more than 20% from 2008 to 2018. The intensification of the EUC is associated with increasing subsurface oxygen concentrations and a thickening of the upper-ocean oxygenated layer in the equatorial Atlantic. These changes counteract climate-warming-induced deoxygenation in the region. The EUC strengthening is found to be mainly forced by trade wind changes in the western tropical North Atlantic. A 60-yr dataset reveals that the recent oxygen increase in the upper equatorial Atlantic is associated with multidecadal variability. This variability is characterized by low oxygen concentrations in the 1990s and early 2000s, and high oxygen concentrations in the 1960s and 1970s. The observed oxygen variability seems to be linked to a compression and expansion of the habitat of tropical pelagic fish, and must be accounted for when evaluating the possible consequences of deoxygenation for marine ecosystems and fisheries.

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Fig. 1: The Atlantic EUC at 23° W.
Fig. 2: Shipboard measurements along the Atlantic Equator in September–October 2019.
Fig. 3: Surface OLT in the tropical Atlantic.

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Data availability

All data supporting the findings of this study are publicly available as referenced within the paper and in Supplementary Tables 1, 4 and 5.

Code availability

All necessary code for the data analysis and preparation of the figures of this manuscript is freely available at https://doi.org/10.5281/zenodo.4478285.

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Acknowledgements

This study was funded by EU H2020 under grant agreement 817578 TRIATLAS project, by the Deutsche Forschungsgemeinschaft as part of the Sonderforschungsbereich 754 “Climate–Biogeochemistry Interactions in the Tropical Ocean” and through several research cruises with RV Meteor and RV Maria S. Merian and by the Deutsche Bundesministerium für Bildung und Forschung (BMBF) as part of the projects NORDATLANTIK (03F0443B) and RACE-Synthese (03F0824C). R. Kopte was supported by the Underway Research Data project of the German Marine Research Alliance. R. Kiko was also supported by a Make Our Planet Great Again grant of the French Agence Nationale de la Recherche under the “Programme d’Investissements d’Avenir” reference ANR-19-MPGA-0012. We thank the captains, crews, scientists and technical groups involved in the different national and international research cruises to the tropical Atlantic that contributed to collecting shipboard and mooring data and making them freely available. Some of the velocity and oxygen observations were acquired within the PIRATA project and the CLIVAR TACE programme.

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Authors and Affiliations

  1. GEOMAR Helmholtz Centre for Ocean Research Kiel, Kiel, Germany

    Peter Brandt, Johannes Hahn, Sunke Schmidtko, Franz Philip Tuchen, Rainer Kiko, Rena Czeschel & Marcus Dengler

  2. Faculty of Mathematics and Natural Sciences, Kiel University, Kiel, Germany

    Peter Brandt

  3. Institute of Geosciences, Kiel University, Kiel, Germany

    Robert Kopte

  4. Laboratoire d’Océanographie de Villefranche, Sorbonne Université, Villefranche-sur-Mer, France

    Rainer Kiko

  5. Instrumentation, Moyens Analytiques, Observations en Géophysique et Océanographie, IRD, Plouzané, France

    Bernard Bourlès

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Contributions

P.B. designed the long-term measurement programme, supervised the data analysis and wrote the first version of the manuscript. J.H. analysed shipboard sections along 23° W. S.S. analysed historical and recent oxygen data. F.P.T. analysed moored velocity data. R. Kopte, F.P.T. and J.H. calculated and analysed EUC transport time series. J.H. and F.P.T. analysed different wind products. R. Kiko, B.B., R.C. and M.D. contributed additional data and expertise on equatorial ocean dynamics and oxygen changes. All authors discussed the results and wrote the manuscript.

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Correspondence to Peter Brandt.

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The authors declare no competing interests.

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Peer review information Nature Geoscience thanks Dongxiao Zhang, Nicolas Gruber and the other, anonymous, reviewer(s) for their contribution to the peer review of this work. Primary Handling Editor: Tom Richardson.

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Supplementary information

Supplementary Information

Supplementary Figs. 1–10, Tables 1–5 and discussion on oxygen changes along the 23° W meridian between 5° S and 14° N

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Brandt, P., Hahn, J., Schmidtko, S. et al. Atlantic Equatorial Undercurrent intensification counteracts warming-induced deoxygenation. Nat. Geosci. 14, 278–282 (2021). https://doi.org/10.1038/s41561-021-00716-1

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