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https://doi.org/10.5194/os-2020-32
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/os-2020-32
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Submitted as: research article 23 Apr 2020

Submitted as: research article | 23 Apr 2020

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This preprint is currently under review for the journal OS.

Importance of El Niño reproducibility for reconstructing historical CO2 flux variations in the equatorial Pacific

Michio Watanabe1, Hiroaki Tatebe1, Hiroshi Koyama1, Tomohiro Hajima1, Masahiro Watanabe2, and Michio Kawamiya1 Michio Watanabe et al.
  • 1Research Institute for Global Change, Japan Agency for Marine-Earth Science and Technology (JAMSTEC), 3173-25, Showa-machi, Kanazawa-ku, Yokohama, Kanagawa, 236-0001, Japan
  • 2Atmosphere and Ocean Research Institute, the University of Tokyo, 5-1-5, Kashiwanoha, Kashiwa, Chiba, 277-8564, Japan

Abstract. In the equatorial Pacific, air–sea CO2 flux is known to fluctuate in response to inherent climate variability, predominantly the El Niño–Southern Oscillation (ENSO). For both investigation of the response of the carbon cycle to human-induced radiative perturbations and prediction of future global CO2 concentrations, representation of the interannual fluctuation of CO2 fluxes in Earth system models (ESMs) is essential. This study attempted to reproduce observed air–sea CO2 flux fluctuations in the equatorial Pacific using two ESMs, to which observed ocean temperature and salinity data were assimilated. When observations were assimilated into an ESM whose inherent ENSO variability was weaker than observations, nonnegligible correction terms on the governing equation of the equatorial ocean temperature caused anomalously false equatorial upwelling during El Niño periods that brought water rich in dissolved inorganic carbon from the subsurface layer to the surface layer. Contrary to observation, this resulted in an unusual upward air–sea CO2 flux anomaly that should not occur during El Niño periods. The absence of such unrealistic upwelling anomalies in the other ESM with the data assimilation reflects better representation of ENSO and the mean thermocline in this ESM without data assimilation. Our results demonstrate that adequate simulation of ENSO in an ESM is crucial for accurate reproduction of the variability in air–sea CO2 flux and hence, in the carbon cycle.

Michio Watanabe et al.

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Michio Watanabe et al.

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Short summary
In the equatorial Pacific, air–sea CO2 flux is known to fluctuate in response to inherent climate variability, predominantly the El Niño–Southern Oscillation. In this study, the same data assimilation scheme is incorporated into two Earth system models and fluctuations of air–sea CO2 flux in the equatorial Pacific were evaluated. Our results demonstrate that adequate simulation of El Niño–Southern Oscillation is crucial for accurate reproduction of the variability in air–sea CO2 flux.
In the equatorial Pacific, air–sea CO2 flux is known to fluctuate in response to inherent...
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