Importance of El Niño reproducibility for reconstructing historical CO<sub>2</sub> flux variations in the equatorial Pacific

Watanabe, Michio; Tatebe, Hiroaki; Koyama, Hiroshi; Hajima, Tomohiro; Watanabe, Masahiro; Kawamiya, Michio

doi:https://doi.org/10.5194/os-16-1431-2020

Articles | Volume 16, issue 6

https://doi.org/10.5194/os-16-1431-2020

© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

https://doi.org/10.5194/os-16-1431-2020

© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Articles | Volume 16, issue 6

Research article

|

18 Nov 2020

Research article |

| 18 Nov 2020

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

Michio Watanabe, Hiroaki Tatebe, Hiroshi Koyama, Tomohiro Hajima, Masahiro Watanabe, and Michio Kawamiya

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Final revised paper (published on 18 Nov 2020)
Supplement to the final revised paper
Preprint (discussion started on 23 Apr 2020)

Interactive discussion

Status: closed

AC: Author comment | RC: Referee comment | SC: Short comment | EC: Editor comment

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- Supplement

RC1: 'Comment from a referee', Anonymous Referee #1, 19 May 2020
- AC1: 'Reply to comments from referee #1', Watanabe Michio, 31 Aug 2020
RC2: 'Comments on Watanabe et al. “Importance of El Niño reproducibility for reconstructing historical CO2 flux variations in the equatorial Pacific”', Anonymous Referee #2, 21 May 2020
- AC2: 'Reply to comments from referee #2', Watanabe Michio, 31 Aug 2020
RC3: 'Interactive comment on “Importance of El Niño reproducibility for reconstructing historical CO2 flux variations in the equatorial Pacific” by Michio Watanabe et al.', Anonymous Referee #3, 25 May 2020
- AC3: 'Reply to comments from referee #3', Watanabe Michio, 31 Aug 2020

Peer-review completion

AR: Author's response | RR: Referee report | ED: Editor decision

AR by Watanabe Michio on behalf of the Authors (02 Sep 2020) Author's response Manuscript

ED: Referee Nomination & Report Request started (09 Sep 2020) by Mario Hoppema

RR by Anonymous Referee #1 (22 Sep 2020)

RR by Anonymous Referee #3 (25 Sep 2020)

Suggestions for revision or reasons for rejection

The revised manuscript is largely improved. The results and figures are well organised and presented in the revised manuscript. I would suggest for publication after clarifying the points below.

Abstract line 12: “observed historical” -> “observed”

Abstract line 20-24: the sentence is unclear and needs to be rephrased.

The 2 sentences in line 16 and line 20 in abstract are repetitive, i.e.,“The simulated CO2 flux anomalies were upward (downward) during El Nino (La Nina) periods. ” and “The simulated CO2 flux anomalies were upward (downward) during El Nino (La Nina) periods.” Suggest to leave one and specify the region: “The simulated CO2 flux anomalies were upward (downward) during El Nino (La Nina) periods in the equatorial Pacific.”

Lines 98-101, “The analysis increment is calculated from ΔXa = Xa(0) − X(0), where Xa(0) is the analysis and X(0) is the model first guess at t = 0; this term is held constant during the analysis interval. For Xa(0) and X(0), we used anomalies from monthly mean climatology during 1961–2000 in observations and models, respectively.” This method description is unclear. Does the IAU method always use the difference between analysis and model first guess at the initial step, i.e., t=0? How long is the analysis interval? Are the anomalies relative to climatology of model or observations?

The figures are largely improved, the new presentation of different component contribution to ΔpCO2 in Fig.2 is compact and informative. I have a question to the calculation as stated in Lines 163-164: “…we estimated ΔX by averaging monthly mean X anomalies regressed on the NINO3-SST anomalies over the entire Nino3 region,…” Why does ΔX involve regression to NINO3-SST anomalies? This part is not stated in equation 2.

As the regression of temperature increment to NINO3-SST shown in Fig. 5 a-b is not linearly additive to the regression of temperature in Fig. 3 to get the temperature regression of the assimilation run. I am curious how the equatorial ocean temperature in assimilation runs regressed to NINO3-SST look like. Maybe the authors could add a figure of results from the assimilation runs as Fig. 3 in supplementary.

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ED: Publish subject to minor revisions (review by editor) (28 Sep 2020) by Mario Hoppema

AR by Watanabe Michio on behalf of the Authors (08 Oct 2020) Author's response Manuscript

ED: Publish as is (08 Oct 2020) by Mario Hoppema

AR by Watanabe Michio on behalf of the Authors (09 Oct 2020)

Short summary

Carbon flux between air and sea is known to fluctuate in response to inherent climate variations. In this study, observed ocean hydrographic data were assimilated into Earth system models, and the carbon flux in the equatorial Pacific was evaluated. Our results suggest that, when observed ocean hydrographic data are assimilated into models for carbon cycle predictions on interannual to decadal timescales, the reproducibility of the internal climate variations in the model itself is important.

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

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