Assimilating synthetic Biogeochemical-Argo and ocean colour observations into a global ocean model to inform observing system design

Ford, David

doi:https://doi.org/10.5194/bg-18-509-2021

Articles | Volume 18, issue 2

https://doi.org/10.5194/bg-18-509-2021

© Author(s) 2021. This work is distributed under
the Creative Commons Attribution 4.0 License.

Special issue:

Biogeochemistry in the BGC-Argo era: from process studies...

https://doi.org/10.5194/bg-18-509-2021

© Author(s) 2021. This work is distributed under
the Creative Commons Attribution 4.0 License.

Articles | Volume 18, issue 2

Research article

|

21 Jan 2021

Research article |

| 21 Jan 2021

Assimilating synthetic Biogeochemical-Argo and ocean colour observations into a global ocean model to inform observing system design

David Ford

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Final revised paper (published on 21 Jan 2021)
Preprint (discussion started on 15 May 2020)

Interactive discussion

Status: closed

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

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- Supplement

RC1: 'Review on Ford 2020', Anonymous Referee #1, 03 Jun 2020
- AC1: 'Reply to Referee #1', David Ford, 31 Jul 2020
RC2: 'review on Ford 2020', Anonymous Referee #2, 14 Jun 2020
- AC2: 'Reply to Referee #2', David Ford, 31 Jul 2020

Peer-review completion

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

ED: Reconsider after major revisions (04 Aug 2020) by Katja Fennel

ED: Reconsider after major revisions (04 Aug 2020) by Katja Fennel (Co-editor-in-chief)

AR by David Ford on behalf of the Authors (15 Oct 2020) Author's response Manuscript

ED: Referee Nomination & Report Request started (19 Oct 2020) by Katja Fennel

RR by Anonymous Referee #2 (11 Nov 2020)

Suggestions for revision or reasons for rejection

The author has done a good job of responding to all the reviewers' comments (including mine), and the manuscript is greatly improved as a result. It reads very well, and includes useful additions from the first version. I have only some minor technical corrections and suggestions, and my recommendation is to accept it.

Minor comments.

Line 138: a model spatial correlation is introduced without a clear link with the previous sentences explaining the DA method. How is the model spatial correlation used by the DA method?

Line 143 which unbalance component is used?

Line 211 and Line 214: OSSEs investigate the potential impacts of future observing systems or new observations of current observing networks. Thus, I wonder whether the reference to “current observing network” (line 211) and to “the assimilative run, which assimilates synthetic observations representing current observing networks” (line 214) can be confusing. I would suggest to refer to future observing networks, which, in this specific study, has been designed starting from a 1-year of Argo trajectories.

Lines 383-384: this sentence is not clear: why should it matter when the FREE-NATURE difference is compared to the error between observation and FREE? Shouldn’t the difference between FREE and NATURE already include all possible (and reasonable) sources of uncertainty in order to be used for OSSEs?

Lines 385-386: this explanation is not entirely convincing. The differences between FREE and OBS (Fig.3i) are up to 0.1 (which would be a rather high acidification signal in just 5 years) while the rather low difference between FREE and NATURE (Fig. 3j) seems possibly related to the initialisations of DIC and alkalinity which are maybe too close.

Line 390: annual zonal section plots of the absolute differences can be added to Fig. 4 (as done in Figure 3). This would greatly facilitate the reading of this section.

Line 355: it seems to me that the choice to show December 2009 results is because the largest DA impacts are at the end of simulation (as it is explained latter). If it is like that, it should be mentioned at this point.

Line 447: I wonder whether the measurement and representation errors added to the synthetic observations in areas where FREE and NATURE are too close force the assimilation of those observations to move the assimilative runs far from the NATURE. This would explain the worsening of assimilative runs in areas with very low FREE-NATURE differences.

Line 459-462: looking at Figures 7s,t and 7x,y, it seems to me that that MAE_red% of pCO2 is worse than that of pH (i.e., red coloured areas in x and y are larger than in s and t). It is not straightforward that pCO2 improves when pH is assimilated, as it is also discussed latter (lines 575-584). It should be mentioned at this point before it is discussed in the final section.

Figures 10, 11 and 12. Could the author reverse the colorscale of these Hovmoeller diagrams? Since the blue colour means reduction of the MAE in figures 7-9, it would increase the readability to have the same also in Fig.10, 11 and 12.
Further, all negative values have a single blue tone. This does not help to understand how relevant negative values are. It would be better to expand negative scale.

Lines 471-473: how significant is this small degradation? The choice of the color scale (i.e., only one blu tone) of Figure 10 seems misleading. Curves of OC, ARGO_1/4_OC and ARGO_FULL_OC of figure 6a are almost indistinguishable.

Lines 488-492: does the author have any explanation for the different impact on MAE reduction in the different oceans? Is there any evidence that can be discussed about the effectiveness of the BGC-Argo network and its assimilation in the different oceanic areas?

Line 684-690: this paragraph is perhaps the only part that rises me some concerns. It is not clear to me what top-down or bottom-up control means in this context. In ecology, these terms are usually referred to the trophic food web interactions (e.g. phytoplankton dynamics is top-down controlled by zooplankton grazing or bottom-up controlled by nutrient supplied).
Instead, these terms are used here to explain some vertical relationships between layers: deep layer and mixed layer. Could the author explain these relationships better?
Further, it seems to me that the greatest improvement of BGC-Argo assimilation over OC occurs at the deep chlorophyll maximum and nitracline depths, which is a pretty nice result to discuss.

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ED: Publish subject to minor revisions (review by editor) (16 Nov 2020) by Katja Fennel

ED: Publish subject to minor revisions (review by editor) (16 Nov 2020) by Katja Fennel (Co-editor-in-chief)

AR by David Ford on behalf of the Authors (30 Nov 2020) Author's response Manuscript

ED: Publish as is (07 Dec 2020) by Katja Fennel

ED: Publish as is (07 Dec 2020) by Katja Fennel (Co-editor-in-chief)

AR by David Ford on behalf of the Authors (07 Dec 2020)

Short summary

Biogeochemical-Argo floats are starting to routinely measure ocean chlorophyll, nutrients, oxygen, and pH. This study generated synthetic observations representing two potential Biogeochemical-Argo observing system designs and created a data assimilation scheme to combine them with an ocean model. The proposed system of 1000 floats brought clear benefits to model results, with additional floats giving further benefit. Existing satellite ocean colour observations gave complementary information.