PONE-D-20-08829

Resolution-dependent variations of sinking Lagrangian particles in general circulation models

PLOS ONE

Dear Mr Nooteboom,

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PLOS ONE

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Reviewers' comments:

Reviewer's Responses to Questions

Comments to the Author

1. Is the manuscript technically sound, and do the data support the conclusions?

The manuscript must describe a technically sound piece of scientific research with data that supports the conclusions. Experiments must have been conducted rigorously, with appropriate controls, replication, and sample sizes. The conclusions must be drawn appropriately based on the data presented.

Reviewer #1: Yes

Reviewer #2: Partly

Reviewer #3: Partly

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2. Has the statistical analysis been performed appropriately and rigorously?

Reviewer #1: Yes

Reviewer #2: Yes

Reviewer #3: Yes

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3. Have the authors made all data underlying the findings in their manuscript fully available?

The PLOS Data policy requires authors to make all data underlying the findings described in their manuscript fully available without restriction, with rare exception (please refer to the Data Availability Statement in the manuscript PDF file). The data should be provided as part of the manuscript or its supporting information, or deposited to a public repository. For example, in addition to summary statistics, the data points behind means, medians and variance measures should be available. If there are restrictions on publicly sharing data—e.g. participant privacy or use of data from a third party—those must be specified.

Reviewer #1: Yes

Reviewer #2: Yes

Reviewer #3: Yes

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PLOS ONE does not copyedit accepted manuscripts, so the language in submitted articles must be clear, correct, and unambiguous. Any typographical or grammatical errors should be corrected at revision, so please note any specific errors here.

Reviewer #1: Yes

Reviewer #2: Yes

Reviewer #3: Yes

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5. Review Comments to the Author

Please use the space provided to explain your answers to the questions above. You may also include additional comments for the author, including concerns about dual publication, research ethics, or publication ethics. (Please upload your review as an attachment if it exceeds 20,000 characters)

Reviewer #1: This paper is a numerical discussion of particle motion in ocean general circulation models, principally as a function of spatial resolution. The conclusions are unsurprising: eddy resolution makes a big difference to long-term particle displacements. My inference is that the paper is directed *not* at numerical modelers, but at paleoclimate people who over-interpret results of crude circulation models. If this inference is correct, it should be made explicit in the abstract, introduction, and probably the title and raises the question of whether it belongs in a paleo-journal.

If the intended audience is not the technically adept numerical modelling community, it is essential to tell the reader that the paper represents errors in Lagrangian displacements in *Eulerian* models. Non-expert readers should be told that although not common, Lagrangian circulation models exist, as do whole books on the subject (e.g., Bennett's). An interesting, technical, question would be how well a Lagrangian GCM can calculate the Eulerian mean flows.)

Why is a backwards in time calculation done? Does some stability issue not arise?

Again, for non-specialist audiences, it would be helpful to point out that skill in calculating the distribution of scalar tracers, such as temperature might be much more accurate in the parameterized Eulerian models---given how the parameterizations are constructed.

Reviewer #2: The manuscript describes how statistics of Lagrangian sinking particles differ when advected off-line in an eddy permitting or coarse resolution ocean model. While realizing the need for the work presented for some communities (paleo?), from a physical oceanography point of view, the content is not novel and mostly qualitative. It is well known that ocean models that are not eddy resolving underestimate horizontal dispersion. The literature on the subject is very large and some references should indeed be added. I note also that the manuscript in its current form reads like a physical oceanography work, and the appeal to the broader and more biologically oriented community of PLOS One is unclear to me.

I do not have major comments on what is in the manuscript (there is nothing wrong per se), but I would have hoped for a more in-depth justification of the work and its limitations considering the lack of novelty, or a more in-depth investigation of the reliability of the high and low resolution results. The outcome of this work confirms deductions that have been made already in the absence of sinking, and many Lagrangian studies have been performed in this regard comparing runs at the exact resolutions used here (to mention few relevant to the content of this manuscript: Putman and He, 2013; Doos, Rupolo and Brodeau, 2011, and many intercomparison papers focused on the Eulerian fields).

In detail:

Firstly, the manuscript never mentions the resolution limitations of the simulation used. 10km is not enough to resolve eddies at high latitudes, therefore the run is eddy permitting. No mention is made of the scales that are not resolved (effectively 30 km and below) in the high resolution case. Nothing is said about the vertical resolution (work by Stewart et al, 2017 and Bracco et al., 2018, both in Ocean Modelling may be of interest). In other words, can we expect to see similar results if we had a 1km resolution ocean model or 100 vertical layers? Accounting for work by, for example, Patrice Klein and others, the likely answer is no. Global runs at 2km have been made and are potentially freely available for analysis (MIT model, OFES…).

Secondly, what would happen is a reanalysis data-set was used instead? The most recent ones are run at 25km->10km near the poles and fields are saved monthly or more frequently. Do the statistics in the 10km run compare well with those from a reanalysis product, where at least surface eddies are constrained by satellite observations and the water column dynamics assimilate the large ARGO dataset? The investigation of the time-dependence of the results (advecting particles using fields saved every month or every 5 days is also superficial and should be expanded).

Thirdly, I understand the argument for justifying the simulation done for small plastic particles, but as far as I know organic particles small enough to have a sinking velocity of 6-20 m/day near the surface do not make it to the ocean bottom as such (vice versa particles that may reach the bottom with these sinking velocities were likely much larger and heavier at formation). Aggregation, grazing and bacterial activities are dominant processes on those sizes. The review by Boyd et al. in Nature in 2019 has a nice image summarizing the fate of sinking particles as function of their size, and shows that sinking is not the dominant process determining the fate of particles as small as those used in the manuscript (they do not reach more than 400-600 m depth in particle form). It is unclear, therefore, what is the paleo relevance that justifies the need for coarse resolution ocean models.

Overall, in my opinion this is an exercise that deserves publication if better motivated, with a more in-depth discussion of implications and limitations, and the exploration of a greater range of sinking velocities, saving times, etc.

Reviewer #3: This manuscript considers the problem of identifying the source location of sinking particles found at the bottom of the ocean and examines the effect of coarse spatial (and to some extent temporal) resolution. The authors find that ocean general circulation models (OGCMs) at a resolution typical of state-of-the-art paleoclimate applications (1 degree) do not capture the distributions of potential source locations well. This can be remedied to some degree by using a Smagorinski parameterisation for the eddy effects that are not directly modeled.

I find the experiment reasonably designed and the manuscript overall well written, although the organization is a bit odd. Some of the discussion could also benefit from a broader perspective, considering alternative explanations, and some of the conclusions are stated too strongly. Therefore, I recommend this submission for publication subject to minor revisions, which should address the detailed comments provided in the attachment.

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Reviewer #1: No

Reviewer #2: No

Reviewer #3: No

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Attachments

Attachment

Submitted filename: Review.pdf

PONE-D-20-08829R1

Resolution dependency of sinking Lagrangian particles in ocean general circulation models

PLOS ONE

Dear Dr. Nooteboom,

Thank you for submitting your manuscript to PLOS ONE. After careful consideration, and based on the comments by the two reviewers, we feel that it has merit but does not fully meet PLOS ONE’s publication criteria as it currently stands. Therefore, we invite you to submit a revised version of the manuscript that addresses the points raised by the reviewers during the review process.

Please submit your revised manuscript by Aug 18 2020 11:59PM. If you will need more time than this to complete your revisions, please reply to this message or contact the journal office at plosone@plos.org. When you're ready to submit your revision, log on to https://www.editorialmanager.com/pone/ and select the 'Submissions Needing Revision' folder to locate your manuscript file.

Please include the following items when submitting your revised manuscript:

• A rebuttal letter that responds to each point raised by the academic editor and reviewer(s). You should upload this letter as a separate file labeled 'Response to Reviewers'.
• A marked-up copy of your manuscript that highlights changes made to the original version. You should upload this as a separate file labeled 'Revised Manuscript with Track Changes'.
• An unmarked version of your revised paper without tracked changes. You should upload this as a separate file labeled 'Manuscript'.

If you would like to make changes to your financial disclosure, please include your updated statement in your cover letter. Guidelines for resubmitting your figure files are available below the reviewer comments at the end of this letter.

If applicable, we recommend that you deposit your laboratory protocols in protocols.io to enhance the reproducibility of your results. Protocols.io assigns your protocol its own identifier (DOI) so that it can be cited independently in the future. For instructions see: http://journals.plos.org/plosone/s/submission-guidelines#loc-laboratory-protocols

We look forward to receiving your revised manuscript.

Kind regards,

Vanesa Magar, Ph.D.

Academic Editor

PLOS ONE

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Reviewers' comments:

Reviewer's Responses to Questions

Comments to the Author

1. If the authors have adequately addressed your comments raised in a previous round of review and you feel that this manuscript is now acceptable for publication, you may indicate that here to bypass the “Comments to the Author” section, enter your conflict of interest statement in the “Confidential to Editor” section, and submit your "Accept" recommendation.

Reviewer #1: (No Response)

Reviewer #3: All comments have been addressed

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2. Is the manuscript technically sound, and do the data support the conclusions?

The manuscript must describe a technically sound piece of scientific research with data that supports the conclusions. Experiments must have been conducted rigorously, with appropriate controls, replication, and sample sizes. The conclusions must be drawn appropriately based on the data presented.

Reviewer #1: Yes

Reviewer #3: Yes

**********

3. Has the statistical analysis been performed appropriately and rigorously?

Reviewer #1: Yes

Reviewer #3: Yes

**********

4. Have the authors made all data underlying the findings in their manuscript fully available?

The PLOS Data policy requires authors to make all data underlying the findings described in their manuscript fully available without restriction, with rare exception (please refer to the Data Availability Statement in the manuscript PDF file). The data should be provided as part of the manuscript or its supporting information, or deposited to a public repository. For example, in addition to summary statistics, the data points behind means, medians and variance measures should be available. If there are restrictions on publicly sharing data—e.g. participant privacy or use of data from a third party—those must be specified.

Reviewer #1: Yes

Reviewer #3: Yes

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5. Is the manuscript presented in an intelligible fashion and written in standard English?

PLOS ONE does not copyedit accepted manuscripts, so the language in submitted articles must be clear, correct, and unambiguous. Any typographical or grammatical errors should be corrected at revision, so please note any specific errors here.

Reviewer #1: Yes

Reviewer #3: Yes

**********

6. Review Comments to the Author

Please use the space provided to explain your answers to the questions above. You may also include additional comments for the author, including concerns about dual publication, research ethics, or publication ethics. (Please upload your review as an attachment if it exceeds 20,000 characters)

Reviewer #1: This revised ms. is much improved in making clear what is the goal, and how it has been addressed. The paper is now clearly addressed to what are expected to be naive paleoceanographers who take model results much too seriously. As such, I would expect the authors to have sent this paper to some place like Quaternary Science Reviews, but that is their business (and that of the Editor of PLOS ONE).

Assuming everyone agrees that the journal is appropriate, the paper can be accepted in principle, but there exist a number of amplifications that would be very useful if the readership really is thought to be uninformed about numerical ocean models.

Readers need to be told that physical oceanographers distinguish between "eddy-permitting" models and "eddy-resolving" ones. This distinction would be crucial to anyone taking the paper seriously,  but it is not even mentioned. Are internal tides, internal waves, sub-mesoscales, of no concern?

The Wasserstein distance is fundamental to their results, but the paper has no explanation except for a reference number. Explain. 

Even in simple laminar flows, particle trajectories are known to be chaotic (e.g., Regier and Stommel, 1979 PNAS). Why is that not an issue here? 

It is likely true that the vertical velocity in the ambient fluid is negligible compared to the sinking velocities.  But in upwelling zones? (See e.g., Liang JGR 2018) Again, useful information for non-modelers. 

The complexity now known about near-bottom flows in the presence of topography needs to be at least mentioned. A large recent literature on this subject and probably very important in the particles landing in sedimentary regions.

The authors keep referring to the "mean" flow. Is there a definable mean flow? Over what time-scale? Same everywhere?

The bolus velocity is mentioned on P. 14. If it was ever defined, I missed it.

Can something be said about the skill of the reference model in reproducing the modern circulation elements, such as the eddy intensities, spatial scales, etc.?  

Consider comparing Fig. 3 to the "ventilation" Fig. 2 of Gebbie and Huybers (GRL 2011). Just needs one sentence.

Is "rectification" (P. 7) the right word? Comes from radio engineering and implies self-interaction producing a mean or low frequency.

Put  horizontal dimensions scales on Fig. 1.

Reviewer #3: The authors have done a good job addressing my previous concerns. I now recommend the manuscript for publication.

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Reviewer #1: No

Reviewer #3: No

[NOTE: If reviewer comments were submitted as an attachment file, they will be attached to this email and accessible via the submission site. Please log into your account, locate the manuscript record, and check for the action link "View Attachments". If this link does not appear, there are no attachment files.]

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Resolution dependency of sinking Lagrangian particles in ocean general circulation models

PONE-D-20-08829R2

Dear Dr. Nooteboom,

We’re pleased to inform you that your manuscript has been judged scientifically suitable for publication and will be formally accepted for publication once it meets all outstanding technical requirements.

Within one week, you’ll receive an e-mail detailing the required amendments. When these have been addressed, you’ll receive a formal acceptance letter and your manuscript will be scheduled for publication.

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Kind regards,

Vanesa Magar, Ph.D.

Academic Editor

PLOS ONE

Additional Editor Comments (optional):

Reviewers' comments:

Reviewer's Responses to Questions

Comments to the Author

1. If the authors have adequately addressed your comments raised in a previous round of review and you feel that this manuscript is now acceptable for publication, you may indicate that here to bypass the “Comments to the Author” section, enter your conflict of interest statement in the “Confidential to Editor” section, and submit your "Accept" recommendation.

Reviewer #1: All comments have been addressed

**********

2. Is the manuscript technically sound, and do the data support the conclusions?

The manuscript must describe a technically sound piece of scientific research with data that supports the conclusions. Experiments must have been conducted rigorously, with appropriate controls, replication, and sample sizes. The conclusions must be drawn appropriately based on the data presented.

Reviewer #1: Yes

**********

3. Has the statistical analysis been performed appropriately and rigorously?

Reviewer #1: Yes

**********

4. Have the authors made all data underlying the findings in their manuscript fully available?

The PLOS Data policy requires authors to make all data underlying the findings described in their manuscript fully available without restriction, with rare exception (please refer to the Data Availability Statement in the manuscript PDF file). The data should be provided as part of the manuscript or its supporting information, or deposited to a public repository. For example, in addition to summary statistics, the data points behind means, medians and variance measures should be available. If there are restrictions on publicly sharing data—e.g. participant privacy or use of data from a third party—those must be specified.

Reviewer #1: Yes

**********

5. Is the manuscript presented in an intelligible fashion and written in standard English?

PLOS ONE does not copyedit accepted manuscripts, so the language in submitted articles must be clear, correct, and unambiguous. Any typographical or grammatical errors should be corrected at revision, so please note any specific errors here.

Reviewer #1: Yes

**********

6. Review Comments to the Author

Please use the space provided to explain your answers to the questions above. You may also include additional comments for the author, including concerns about dual publication, research ethics, or publication ethics. (Please upload your review as an attachment if it exceeds 20,000 characters)

Reviewer #1: I thought the previous comments were sufficiently minor that I didn't have to see this 3rd version Nonetheless,I have read it again. The authors do seem to have met the most important criticisms and the paper should beaccepted.

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7. PLOS authors have the option to publish the peer review history of their article (what does this mean?). If published, this will include your full peer review and any attached files.

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Reviewer #1: No