PCLM-D-25-00150

Characterizing compound physical and biogeochemical extremes in the California Current Large Marine Ecosystem

PLOS Climate

Dear Dr. Freeman,

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

Both reviewers see merit in your study and recommended a potential publication in PLOS Climate of a revised manuscript. I also find you manuscript clear, well-presented and easily readable.

While many requested reviews are relatively minor, there are important issues that should be addressed before I can accept your contribution. More specifically, Reviewer 2 provided important recommendation regarding the methodological details and advice for a better evaluation of the model skills. The reviewer also raised that some literature from the CCLME was missing and potential problems with the Hypoxic Layer Depth's definition. Please carefully address these comments and/or provide a rebuttal when appropriate.

Please submit your revised manuscript by Aug 14 2025 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 climate@plos.org. When you're ready to submit your revision, log on to https://www.editorialmanager.com/pclm/ and select the 'Submissions Needing Revision' folder to locate your manuscript file.

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Guidelines for resubmitting your figure files are available below the reviewer comments at the end of this letter.

We look forward to receiving your revised manuscript.

Kind regards,

Frédéric Cyr

Academic Editor

PLOS Climate

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Additional Editor Comments (if provided):

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Reviewers' comments:

Reviewer's Responses to Questions

Comments to the Author

1. Does this manuscript meet PLOS Climate’s publication criteria? Is the manuscript technically sound, and do the data support the conclusions? The manuscript must describe methodologically and ethically rigorous research with conclusions that are appropriately drawn based on the data presented.

Reviewer #1: Yes

Reviewer #2: Partly

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

Reviewer #1: Yes

Reviewer #2: I don't know

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3. Have the authors made all data underlying the findings in their manuscript fully available (please refer to the Data Availability Statement at the start of the manuscript PDF file)?

The PLOS Data policy requires authors to make all data underlying the findings described in their manuscript fully available without restriction, with rare exception. 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:No

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

PLOS Climate 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

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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: Overall, the paper is excellent. The figures are clear and very well thought out. The writing helped guide the reader through the science and explained the figures and meaning behind the different metrics very well.

There were just a few minor items to note:

-Figure 2 caption, there is mention of a carrot, I believe the spelling should be caret instead.

- Figure 6 caption, which zone had the outlier in the 9.75-10 range?

- Figure 10, just out of curiosity (no need to change the figure), why was volume of water not looked at? Do you think looking at volume instead of area would have changed the result?

- Hindcast evaluation/Supp figure 1: what about false positives? How often did GLORYS show an extreme event that was not in the observations?

- Supp Figure 4, the caption indicates there should be 4 columns of figures, but I only see three. The column with the MHW-LCX-SHX is possibly missing?

- Supp Figures 6 and 8, the yaxis units could be updated so the -2 can be a superscript (similar to Figure 9 Chl units) to be more consistent with the figures in the paper

Reviewer #2: Overall:

Freeman et al. analyzed overlapping extreme conditions—including high temperatures, low oxygen, and reduced chlorophyll—in the California Current marine ecosystem from 1996 to 2019 using a new product they generated using output from the global model named GLORYS. The authors found that:

• Marine heatwaves and low chlorophyll commonly occur during strong El Niño phases.

• Nearshore, shallow low-oxygen events typically link to La Niña.

• The most common compound event near shore is heatwave plus low chlorophyll; triple events with all three extremes are rare.

These results are novel and a worthy contribution to the community but many concerns exist in the approach and methods that require major revisions to clarify and potentially correct to understand the results. Additionally, the author team insufficiently cites or refer to currently wide body of work from existing BGC models in the region that have been generated by researchers external to NOAA, which need to be reviewed and included in this work. Below our feedback is detailed and separated into more specific comments. Upon addressing these concerns the work may be publishable in this journal.

Major comments:

1. GLORYS reanalysis and BGC model output differ in their resolution and output timing quite a bit. Notably the temperature and salinity from the “free-run” that generates the BGC are not provided as output by GLORYS – making the BGC product difficult to use. In contrast the reanalysis product has T and S but no BGC variables with it. The current description of how the new product which merges these two data streams together is insufficient and makes reviewing the rest of the work adequately difficult. Please add additional details on this that motivate the interpolation described in the methods describing how you generate GLORYS-BGC paying attention to the time and space details of the interpolation. These are both important to the story as you are investigating short term event variability in a shelf system.

2. To justify the use of this newly generated product, the authors need to provide more obs-model evaluations to show the model can skillfully capture extreme events throughout the model domain explored across the CCS. Currently the authors only showcase the CALFCOFI region. While climatology comparison (e.g., https://documentation.marine.copernicus.eu/QUID/CMEMS-GLO-QUID-001-029.pdf) may appear reasonable, point-to-point time-series comparison (e.g., from moorings) or vertical profiles (e.g., from bottle data) will likely show more disagreement. Indeed the comparisons shown for HLD in the supplement are not confidence inducing. How does this new product fare further north? The Newport line has a lot of observations. So does Line P. As GLORYS is a global model, surely the domain is not limited to 45 degrees N. It is also not clear why the authors chose to limit their results to this area. More justification for this approach, details as to how the interpolation mentioned are needed in the methods, as well as evaluation further north in the CCS are required. The BGC output from GLORYS is not a reanalysis product (provided at higher resolution) and is generally too coarse for nearshore BGC event dynamics as it does not resolve shelf/coastal processes in many instances in the CCS. The spatial resolution of GLORYS-BGC is 25 km, meaning only 3 grid points across the critical nearshore band (0-75 km). This coarse resolution might not be sufficient to resolve key shelves and nearshore processes, particularly short-term events (e.g., hypoxia), as such, using observational time-series (e.g., from coastal moorings) to showcase that model is skilled in representing event-scale dynamics is crucial.

3. We have concerns about how the authors have defined and treated HLD. Replacing missing HLD values with bottom depth is conceptually problematic, as it contradicts the definition of hypoxia. As noted in lines 138–144, shelf waters often do not experience hypoxia during winter and spring, and even during the upwelling season, hypoxic events are episodic in both space and time. Therefore, establishing a HLD “climatology” across the nearshore band could misrepresent the actual dynamics. In Fig. 8(c) and S9 Fig., where the climatological HLD in nearshore waters is shown deeper than 100 m continuously with time, which likely mischaracterizes hypoxia, especially in locations shallower than 100 m, using area-weighted, regionally average will further amplify this misrepresentation. We suggest the authors consider isolating the calculation to the upwelling season only and revisit the climatology definition to better consider the challenge of regions that never experience HLD

4. The authors do not consider the vertical structure of the anomalies in their compound event analysis. MHWs, hypoxia can and often are prominent in the subsurface, and their vertical extents often differ Although lines 115–117 acknowledge this, the manuscript currently emphasizes more of co-occurrence in time, not in vertical alignment. Chlorophyll anomalies may be biased the most because of this treatment as the blooms may simply be deeper at various times. For example, as shown in Fig.2 the LCX are not vertically co-located with SHX. This vertical decoupling might have distinct ecological implications. While SMHW and LCX are shown to be the most common combo, important subsurface nuances are not discussed. Please add additional discussion of the subsurface as this is novel and would benefit the community.

5. The authors also present monthly pH anomalies in the discussion section, which even if intended to present relative changes, should be treated with cautious the carbon component in GLORYS has not been fully validated to date, and without a robust model–observation evaluation, the reliability of the pH output remains uncertain. I recommend removing the content related to pH or ocean acidification entirely in the manuscript unless more robust evaluation is provided, and focusing only on temperatures, oxygen, nutrients, and chlorophyll, which is possible but not present in the current manuscript. When omitting it adding in the mention that while the pH exists, the output is not at a fine enough temporal resolution and its limited to the surface should be included in the methods.

6. The literature review is not sufficient and largely only cites only the author teams’ own work. The author team would benefit from including the wide list of non-NOAA external research that has been conducted in the CCS notably on biogeochemistry in the introduction and discussion. One comment in the conclusions is particularly symptomatic of this issue: “Despite these complexities, to first order, water column dissolved oxygen–and thus, HLD variability–is not fully understood, attributable to a historical lack of long-term, sustained observations across broad spatial scales on these timescales.”

This is problematic as there are a lot of observations in the region, but the author team only uses CALCOFI and exclude most of the observations within the domain. Additionally, the team is using a very coarse model product. Is it possible that the product can’t be used to explore the HLD? There have been several works characterizing oxygen variability well in simulations in the CCS – Damien et al. (2024), Deutsch et al. (2021), Renault et al (2021), Kessouri et al (2021), Howard et al. (2020a, 2020b), Cheresh and Fiechter (2020), Dussin et al. (2019), and Siedlecki et al (2015) to name a few – but these uncited models were much higher resolution than the derived model product analyzed here. This point deserves attention in the discussion, because if the resolution is the issue that is also worth noting to the community.

7. The acronyms throughout are quite distracting. It is possible they are too long – but it detracts from the reader’s understanding at the moment. Reducing them, renaming them, or using headings or subsections to discuss different variables may help.

8. There have been several notable and almost “famous” MHW events in this region – the authors don’t explore any of these as potential compound events within. Some discussion on this seems warranted. Should these communities scientifically overlap? Or are compound events entirely different? Figure S1 suggests the author team has the information from this product to address this. This author team is well poised to weigh in on this and that would be important for the broader community to hear.

Minor comments:

Line 102–104: The manuscript states that the model values are realistic; however, this claim should be supported with additional observational–model comparison statistics to validate the model’s performance, ideally from moorings. As this product is new and focused on the BGC, so should the evaluation.

Line 110: What is the treatment for locations shallower than 100 m when integrating chlorophyll over 0–100 m?

Line 138-144: For HLD calculations, restricting analysis to the upwelling season (April–September) in the N-CCS may yield more scientifically correct results for the nearshore band. Regions further south also experience seasonality in hypoxia which should also be considered.

Line 410: the figure citations need to be checked, should it be Fig. 8 and S7–S9 Figs? Similarly, line 415—should it be Fig. 8 vs. S6 Fig. ? Consider checking figure citations throughout the manuscript.

Line 414-416, While alongshore lag in HLD timing is apparent from S9, the manuscript does not conveniently show a lag between offshore and nearshore bands. Differences in duration and intensity are noted, but a temporal lag is not clearly supported.

Line 465: “Turi et” should be corrected to “Turi et al.”

Lines 477–523: Section 3.4 (“The most widespread extremes”) would flow more logically if placed immediately after Section 3.2 (“The most common extremes”).

Line 536: GLORYS-BGC product for oxygen and nutrients needed to be evaluated, more statistics need to be provided in the supplement to showcase that it is possible to use GLORYS in this way.

Line 598, The HLD and CUTI anomalies in 2015–2016 differ from those in 1997–1998; this distinction should be more clearly discussed.

Figures.

Fig. 9: Are the data shown regionally averaged over the nearshore (0–75 km) coastal band, as did in Fig. 8? Please clarify.

Fig. 11, I recommend adding El Niño and La Niña labels to panels (b) and (c).

Fig. 11: Although the Aleutian Low is shown, its role is not sufficiently discussed. Please elaborate on its mechanism and relevance in the text.

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Reviewer #1: No

Reviewer #2: No

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Characterizing compound physical and biogeochemical extremes in the California Current Large Marine Ecosystem

PCLM-D-25-00150R1

Dear Dr. Freeman,

We are pleased to inform you that your manuscript 'Characterizing compound physical and biogeochemical extremes in the California Current Large Marine Ecosystem' has been provisionally accepted for publication in PLOS Climate.

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Best regards,

Frédéric Cyr

Academic Editor

PLOS Climate

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Additional Editor Comments (if provided):

Thanks for the careful review. The paper is well written and the figures are of great quality. I am happy to accept it as is. Congratulations.

Reviewer Comments (if any, and for reference):