PCLM-D-24-00289

Marine cold-spells in the California Current System: Modeling changes in frequency and impacts on endangered species habitat

PLOS Climate

Dear Dr. Frazer,

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.

Two main aspects justify, in my view, the decision for a major review. First, and among other things, assessing performance of the numerical simulations (both the reanalysis and the projections) seems crucial to this study. I would ask you to carefully take into account the comments from Reviewer 2 regarding this aspect. If you disagree, or cannot perform all the suggestions formulated, please acknowledge the limitations of your study and provide clear justifications for not doing those changes. Second, both reviewers noted that the manuscript could be better organized, and the background information better communicated. Please take into account these comments and consider a reorganization of your manuscript. Finally, do not forget to address all other comments provided by the reviewers.

Please submit your revised manuscript by May 17 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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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?>

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

Reviewer #1: No

Reviewer #2: Yes

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

Reviewer #1: Yes

Reviewer #2: Yes

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Reviewer #1: This work seeks to detect potential expansion of thermal refugia created by marine cold-spells for endangered species in marine protected areas of the California Current System. The authors use SST to detect marine cold spells, then use species distribution models to identify potential habitat changes through the 21st century. This work highlights the importance of eastern boundary upwelling systems as thermal refugia, which may mitigate climate driven warming trends in the future.

Major comments:

Background information could be organized more logically to include the more detailed description about target species (currently in the methods) in the introduction, and more detailed study area (eg lines 85-90) in the methods.

The concept of shifting baselines and rationale for presenting analyses of fixed baseline and detrended MCSs should be included in the introduction. How and why should the two methods differ, and why would it be useful to do both? This seems to be a central focus of the paper and should be more clearly justified.

Clarify hypotheses about anticipated effects of MCS on whales. You've mentioned this for turtles on line 139-142, do you expect impacts on whales to be similar?

Brief details of the established SDMs would be helpful for readers unfamiliar with those efforts. Were habitat covariates useful to fit these models or were SDMs strongly driven by spatial and temporal variation? For example, was seasonality included in the turtle model as it was in the whale model?

Regarding the statement that detrended MCSs are projected to represent warmer SSTs in the far future (line 457): given greater amplitude and variability of cold AND warm temperatures in the future, do you anticipate that warm conditions will overwhelm this signal?

I may have missed it, but on lines 480-481 you mention that SST in the far future may be warmer than the suitable range for leatherback turtles. Has a similar analysis been done for MHWs? It seems like MHWs would have a much greater impact on future habitat than MCS.

Minor comments:

line 70 - clarify why changes in location and intensity of MCSs is uncertain?

lines 85-90: consider moving this detailed study area information to the methods

line 120: provide a citation for the thermoregulatory capabilities of these species

line 125: provide a citation for these datasets

line 151: provide a justification for using RCP8.5 but not others. Why is this the best/only choice presented?

line 191 "While many environmental conditions contribute to SDMs...": does this mean other covariates used in the established SDMs should be used in this analysis too?

line 229: use of 10th percentile is not mentioned in the your MCS definition on lines 208-211

line 231 "Hereafter...": you have mentioned fixed baseline earlier in the manuscript, you should define it upon first mention.

line 254-255: help the reader interpret what these habitat thresholds mean, are 0.503 and 0.502 biologically meaningful?

line 292: you focused on Monterey Bay - is it representative of other locations?

Fig 6: Consider inverting the color gradient from cool to warm colors (low to high values) for conventional purposes

line 457-459: clarify this sentence

line 469: replace "less strong" with "weaker"

line 493 "these evens diverge greatle in frequency...": compared to what?

line 501: replace "be providing" with "provide"

line 511 "GAM may provide more linear predictions": clarify what you mean by this, GAM is used to predict non-linear responses

line 515: clarify what "species memory" is

Reviewer #2: Review of Frazer et al. (PCLM-D-24-00289)

General Comments

This study examines the potential role that marine cold spells (MCSs) may play in creating thermal refugia for blue whales and leatherback turtles off the coast of California under future climate change. The authors provide a number of comparisons in their study: three different forcing conditions, two definitions for MCSs, gridded regional analyses vs averages over National Marine Sanctuaries. I think focussing on the potential action of MCSs as climate refugia is interesting and unique, but I find some of the comparisons don’t add a lot and make the discussion more complicated. My biggest concern is that the results are entirely predicated on accurately representing both the recent state of conditions in this location as well as their changes under climate change, but the manuscript currently provides no evaluation of the model data products being used in this study. As a result, I find it impossible to assess how reliable their predictions, and therefore their conclusions, are.

I have a few recommendations for the authors. First, they use two different types of model products. The first is a historical regional reanalysis that has been previously published. It would be helpful if the authors summarize the performance of the reanalysis over their region. In particular, describing how well the model represents sub-surface temperatures is key, since they argue that marine cold spells in this region predominantly correspond with upwelling events. If the sub-surface temperature field isn’t represented well, then the upwelled conditions won’t be either.

Second, applying new forcings and omitting data assimilation from the regional model in order to make future predictions results in a different model configuration, whose biases will not correspond to those of the reanalysis. Fortunately, they run these simulations from 2010 onwards, so there is the opportunity to evaluate the new model products against data over the intervening years (2010-now). Identifying biases over this period will help to establish confidence in the ability of these new model products to realistically represent conditions close to those similar to present day and highlight potential areas of mismatch. There’s not much that can be done to evaluate future predictions, but it is nice that the authors picked 3 CMIP5 simulations that span the range of the multi-model ensemble.

Finally, it feels like the logic becomes somewhat convoluted in places in the manuscript. However, I think the results can be presented more clearly. Given the discussion points primarily to temperature being the dominant control on habitat suitability, I think the crux of the results is this: in regions where mean conditions are on the warm side relative to the species’ preferred habitats, MCSs may provide relief and increase habitability. In places where mean conditions are on the cold side relative to their preferred habitats, MCSs will reduce the habitability. Thus, a simpler way to organize the results would be to present the habitat suitability for mean conditions at present and in the future to establish where too-warm temperatures are affecting habitability and then show the spatial distribution of MCSs during these two periods to see whether they overlap. These results can be confirmed by running the habitat models under these MCS conditions as presented currently.

Note: I’m unable to evaluate the habitat suitability modelling, as that’s outside my area of expertise.

Additional comments below.

Major Comments

Line 99-104 and 141-142 Are all MCSs in the region associated with upwelling anomalies? How much is known about MCSs in this region or in similar locations? Currently the Introduction describes the Eastern Boundary Upswelling System and how it is anticipated to be affected by climate change, but there isn’t an introduction to MCSs in the region and their characteristics.

I understand from this manuscript that the calculation of MCS was based on the method in Schlegel et al (2021), who based their definition on the converse of the definitions of oceanic MHWs in Hobday et al (2016 and 2018), among other sources. The description of this calculation is not presented very clearly here. I’m aware that Hobday’s method includes smoothing of the percentiles, etc. Were these methods applied here?

Line 215-216 and 233-234 I don’t understand from these descriptions how the detrending was performed. Regressions are mentioned, but what SSTs are regressed against is not clear. An equation would be helpful.

Lines 226-241 I see there being two methodological questions here: what is the impact of the changing background conditions to the identification of MCSs (which could be identified by just calculating MCSs relative to a sliding window of conditions over the prior 3 decades), and what is the impact of background climate conditions changing during the 30yr window over which climate variability is assessed (where detrending would apply). The relevance of the latter question depends on how important these sub-30yr trends are to conditions in this region. As to the former, I don’t really see the value of applying the criteria based on the fixed baseline period to future years unless the habitable temperature ranges for these species are limited to these conditions. You’ll just get the expected result that the number and duration of MCSs decrease when background conditions warm. It doesn’t feel like it adds much to the discussion here, and it makes the argument of the paper much more complicated. If the authors want to check this result, that is reasonable as presented in Section “Implications for oceanography and ecology”, but then I suggest leaving it out of the other sections and focusing on applying the WMO definition with sliding windows for the remainder of the discussion.

Figure 3B’s color bar covers a much smaller range than is used for the other plots. In fact, it’s so small that variations that are highlighted here as significant would be indistinguishable in the other sub-plots. Are these results really significant relative to the uncertainties at this level, or is Figure 3B just showing noise?

While this is outside my area of expertise, it would be nice to see some spatial comparisons between where blue whales and leatherback turtle habitat is modelled to be optimal in the historical period against the tagging data mentioned.

Minor Comments

Line 86 Olympic Coast NMS is not plotted in Figure 1 or included in Table 1. I suggest it just not be mentioned and instead say the focus are NMSs off the coast of California.

Line 100 I started losing track of all of the acronyms in this paper. I suggest the ones that get used a lot (e.g. CCS, MCS,…) be kept, but for others like EBUS that are only used a few times or in one place, they be avoided.

Line 166 I didn’t understand this description

Line 172-173 more precise description of these results would be helpful => did they “agree” in all locations and conditions and based on what criteria?

Line 192 “relatively high importance” is not very informative. What % of explained variance did you get from SST and bathymetry?

Line 208 I don’t understand why the threshold is not articulated here. In both cases of fixed baseline and detrended baseline cases, the threshold was set at the 10th percentile, which is a standard for marine cold spells.

Line 255 Are these thresholds really significant to the third digit? Is a value of 0.503 actually different than 0.502?

Line 258 I find the description “typical condition” to be very uninformative, and it’s not defined until the next paragraph. Why not just say the monthly average over the decade?

Line 270 Table S2 is provided without caption, which makes it hard to interpret by a non-expert.

Line 329-330 why not use the language used on the plot? Mean MCS duration in days

Line 346 typo: says “Fig 4” instead of “Fig 5”

Figure 5 it’s hard to see the red against some of the background colors. Could another color be used? Perhaps white?

Line 386 typo “mixed responses”

Figure 1 Suggestion: including the bathymetry on this plot, and removing it from Figure 3, which is much busier

Figure 3: I found it hard to estimate the values associated with the colors in the plot. Perhaps a discrete color bar would be clearer?

Figure 4: missing units on Mean Temp anomaly color bar label

Figure 7: I found it difficult to figure out what was going on in this figure, and the lack of label on the y axes didn’t help. For example, the black lines represent response curves, but the legend label says they represent SSTs. The temperature distributions would have a different variable on the y axis than response curves. Perhaps the figure would be more interpretable if the SST distributions were flattened and attached to the plot along the bottom or top? (E.g. something like this https://hossainlab.github.io/dataviz/notebooks/SB01-Distribution%20Plots.html) Also, note that the x axis doesn’t have any units.

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Reviewer #1: No

Reviewer #2: No

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PCLM-D-24-00289R1

Marine cold-spells in the California Current System: Modeling changes in frequency and impacts on endangered species habitat

PLOS Climate

Dear Dr. Frazer,

Thank you for submitting your manuscript to PLOS Climate. After careful consideration, one Reviewer still has minor comments that should be easily addressed Therefore, we invite you to submit a revised version of the manuscript that addresses the points raised during the review process.

Please submit your revised manuscript by Nov 15 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.

Please include the following items when submitting your revised manuscript:

• A rebuttal letter that responds to each point raised by the 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'.

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

Journal Requirements:

If the reviewer comments include a recommendation to cite specific previously published works, please review and evaluate these publications to determine whether they are relevant and should be cited. There is no requirement to cite these works unless the editor has indicated otherwise. 

Please review your reference list to ensure that it is complete and correct. If you have cited papers that have been retracted, please include the rationale for doing so in the manuscript text, or remove these references and replace them with relevant current references. Any changes to the reference list should be mentioned in the rebuttal letter that accompanies your revised manuscript. If you need to cite a retracted article, indicate the article’s retracted status in the References list and also include a citation and full reference for the retraction notice.

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Reviewers' comments:

Reviewer's Responses to Questions

Comments to the Author

Reviewer #2: All comments have been addressed

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publication criteria?>

Reviewer #2: Yes

**********

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

Reviewer #2: Yes

**********

4. 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

Reviewer #2: Yes

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

Reviewer #2: Yes

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Reviewer #2: Just a few remaining minor comments. Line numbers are based on draft without track changes.

Line 105 first use of SDM needs definition

Fig.1 Why is Cordell Bank not shown on Fig 1 if it’s included in this study?

Table 1 there is strange formatting occurring in this table, with horizontal lines between some rows and not others

Line 264-265 AUC and TSS not defined

Line 390-391 and Figure 4b to what degree is multi-decadal variability affecting the estimation of the trend in detrended cold-spell days? It looks like there isn’t much change except for a particularly strong decade in the 2080s.

Line 398 typo: Fig 4 should be Fig 5

Line 409 missing closing parenthesis

Line 443-444 unclear what standard deviation is evaluated over here => is it the standard deviation obtained when average over daily habitat values to create decadal means?

Line 499 is this meant to be a reference to Fig 3?

Line 563 didn’t notice that this point was made elsewhere in the paper, so present tense may be more appropriate here

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Reviewer #2: No

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Marine cold-spells in the California Current System: Modeling changes in frequency and impacts on endangered species habitat

PCLM-D-24-00289R2

Dear Ms. Frazer,

We are pleased to inform you that your manuscript 'Marine cold-spells in the California Current System: Modeling changes in frequency and impacts on endangered species habitat' has been provisionally accepted for publication in PLOS Climate.

Before your manuscript can be formally accepted you will need to complete some formatting changes, which you will receive in a follow-up email from a member of our team. 

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Thank you again for supporting Open Access publishing; we are looking forward to publishing your work in PLOS Climate.

Best regards,

Frédéric Cyr

Academic Editor

PLOS Climate

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Additional Editor Comments (if provided):

Reviewer Comments (if any, and for reference):