the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 23 Jul 2025
Emerging Climate Signals in Oxygen Minimum Zones
Abstract. The ocean is losing oxygen due to anthropogenic climate change. This loss is particularly worrying when it occurs in naturally low-oxygen regions, such as the Oxygen Minimum Zones (OMZs) found at mid-depth in tropical oceans, because the expansion of OMZs reduces habitable space for marine life and threatens oxygen-dependent ecosystems. However, detecting the emergence of climate-driven signals is challenging due to internal variability. Here, we isolate externally forced signals of OMZ volume change and regional deoxygenation, and determine their time of emergence using the IPSL-CM6A-LR Large Ensemble. We apply time of emergence analysis to identify when climate-driven signals become statistically distinguishable from natural variability. Our results show that OMZ edges consistently expand, with emergence occurring in the second half of the 20th century, which is in phase with regional mean deoxygenation in the tropical Pacific and tropical Atlantic. In contrast, we reveal a marked spatial asymmetry in the emergence of OMZ core and hypoxic volumes between the northern and southern parts of OMZs. While OMZ core volumes in the tropical North Pacific and hypoxic volumes in the tropical North Atlantic expand, their southern counterparts contract due to a sudden, ventilation-driven oxygen increase from the Southern Ocean at the start of the 21st century. Uncertainties in emergence timing range from 20 to 30 years across ensemble members, and increase substantially in regions influenced by abrupt changes in OMZ ventilation. By linking the emergence of regional deoxygenation to that of OMZ volume changes, climate-driven expansions of OMZ volumes are likely already beginning to emerge, with distinct dynamics between northern and southern tropical oceans.
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Status: final response (author comments only)
- RC1: 'Comment on egusphere-2025-2805', Anonymous Referee #1, 14 Sep 2025
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CC1: 'Comment on egusphere-2025-2805', Sam J. Ditkovsky, 14 Oct 2025
Dear authors,
Thank you for the interesting study. It’s great to see that the oxygen water mass framework is inspiring new work. However, there are some key ways in which the methodology in Section 2.3 of this manuscript differs from that of Ditkovsky and Resplandy (2025; hereafter D&R25) regarding regional bias correction. That is not to say that the methods of the present study are incorrect in any way, but they perhaps need justifications beyond references to D&R25.
Key differences in the methods of the present study compared to D&R25 are the regional constraints on OMZs and the direct mapping of simulated oxygen concentrations onto observed concentrations. (1) In D&R25, the oxygen-percentile framework is applied to the global ocean, but in the present study it is applied to local OMZ regions. When the oxygen-percentile framework is applied to constrained regions with open horizontal and vertical boundaries rather than to the global ocean, some key generalities are lost. For example, one can no longer assume that mixing-driven and redistribution-driven changes will sum to zero. (2) The present study goes beyond the scope of D&R25 by directly mapping modeled oxygen concentrations onto observed concentrations. This is a salient extension of the methodology, especially given the extreme biases in the IPSL model that are being corrected. Given both of these considerations, the approach of the present study feels much closer to a quantile-mapping bias correction (as used in e.g. precipitation studies) rather than a water mass framework. The authors may want to consult some papers dedicated to these approaches (e.g. Cannon et al., 2015, Journal of Climate) for some variations on this approach and the strengths and weaknesses of such approaches.
In my interpretation, an oxygen-based bias correction is sound if the biases in the IPSL model can be attributed primarily to biogeochemical processes, rather than physical processes. An oxygen-based coordinate system is meant to capture the behavior of physical pathways, taking advantage of the fact that remineralization differentiates waters along ventilation pathways. If the model simulates realistic ventilation pathways, then even unrealistic remineralization rates will differentiate waters along that pathway in oxygen-percentile space. However, if the model does not simulate realistic ventilation pathways– for example, if there is no effective shadow zone in the model– then the mapping of simulated regimes onto observations seems to me to have little physical significance. So, if the authors can support the claim that the oxygen biases in the IPSL model come primarily from biogeochemical processes, I believe that would strengthen the study significantly. One approach to this (but certainly not the only) could be evaluating temperature and salinity distributions in oxygen-percentile space for each region.
As a final note, the terms “geographic-space” and “ventilation-space” coined in D&R25 seem to be used incorrectly in the methods (Section 2.3) of the present study. To clarify, a result in geographic space is any distribution which uses latitude, longitude and depth as coordinates. So, both oxygen and oxygen-percentiles are represented in geographic space when shown on a map or section. Meanwhile, a result in ventilation-space is any distribution that uses an oxygen-based (or some other non-conservative) tracer coordinate. The frequency distribution of oxygen concentration values (as used in Busecke et al. 2022) and the oxygen-percentile relation are both examples of distributions in ventilation space.
I hope this is helpful, and good luck with the manuscript!
Best wishes,
Sam Ditkovsky
Cannon, A. J., Sobie, S. R., & Murdock, T. Q. (2015). Bias correction of GCM precipitation by quantile mapping: how well do methods preserve changes in quantiles and extremes?. Journal of Climate, 28(17), 6938-6959.
Citation: https://doi.org/10.5194/egusphere-2025-2805-CC1 -
RC2: 'Comment on egusphere-2025-2805', Anonymous Referee #2, 15 Jan 2026
First let me congratulate the authors on a generally well-written paper. The are a few minor quirks with the English, but on the whole it is very good.
I have a few main conceptual points and long list of minor quibbles.
(1) The subarctic Pacific (SAP) is largely ignored, and possibly the title and Abstract should be altered to better reflect the exclusive focus on tropical and subtropical latitudes. There is a strong OMZ in the subarctic Pacific, which generally ESMs do a poor job of reproducing (Figure 2). It is ignored in the subsequent analysis (Figure 5), yet the casual reader of the title and Abstract could easily infer that the analysis is global. In Figure 5, data outside the boxes are included for the observations but not the model, and the caption does not explain why or make any mention of it.
(2) The exact method or criterion for choosing variable intervals along the piControl is not explained (118). The interval varies between 20 and 40 years and there is no explanation of how the specific years were chosen. Possibly this is explained in Bonnet et al 2021, but a brief summary of the core conceptual approach is warranted here.
(3) 20 umol/kg seems high for a choice of threshold to define the OMZ core, and is similarly not explained. I think the assertion that nitrous oxide is produced in <20 uM is a misreading of Ji et al.. Possibly there is some enhancement of N2O production from nitrification in this concentration range, but there is little or no denitrification above 6 uM (e.g., Devol 2008, 10.1016/B978-0-12-372522-6.00006-2). I think that almost all of the net N2O production in OMZs occurs below 6 uM.
(4) I often counsel authors to try alternate methods of data presentation such as histograms or scatterplots, rather than relying solely on visual comparison of colour maps. This seems like an obvious case. A statement like "Across all regions, the IPSL simulations systematically overestimate oxygen concentrations compared to observations" (190-191) seems made for such an analysis. If some histograms and/or scatterplots were included as Supplementary figures the reader could more easily and quantitatively evaluate this bias. (Note also that this passage makes no mention of the SAP.)
On 344 we have "Internal variability of OMZ volumes remains stable throughout the SSP2-4.5 scenario (Figure 6)" This may be true but it's not clear that the reader can verify it from the figure. Another possible example of where additional Supplementary figures with different data-presentation approaches could be useful.
(5) I was also surprised not to see any reference to the data product of Kwiecinski and Babbin (10.1029/2021GB007001). I think this is not a 'gridded' data product in the sense that WOA uses optimal interpolation to produce a continuous field, but it does address some of the deficiencies of the WOA in the ETNP and ETSP (see 150-151). I don't want to call for major new analysis at this stage, but it would nice if it were at least mentioned in the Discussion (e.g., what are the implications of alleviation of these biases for the paper's conclusions?)
(6) The methodology for emergence of individual ensemble members is not really explained (Figure 9). Because the ensemble mean has fairly low internal variability, its emergence from the envelope is usually monotonic. But individual ensemble members are more likely to meander in and out of the envelope for some time, and the text should clearly state whether it is the first or the last crossing of the envelope upper/lower bound that is recorded as ToE. Also on 456-457 it states that "They are members with either stronger forced trends or lower internal variability". Does this make sense? I think it does not. Why would one ensemble member have a stronger forced signal than another? Is not the whole premise of the experiment that the forced signal is common to all members? Some reconsideration of the wording is warranted here. Also what does it mean to say that the ensemble mean 'underestimates' ToE vs the median (460)? How do we know which is 'right'? Wouldn't it be better to just say that one is generally earlier/later?
(7) The Conclusion states that "extratropical ventilation pathways play a key role in maintaining oxygen levels". But this paper does not directly address ventilation pathways or mechanisms. It examines regional mean values of AOU and water age to infer that the ventilation pathways or mechanisms identified in some of the cited literature are important. There's nothing wrong with this but I would suggest that the Abstract and Conclusion be reworded to reflect what was actually done. Other than this the Discussion and Conclusions are generally good, although I find section 4.3 a bit vague and confusing (what does 'dispersion' mean in this context?), and 4.5 a bit repetitive. A little effort here could makes these both shorter and clearer.
Terminology/formatting
There are a few quirks about the way that numbers and units are represented, like not leaving a space between a number and its unit (e.g., 1000m), or using a . instead of space (e.g., mol.kg-1). Both of these occur numerous times.
O2 and AOU are usually italicized and should not be.
The sign convention is confusing. AOU is usually expressed as positive, i.e., if I say that AOU is 40 uM it is implicitly understood that O2sat>O2. There are numerous places in this MS where e.g., AOU is said to have decreased by -X uM (e.g., 407-410), which really means that it increased by X uM, which is the opposite of what is intended (see also e.g., 206-207, 226, 229, 381-382, 442-445, 449).
The number of significant figures is sometimes inconsistent and sometimes excessive. Generally it is good practice to use a consistent number of sf rather than of decimal places (e.g., 2.5+/-0.90). For example, on 225-229 some numbers have 2sf, some 3 and some 4. I think 2 is all that is justified by the actual precision of the data. On e.g., 291 the number of sf seems excessive. On 320-342 most numbers have 3sf, some have 2. I think 2 is all that is really justified or necessary here. Same for 381-386 and 408-415. On 426-449 some numbers have 4sf and some 3. 3 might be justifiable but 2 is probably adequate, and most of these numbers don't really require scientific notation, e.g., one could write 32.57e-1 as 3.3 or 3.78e-2 as 0.038.
There are quite a lot of cases where an unnecessary 's' is tacked onto a word (e.g., confuses singular and plural subject), or where there should be an 's' but it is missing (e.g., 55, 219, 295, 364, 365, 370, 373, 513, 526, 562).
There are quite a few "microparagraphs" consisting of single sentence (e.g., 237, 283, 362). Consider joining these to the preceding paragraph.
There are several figures where the subpanels are numbered abcde… but these are not mentioned in the caption (Figs 4, 5, 6, 7, 8, 9). The subpanels are referenced in the text, and in general there is no ambiguity. I don't know if this journal has a policy on this, but generally if you are going to number the subplots it is usual to define the labels in the caption.
The phrase, "In contrast" appears 13 times. I prefer, "By contrast", but I question whether all of the uses of this phrase are necessary at all. On 96 for example, I think it could be deleted without losing anything important. I would recommend to excise as many as possible.
On 382 for example, we have "These losses are stronger than those observed in their southern counterparts" I find this terminology vague and I think it would be easier on the reader to say e.g., "These losses are greater in the northern hemisphere OMZs than in the southern hemisphere OMZs of the same basin". Large gain in clarity for small number of extra words. (see also 389, 429, 485, 493, 523, 525).
On 178 and elsewhere: don't use 'significant' as a generalized term of emphasis (12 total occurrences; not all are necessarily inappropriate but some are)
Some details
15 "likely already beginning" is this consistent with IPCC-approved use of "likely" or more colloquial?
24-25 I am supportive of citing seminal historical papers like Luyten. The ur-reference on this topic is Wyrtki 1962 (Deep Sea Res 9: 11), which could possibly be included here.
35 change "severity" to "intensity"
42 "parameterization" misspelled
44 delete "the" before "Phase 5"
72 change "extract" to "identify"
105, 109 change "oceanic" to "ocean"
110 change "at 1/3" to "to 1/3"
123 change "then" to "and"
125 and "ensemble" after "IPSL-CM6A-LR"
127 and elsewhere "inter-member" strikes me as an unnecessary jargon term that could be avoided (~20 total occurrences). " Large Ensemble inter-member mean" could just be "Large Ensemble mean" and "the inter-member spread" could be "the spread among members".
182-183 change "the IPSL Hypoxic waters account for only 10% of the observed Hypoxic waters" to "in the IPSL model the volume of Hypoxic water is only 10% of that in the observational data product"
184-186 this sort of speculation properly belongs in the Discussion
189 "it fails" unclear antecedent
193-194 "CMIP6 models fail to capture OMZ core waters in the North Indian Ocean, with an oxygen minimum in the Arabian Sea rather than in the Bay of Bengal" appears to have the place names reversed
203 and elsewhere "Storch and Swiers 1999" both authors' names are misspelled. Zwiers is spelled with a "Z" and von Storch's surname is "von Storch"
203-204 "A 90% confidence interval is applied, accounting for the reduction in degrees of freedom" A vague and essentially meaningless statement. Reword and explain clearly what methods were used and what assumptions were made.
221 "fail to reproduce the observed dipole" "dipole is a jargony word that is probably unnecessary, and it isn't really clear what it refers to here
249 Equation (2): The text below the integral signs is not a limit of integration as is usual practice. It might be better to just drop the equation and explain this in words: V is the total volume of water with O2<O2*. I don't think the equation adds much value.
255 change "29st" to "29th" (I think French does not have this unfortunate quirk, it's just 'e' across the board)
269 should this specify OMZ core? (Figure 2)
297, 378, 488 "dependent" misspelled (https://www.grammarly.com/commonly-confused-words/dependant-vs-dependent)
297 "prevents underestimating variability" vague; reword
309 add "with" after "along"
321 "the tropical North Pacific shows the fastest growth" I'm not sure this is a meaningful comparison, as the volumes in the preindustrial climate vary. If it is the largest to begin with, does comparing rates of expansion in m^3/y really indicate the "fastest growth"?
333 add "about" before "2004"? I'm not sure this number is known this precisely. (see also 422)
349 "OMZ Core volumes being smaller" not clear if this refers to the mean or the variance
351 "relative variability of North Atlantic Hypoxic volume is the highest across all regions" appears to contradict the statement on 477 that "earlier emergence is not due to faster expansion, but rather to lower internal variability of the tropical Atlantic Low-oxygen volumes"; possibly these do not refer to the same [O2] ranges
363 , should be a ; (after "simulation")
375 elsewhere O2sat does not have a comma
381 "dissolved" misspelled
392 missing . after parenthesis
398 "increases in the tropical South Pacific and the tropical South Pacific" ???
400 "decrease signal" is another unnecessary bit of jargon where clarity could be increased with only a small amount of rewording. For example, what if they reworded "In all regions, the O2sat decrease signal emerges after the deoxygenation signal (Figure 7, Table 3). These signal decline gradually until 2000, after which it accelerates sharply" as "In all regions, O2sat emerges after deoxygenation (Figure 7, Table 3). Both O2 and O2sat decline gradually until about 2000, after which the decline accelerates sharply"
402 "the decline rate increases by a factor 10" Another assertion that is probably true but the reader can not necessarily verify from the plot. Possibly include a Supplemental table that would present the actual statistics.
404 and elsewhere I'm not sure any of the uses of "inflection point" (or "inflections" on 465) are valid or necessary. This term has a specific meaning in mathematics, i.e., the point at which d2X/dt2 changes sign (e.g., if supralinear growth become sublinear)
431 Figure 8 b, d, e should be d, h, j?
488-490 "Moreover, Northern Hemisphere ecosystems may face earlier and more intense disruptions from expanding low-oxygen volumes, while Southern Hemisphere systems may experience more gradual but persistent changes." Vague; reword
499 delete "such as the future ocean,"
500 delete "through"
501 "via the Agulhas Current" could use a literature ref
504 change "show" to "shown"
505 delete "showed"
507 add "Ocean" after "Indian"
508 add "model" after "IPSL"
512 according to the title, Schmidt et al., 2021 is about CMIP5 models. I'm not sure this is useful as a basis for generalizing about the present ensemble. Some CMIP5 models (notably IPSL-CM5A-LR) had very coarse resolution (ORCA2) and very weak ventilation of some intermediate ocean areas. In my experience the difference between ORCA2 and ORCA1 in terms of ventilation processes is large.
608 Code availability section not completed
660 Ditkovsky reference incomplete
Figure 1 caption delete "millennial polynomial"
Figure 3 the boxes referenced in the caption are not visible on the maps
Table 1 why not show values for the WOA as well?
Figure 6 consider using different y axes for different panels; also "The time of emergence …b occurs when (solid line) the ensemble mean exceeds (coloured area) the standard deviation over the Large Ensemble" may be true but it's not clear how the reader verifies this, as the envelope of preindustrial variability is not shown on the plots.
Table 2 change "has not emerged from twice the ensemble standard deviation" to "has not emerged at the level of twice the ensemble standard deviation"; change "absence of OMZ volume" to "absence of OMZ core"
Table 3 2986 should be 1986?
Figure 8 caption seems to imply that the criterion for emergence is 1sd rather than 2
Figure 10 For the North Atlantic, the dark blue star is not visible. The text states that O2 emerges before OMZ volume (521) but it appears they are concurrent in this case.
Citation: https://doi.org/10.5194/egusphere-2025-2805-RC2
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The manuscript "Emerging Climate Signals in Oxygen Minimum Zones" by Delteil et al provides an a comprehensive analysis of oxygen minimum zones across the worlds oceans within the IPSL large ensemble including both spatial and volumetric metrics, correlating mechanistic drivers such as solubility, nutrients, and age and multiple metrics of time of emergence of change. It is well written with only a few points requiring attention specified below. I recommend publication after attention to these technical issues.
Specific comments:
24 - This is also true in regions where temperature is high reducing solubility