Linear trends of temperature, salinity, and oxygen in the North Pacific based on GOBAI-O<sub>2</sub> over 2 decades and their controlling factors

Ishizu, Miho; Ogata, Tomomichi

doi:10.5194/egusphere-2025-2746

Preprints

https://doi.org/10.5194/egusphere-2025-2746

Preprints

28 Jul 2025

| 28 Jul 2025

Linear trends of temperature, salinity, and oxygen in the North Pacific based on GOBAI-O₂ over 2 decades and their controlling factors

Miho Ishizu and Tomomichi Ogata

Abstract. Oxygen concentrations in the ocean are believed to be declining due to global warming. However, our understanding of its variability remains limited compared to physical parameters such as temperature and salinity, because oxygen is difficult to observe with high spatial and temporal resolution. In this study, we analyzed linear trends in potential temperature, salinity, and dissolved oxygen in the North Pacific over the past two decades (2004–2023), using the GOBAI-O₂ data. We then compared oxygen trends with physical parameters to investigate the spatial pattern of linear changes across the region. The oxygen trends derived from GOBAI-O₂ were consistent with those observed along hydrographic lines that have been relatively frequently and continuously surveyed by ship-based observations. Although an overall declining trend in dissolved oxygen was evident, localized increases were observed in certain density layers. By examining the associated physical conditions, we found that the spatial heterogeneity of the oxygen trends can be attributed to known oceanographic processes, such as the southward retreat of the oxygen minimum layer and the northward migration of a recently identified front separating the subtropical and subarctic gyres. Our findings highlight the utility of GOBAI-O₂ data in linking physical changes to previously unrecognized biological and biogeochemical patterns in the ocean.

Received: 10 Jun 2025 – Discussion started: 28 Jul 2025

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Journal article(s) based on this preprint

22 Jun 2026

Revealing hidden oxygen variability in the North Pacific: a two-decade analysis using GOBAI-O₂

Miho Ishizu and Tomomichi Ogata

Biogeosciences, 23, 4037–4056, https://doi.org/10.5194/bg-23-4037-2026,https://doi.org/10.5194/bg-23-4037-2026, 2026

Short summary

Miho Ishizu and Tomomichi Ogata

Interactive discussion

Status: closed

RC1:
'Comment on egusphere-2025-2746', Anonymous Referee #1, 06 Aug 2025

This paper investigated the linear trend of oxygen concentration in the North Pacific and their relation to the circulation change. Deoxidization is an important issue not only for our scientific community but also for the broader society. Thus, many papers tackled in the issue (ex. Ito et al. 2017, 2024; Kolodziejczyk et al. 2024). However, in the manuscript, there is insufficient discussion to clarify the relationship with those previous studies. Furthermore, uncertainty is also inadequately discussed. Spatial distributions of uncertainty in mapped oxygen and that in linear regression are given, but there is no description how these uncertainties affect the results. Since oxygen concentration in GOBAI-O2 are estimated by using temperature and salinity information, uncertainties need to be fully investigated before discussing physical implications.

Minor comments: Line 364, "areal areas"?

Citation: https://doi.org/10.5194/egusphere-2025-2746-RC1
- AC1: 'Reply on RC1', Miho Ishizu, 05 Sep 2025
  
  Thank you for your comments. We will consider your comments and reflect them in the revised manuscript.
  
  Citation: https://doi.org/10.5194/egusphere-2025-2746-AC1
RC2:
'Comment on egusphere-2025-2746', Anonymous Referee #2, 29 Sep 2025

The manuscript examines two decades of linear trends in temperature, salinity, and dissolved oxygen in the North Pacific using the GOBAI-O2 dataset. This timely topic addresses the significant issue of ocean deoxygenation, and the emergence of biogeochemical Argo products offers new avenues for basin-scale analysis. The results align with previous ship-based observations, while also highlighting spatial heterogeneity in oxygen trends attributed to circulation changes like the retreat of the oxygen minimum layer and the northward migration of the subtropical–subarctic front.
The study is potentially valuable, but the manuscript in its present form does not sufficiently establish what is new compared to earlier syntheses. The discussion of previous work, notably Ito et al. (2017, 2024), Sasano et al. (2014), and Kolodziejczyk et al. (2024), is too brief. The paper sometimes reads as a confirmation exercise, whereas it should highlight more clearly what GOBAI-O2 reveals that was inaccessible with earlier datasets.
A more serious concern is the treatment of uncertainty. While fields of uncertainty are presented, their implications for the robustness of the detected trends are not discussed. In several regions the uncertainty is of the same order as the trends themselves. Furthermore, recent work has identified a systematic negative bias in oxygen from air-calibrated BGC-Argo floats relative to shipboard reference profiles, with offsets of roughly –2.7 µmol kg-1 at depth, which propagate into carbonate system estimates such as pCO2 (+3.2 µatm; Bushinsky et al., 2025). Since GOBAI-O2 is trained on these float observations, such biases must be considered explicitly. Without a careful discussion of these uncertainties, the attribution of observed patterns to physical processes remains premature.
On the methodological side, the decision to interpolate GOBAI-O2 fields to 1 m vertical resolution requires justification, as it may artificially smooth variability. The decomposition of oxygen change following equation (2) is interesting but not easy to follow; a clearer derivation or schematic would help. Some of the proposed mechanisms, for example the role of NEC/NECC migration, are plausible but speculative unless supported by independent evidence such as altimetry or reanalyses. Finally, the exclusive focus on linear trends risks obscuring the role of low-frequency climate variability such as the PDO or NPGO.
The language and presentation require significant improvement. Numerous grammatical errors hinder readability, and several figures, especially the large multi-panel maps, are overly dense. A careful edit and improved figure design would considerably enhance clarity.
In conclusion, the dataset is of great interest and the paper has merit, but substantial revision is needed. The authors should better situate their findings relative to previous work, integrate uncertainties and known biases into their interpretation, clarify methodological choices, and moderate or substantiate their physical explanations. With these improvements, the manuscript could become a valuable contribution to our understanding of oxygen variability in the North Pacific.

Citation: https://doi.org/10.5194/egusphere-2025-2746-RC2
- AC2: 'Reply on RC2', Miho Ishizu, 16 Oct 2025
  
  Thank you for your comments. We will consider your comments and reflect them with point-by-point responses in the revised manuscript.
  
  Citation: https://doi.org/10.5194/egusphere-2025-2746-AC2

Interactive discussion

Status: closed

RC1:
'Comment on egusphere-2025-2746', Anonymous Referee #1, 06 Aug 2025

This paper investigated the linear trend of oxygen concentration in the North Pacific and their relation to the circulation change. Deoxidization is an important issue not only for our scientific community but also for the broader society. Thus, many papers tackled in the issue (ex. Ito et al. 2017, 2024; Kolodziejczyk et al. 2024). However, in the manuscript, there is insufficient discussion to clarify the relationship with those previous studies. Furthermore, uncertainty is also inadequately discussed. Spatial distributions of uncertainty in mapped oxygen and that in linear regression are given, but there is no description how these uncertainties affect the results. Since oxygen concentration in GOBAI-O2 are estimated by using temperature and salinity information, uncertainties need to be fully investigated before discussing physical implications.

Minor comments: Line 364, "areal areas"?

Citation: https://doi.org/10.5194/egusphere-2025-2746-RC1
- AC1: 'Reply on RC1', Miho Ishizu, 05 Sep 2025
  
  Thank you for your comments. We will consider your comments and reflect them in the revised manuscript.
  
  Citation: https://doi.org/10.5194/egusphere-2025-2746-AC1
RC2:
'Comment on egusphere-2025-2746', Anonymous Referee #2, 29 Sep 2025

The manuscript examines two decades of linear trends in temperature, salinity, and dissolved oxygen in the North Pacific using the GOBAI-O2 dataset. This timely topic addresses the significant issue of ocean deoxygenation, and the emergence of biogeochemical Argo products offers new avenues for basin-scale analysis. The results align with previous ship-based observations, while also highlighting spatial heterogeneity in oxygen trends attributed to circulation changes like the retreat of the oxygen minimum layer and the northward migration of the subtropical–subarctic front.
The study is potentially valuable, but the manuscript in its present form does not sufficiently establish what is new compared to earlier syntheses. The discussion of previous work, notably Ito et al. (2017, 2024), Sasano et al. (2014), and Kolodziejczyk et al. (2024), is too brief. The paper sometimes reads as a confirmation exercise, whereas it should highlight more clearly what GOBAI-O2 reveals that was inaccessible with earlier datasets.
A more serious concern is the treatment of uncertainty. While fields of uncertainty are presented, their implications for the robustness of the detected trends are not discussed. In several regions the uncertainty is of the same order as the trends themselves. Furthermore, recent work has identified a systematic negative bias in oxygen from air-calibrated BGC-Argo floats relative to shipboard reference profiles, with offsets of roughly –2.7 µmol kg-1 at depth, which propagate into carbonate system estimates such as pCO2 (+3.2 µatm; Bushinsky et al., 2025). Since GOBAI-O2 is trained on these float observations, such biases must be considered explicitly. Without a careful discussion of these uncertainties, the attribution of observed patterns to physical processes remains premature.
On the methodological side, the decision to interpolate GOBAI-O2 fields to 1 m vertical resolution requires justification, as it may artificially smooth variability. The decomposition of oxygen change following equation (2) is interesting but not easy to follow; a clearer derivation or schematic would help. Some of the proposed mechanisms, for example the role of NEC/NECC migration, are plausible but speculative unless supported by independent evidence such as altimetry or reanalyses. Finally, the exclusive focus on linear trends risks obscuring the role of low-frequency climate variability such as the PDO or NPGO.
The language and presentation require significant improvement. Numerous grammatical errors hinder readability, and several figures, especially the large multi-panel maps, are overly dense. A careful edit and improved figure design would considerably enhance clarity.
In conclusion, the dataset is of great interest and the paper has merit, but substantial revision is needed. The authors should better situate their findings relative to previous work, integrate uncertainties and known biases into their interpretation, clarify methodological choices, and moderate or substantiate their physical explanations. With these improvements, the manuscript could become a valuable contribution to our understanding of oxygen variability in the North Pacific.

Citation: https://doi.org/10.5194/egusphere-2025-2746-RC2
- AC2: 'Reply on RC2', Miho Ishizu, 16 Oct 2025
  
  Thank you for your comments. We will consider your comments and reflect them with point-by-point responses in the revised manuscript.
  
  Citation: https://doi.org/10.5194/egusphere-2025-2746-AC2

Peer review completion

AR – Author's response | RR – Referee report | ED – Editor decision | EF – Editorial file upload

ED: Reconsider after major revisions (30 Oct 2025) by Koji Suzuki

AR by Miho Ishizu on behalf of the Authors (24 Nov 2025) Author's response Author's tracked changes Manuscript

ED: Referee Nomination & Report Request started (25 Nov 2025) by Koji Suzuki

RR by Anonymous Referee #2 (25 Nov 2025)

Suggestions for revision or reasons for rejection

The authors have made a genuine effort to address the reviewer concerns and the manuscript is notably improved; however, several key issues remain unresolved, including the absence of a statistical assessment of trend robustness relative to uncertainty, an incomplete treatment of known optode biases, mechanistic interpretations that extend beyond what the data can firmly support, an only partly developed articulation of what is genuinely novel in this work, and persistent language and clarity problems. Overall, I recommend major revision (though this could be reduced to minor if these issues are satisfactorily addressed). The study has strong potential, but it will require more rigorous handling of uncertainty and a more cautious, evidence-based interpretation before it can serve as a reliable contribution to our understanding of North Pacific oxygen variability.

General Assessment
The manuscript is significantly improved. The authors have:
-added extensive comparisons with Ito et al. (2017) and briefly with Ito 2024 and Kolodziejczyk 2024.
-incorporated uncertainty fields, described their origin (Sharp et al., 2023), and added figures showing uncertainty alongside trends.
-added methodological justification for the 1-m vertical interpolation, including sensitivity tests (Figures S1–S2).
-clarified the decomposition of oxygen change and added a schematic.
-expanded discussion of physical mechanisms (northward front migration, OML retreat).
-improved figure presentation in several places.

These are meaningful improvements. However, several issues persist that limit the manuscript’s robustness, especially regarding uncertainty interpretation, sensor bias, and the strength of physical attribution.

Major remaining issues
1. The manuscript now includes uncertainty fields and explains the GOBAI-O2 uncertainty components. However the interpretation of uncertainty relative to trends is still weak. On several figures (e.g., Fig. 1O–U), the magnitude of oxygen trends is comparable to the mean uncertainty, yet the discussion continues to present the spatial patterns as physically meaningful without a formal test of significance. No statistical test (e.g., trend significance vs. local uncertainty, confidence intervals, fdr correction, etc.) is performed. The authors state that “the observed trends are spatially coherent and smoothly connected,” but spatial coherence cannot substitute for uncertainty quantification. The authors need to explicitly state where trends are robust vs. indistinguishable from uncertainty. Even a simple criterion (e.g., |trend| > mean uncertainty) or a bootstrap approach would be acceptable.

2. In the response, the authors acknowledge known negative biases (~–2.7 μmol kg-1) from air-calibrated floats, but in the manuscript this appears in one paragraph with no assessment of its impact on long-term trends, regional patterns (e.g., oxycline-intensifying bias) or comparisons with shipboard observations. Given that GOBAI-O2 is trained directly on such floats, the bias issue cannot simply be acknowledged, it must be integrated into the uncertainty interpretation. Consider adding a subsection explaining how a systematic offset would (or would not) affect the trend estimates, particularly in regions where variability is small. If not quantified, the authors should explicitly state the limitation.

3. The revised manuscript still pushes some mechanistic explanations too far relative to the observational evidence provided.
Examples:
-The link between the tropical OML retreat and NECC weakening/poleward shift (Section 3.3.2) is plausible but not demonstrated. The use of OFES output is too qualitative to support the claims.
-The northward migration of the subtropical–subarctic front is invoked repeatedly, but no independent metric (e.g., SSH fronts, SSS gradients, isopycnal outcrop latitude) is quantitatively analyzed in the manuscript.
The authors should soften language (“suggests”, “is consistent with”, “is likely influenced by”) unless independent evidence is shown in the figures.

4. The comparison with Ito et al. (2017) is improved, but the manuscript still reads at times as confirmation of known patterns rather than revealing new insights emerging uniquely from GOBAI-O2.
Consider adding a short paragraph in the Discussion explicitly stating:
-What patterns are new,
-Why these patterns are only recoverable with BGC-Argo and GOBAI-O2,
-How this contributes to advancing our understanding beyond previous basin-scale syntheses.

5. The new schematic (S5) helps, but the main text remains mathematically dense and could be streamlined. Additionally, several terms include both ∂z/∂t and ∂X/∂z interactions that need clearer physical interpretation.
Consider keeping the decomposition but include a brief physical interpretation table in the main text (e.g., “Term (ii): vertical heave; Term (iii): solubility change”, etc.).

6. The text is improved compared to the first submission but still contains numerous grammatical errors (e.g., inconsistent tense, missing articles, awkward constructions). For example:
-“the magnitudes of the negative latitudes not necessary rely on the latitudes only” (p. 11)
-“shifts horizontally rather than vertically” → ambiguous phrasing
-“observed trends are connected smoothly each other” → needs rewriting
Maybe have professional English editing before acceptance.

Minor Issues
A few figure panels are still crowded (particularly large multipanel figures).
Axis labelling in several uncertainty plots (e.g., Fig. 1v–bb) is small and difficult to read.
Discussion of regional sensor coverage could benefit from a map of float sampling density.

Hide

RR by Anonymous Referee #1 (03 Dec 2025)

ED: Reconsider after major revisions (12 Dec 2025) by Koji Suzuki

Dear Authors,

We have received two reviews of your revised manuscript from the referees who examined the original one. Both still have many concerns, such as the quantitativeness and robustness of your data analyses. Therefore, we encourage you to submit a further revised manuscript with point-by-point responses to the referees' comments.

Thank you for your efforts to improve the manuscript.

Kind regards,

Koji Suzuki
Associate Editor

---
Referee #1
The authors have made a genuine effort to address the reviewer concerns and the manuscript is notably improved; however, several key issues remain unresolved, including the absence of a statistical assessment of trend robustness relative to uncertainty, an incomplete treatment of known optode biases, mechanistic interpretations that extend beyond what the data can firmly support, an only partly developed articulation of what is genuinely novel in this work, and persistent language and clarity problems. Overall, I recommend major revision (though this could be reduced to minor if these issues are satisfactorily addressed). The study has strong potential, but it will require more rigorous handling of uncertainty and a more cautious, evidence-based interpretation before it can serve as a reliable contribution to our understanding of North Pacific oxygen variability.

General Assessment
The manuscript is significantly improved. The authors have:
-added extensive comparisons with Ito et al. (2017) and briefly with Ito 2024 and Kolodziejczyk 2024.
-incorporated uncertainty fields, described their origin (Sharp et al., 2023), and added figures showing uncertainty alongside trends.
-added methodological justification for the 1-m vertical interpolation, including sensitivity tests (Figures S1–S2).
-clarified the decomposition of oxygen change and added a schematic.
-expanded discussion of physical mechanisms (northward front migration, OML retreat).
-improved figure presentation in several places.

These are meaningful improvements. However, several issues persist that limit the manuscript’s robustness, especially regarding uncertainty interpretation, sensor bias, and the strength of physical attribution.

Major remaining issues
1. The manuscript now includes uncertainty fields and explains the GOBAI-O2 uncertainty components. However the interpretation of uncertainty relative to trends is still weak. On several figures (e.g., Fig. 1O–U), the magnitude of oxygen trends is comparable to the mean uncertainty, yet the discussion continues to present the spatial patterns as physically meaningful without a formal test of significance. No statistical test (e.g., trend significance vs. local uncertainty, confidence intervals, fdr correction, etc.) is performed. The authors state that “the observed trends are spatially coherent and smoothly connected,” but spatial coherence cannot substitute for uncertainty quantification. The authors need to explicitly state where trends are robust vs. indistinguishable from uncertainty. Even a simple criterion (e.g., |trend| > mean uncertainty) or a bootstrap approach would be acceptable.

2. In the response, the authors acknowledge known negative biases (~–2.7 μmol kg-1) from air-calibrated floats, but in the manuscript this appears in one paragraph with no assessment of its impact on long-term trends, regional patterns (e.g., oxycline-intensifying bias) or comparisons with shipboard observations. Given that GOBAI-O2 is trained directly on such floats, the bias issue cannot simply be acknowledged, it must be integrated into the uncertainty interpretation. Consider adding a subsection explaining how a systematic offset would (or would not) affect the trend estimates, particularly in regions where variability is small. If not quantified, the authors should explicitly state the limitation.

3. The revised manuscript still pushes some mechanistic explanations too far relative to the observational evidence provided.
Examples:
-The link between the tropical OML retreat and NECC weakening/poleward shift (Section 3.3.2) is plausible but not demonstrated. The use of OFES output is too qualitative to support the claims.
-The northward migration of the subtropical–subarctic front is invoked repeatedly, but no independent metric (e.g., SSH fronts, SSS gradients, isopycnal outcrop latitude) is quantitatively analyzed in the manuscript.
The authors should soften language (“suggests”, “is consistent with”, “is likely influenced by”) unless independent evidence is shown in the figures.

4. The comparison with Ito et al. (2017) is improved, but the manuscript still reads at times as confirmation of known patterns rather than revealing new insights emerging uniquely from GOBAI-O2.
Consider adding a short paragraph in the Discussion explicitly stating:
-What patterns are new,
-Why these patterns are only recoverable with BGC-Argo and GOBAI-O2,
-How this contributes to advancing our understanding beyond previous basin-scale syntheses.

5. The new schematic (S5) helps, but the main text remains mathematically dense and could be streamlined. Additionally, several terms include both ∂z/∂t and ∂X/∂z interactions that need clearer physical interpretation.
Consider keeping the decomposition but include a brief physical interpretation table in the main text (e.g., “Term (ii): vertical heave; Term (iii): solubility change”, etc.).

6. The text is improved compared to the first submission but still contains numerous grammatical errors (e.g., inconsistent tense, missing articles, awkward constructions). For example:
-“the magnitudes of the negative latitudes not necessary rely on the latitudes only” (p. 11)
-“shifts horizontally rather than vertically” → ambiguous phrasing
-“observed trends are connected smoothly each other” → needs rewriting
Maybe have professional English editing before acceptance.

---
Referee #2
While the revised manuscript now includes many figures showing uncertainties, the authors describe only qualitative features. These uncertainties should be quantitatively compared with the magnitude of the trends, and then the signals that exceed the uncertainty should be discussed. Since oxygen concentration in GOBAI-O2 is estimated by using temperature and salinity information, detected signals should be fully evaluated by taking their uncertainties into account before discussing physical implications.

Hide

AR by Miho Ishizu on behalf of the Authors (20 Jan 2026) Author's response Author's tracked changes Manuscript

ED: Referee Nomination & Report Request started (25 Jan 2026) by Koji Suzuki

RR by Anonymous Referee #1 (27 Jan 2026)

RR by Anonymous Referee #2 (06 Feb 2026)

Suggestions for revision or reasons for rejection

The authors clearly engaged with the reviews and implemented substantial changes (uncertainty discussion, significance tests, novelty clarification, softened claims). However, several of the original major concerns are addressed procedurally rather than substantively, and some still limit robustness.

The authors have added statistical significance tests for the trends, which is a welcome and necessary improvement. However, this addition does not fully resolve the central concern regarding the robustness of the diagnosed patterns relative to the underlying dataset uncertainty. The t-tests demonstrate where trends differ significantly from zero, but they do not establish whether the magnitudes of those trends are meaningfully distinguishable from the structural uncertainty in GOBAI-O2. In several regions, the trend magnitude remains comparable to the stated uncertainty, and this issue is not directly quantified. The authors also declined to relate trend magnitude to uncertainty on the grounds of unit mismatch, yet it would be straightforward to convert gridded uncertainty into trend uncertainty through propagation or confidence intervals. As a result, it remains unclear where spatial patterns can be interpreted as physically robust versus where they may be indistinguishable from dataset uncertainty. In addition, the uncertainty fields are described but not propagated into the trend confidence intervals or used to define spatial robustness masks, and some discussion sections still treat statistically significant patterns as physically meaningful without clearly distinguishing between statistical significance, robustness relative to dataset uncertainty, and regions that are primarily ML-inferred rather than directly observed. Overall, this core issue is improved but not fully resolved and remains a matter of scientific robustness.

The treatment of potential sensor bias has also been improved but remains largely qualitative. The revised manuscript now acknowledges the reported negative bias (~–2.7 μmol kg-1) associated with some air-calibrated BGC-Argo floats and notes that this could influence the magnitude and spatial structure of the diagnosed trends. However, this discussion is not accompanied by any quantitative sensitivity analysis, regional assessment, or estimate of how such biases might affect inferred trends. There is also no substantive comparison with shipboard reference sections beyond a general qualitative statement. The authors argue that a quantitative assessment is not feasible and therefore simply note the limitation. While this may be acceptable in some contexts, the manuscript relies heavily on a derived ML product trained on float data, and the lack of quantitative evaluation leaves the treatment methodologically thin. This concern is therefore only partially addressed and does not yet meet the standard typically expected for resolution of a major revision.

Finally, the physical interpretation has been moderated but still at times extends beyond what is directly demonstrated. The authors have softened their language and now frame mechanistic explanations more cautiously, which is appropriate. Nevertheless, several interpretations continue to rely primarily on consistency arguments rather than independent quantitative evidence. The OFES-based discussion remains largely qualitative, and the evidence for frontal migration is limited to the isopycnal outcrop analysis, which is defensible but not especially strong. These interpretations are likely acceptable if presented strictly as hypotheses consistent with the observations rather than as demonstrated mechanisms, but in their current form they remain somewhat borderline. Overall, this aspect is mostly addressed but not yet fully convincing.

Hide

ED: Reconsider after major revisions (12 Feb 2026) by Koji Suzuki

Dear Authors,

We received two reviews of your revised manuscript. The referees and I were pleased to find your improvements, including the addition of statistical significance testing. However, we consider that major issues remain, particularly the quantitative interpretation of the trends and their associated uncertainties (see below), which must be resolved before publication. We therefore encourage you to submit a further revised manuscript in response to the two referees' comments. Thank you.

Kind regards,

Koji Suzuki
Associate Editor

---
Referee #1
The authors clearly engaged with the reviews and implemented substantial changes (uncertainty discussion, significance tests, novelty clarification, softened claims). However, several of the original major concerns are addressed procedurally rather than substantively, and some still limit robustness.

The authors have added statistical significance tests for the trends, which is a welcome and necessary improvement. However, this addition does not fully resolve the central concern regarding the robustness of the diagnosed patterns relative to the underlying dataset uncertainty. The t-tests demonstrate where trends differ significantly from zero, but they do not establish whether the magnitudes of those trends are meaningfully distinguishable from the structural uncertainty in GOBAI-O2. In several regions, the trend magnitude remains comparable to the stated uncertainty, and this issue is not directly quantified. The authors also declined to relate trend magnitude to uncertainty on the grounds of unit mismatch, yet it would be straightforward to convert gridded uncertainty into trend uncertainty through propagation or confidence intervals. As a result, it remains unclear where spatial patterns can be interpreted as physically robust versus where they may be indistinguishable from dataset uncertainty. In addition, the uncertainty fields are described but not propagated into the trend confidence intervals or used to define spatial robustness masks, and some discussion sections still treat statistically significant patterns as physically meaningful without clearly distinguishing between statistical significance, robustness relative to dataset uncertainty, and regions that are primarily ML-inferred rather than directly observed. Overall, this core issue is improved but not fully resolved and remains a matter of scientific robustness.

The treatment of potential sensor bias has also been improved but remains largely qualitative. The revised manuscript now acknowledges the reported negative bias (~–2.7 μmol kg-1) associated with some air-calibrated BGC-Argo floats and notes that this could influence the magnitude and spatial structure of the diagnosed trends. However, this discussion is not accompanied by any quantitative sensitivity analysis, regional assessment, or estimate of how such biases might affect inferred trends. There is also no substantive comparison with shipboard reference sections beyond a general qualitative statement. The authors argue that a quantitative assessment is not feasible and therefore simply note the limitation. While this may be acceptable in some contexts, the manuscript relies heavily on a derived ML product trained on float data, and the lack of quantitative evaluation leaves the treatment methodologically thin. This concern is therefore only partially addressed and does not yet meet the standard typically expected for resolution of a major revision.

Finally, the physical interpretation has been moderated but still at times extends beyond what is directly demonstrated. The authors have softened their language and now frame mechanistic explanations more cautiously, which is appropriate. Nevertheless, several interpretations continue to rely primarily on consistency arguments rather than independent quantitative evidence. The OFES-based discussion remains largely qualitative, and the evidence for frontal migration is limited to the isopycnal outcrop analysis, which is defensible but not especially strong. These interpretations are likely acceptable if presented strictly as hypotheses consistent with the observations rather than as demonstrated mechanisms, but in their current form they remain somewhat borderline. Overall, this aspect is mostly addressed but not yet fully convincing.

---
Referee #2
As mentioned previously, the uncertainties should be quantitatively compared with the magnitude of the trends, and then the signals that exceed the uncertainty should be discussed. As the comment from the first reviewer, |trend| > mean uncertainty would be acceptable. Trend/yr multiplied by data period can be compared with mean uncertainty.

Hide

AR by Miho Ishizu on behalf of the Authors (03 Mar 2026) Author's response Author's tracked changes Manuscript

ED: Referee Nomination & Report Request started (06 Mar 2026) by Koji Suzuki

RR by Anonymous Referee #2 (17 Mar 2026)

Suggestions for revision or reasons for rejection

This revised manuscript is improved compared with the previous version. The topic is important, the large-scale patterns are interesting, and the authors have clearly made a serious effort to respond to the earlier comments. In particular, the additions on significance testing, uncertainty, and the discussion of frontal migration make the paper stronger than before.

That said, I still do not think all of the key concerns are fully resolved. My overall view is that the manuscript is getting close, but I still have several substantive concerns about reproducibility, uncertainty treatment, and the strength of some of the interpretations.

The first point is that I could not find an explicit statement of which exact GOBAI-O2 version was used. The manuscript cites the dataset and describes the product generally, but I did not see a clear release/version number in the Methods or Data Availability sections. I think this is important for reproducibility, especially because different GOBAI-O2 versions can give noticeably different regional trend patterns (some versions (pre-2.3) even disagree on the sign of trend in oxygen change!). Since the conclusions here depend on relatively subtle spatial oxygen trends, the exact dataset version should be stated explicitly. Ideally the authors should also comment on whether the main results are sensitive to dataset version.

Second, the uncertainty treatment is improved, but I still think the wording is stronger than what is actually demonstrated. The new robustness metric comparing 20-year trend magnitude with uncertainty is useful, and it definitely helps. However, I do not think this is the same thing as formally propagating uncertainty into the trend estimates. As written, the manuscript sometimes gives the impression that this has been done more rigorously than it actually has. I think the current approach is acceptable as a heuristic or screening metric, but the language should be revised so that this is presented more carefully. In particular, the paper should avoid moving too quickly from “statistically significant” to “physically robust,” since those are not equivalent in a reconstructed ML-based product.

Third, some of the conclusions still read as more definitive than the evidence allows. The descriptive patterns themselves are interesting and likely robust at basin scale, but several of the mechanistic interpretations still seem to rely mainly on consistency arguments using OFES, GODAS, and previous literature, rather than direct quantitative attribution. I think that is fine, but the paper should distinguish more clearly between observed trend patterns, interpretations supported by consistency with other datasets, and mechanisms that remain hypotheses. A more cautious tone in some parts of the discussion would improve the paper.

Related to this, the new frontal-migration analysis is helpful and makes that part of the argument more convincing than before. However, the agreement across datasets is not completely uniform, and I do not think the results support an equally strong statement everywhere. I would encourage the authors to acknowledge this more directly and emphasize that the evidence supports a broad tendency rather than quantitatively consistent behavior at all locations and depths.

The discussion of possible sensor bias is also better than before, but it still remains fairly qualitative. I appreciate that the authors now acknowledge this limitation more clearly, and I do not necessarily think a full new sensitivity analysis is required if that is genuinely not feasible. Still, I think the manuscript should remain cautious when interpreting trends that may be influenced by unresolved observational or reconstruction uncertainties.

Overall, I think the manuscript is substantially improved and may become publishable, but I do not think it is fully there yet. My strongest requests are that the authors explicitly state the exact GOBAI-O2 version used and that they revise the wording around uncertainty propagation and robustness so that it matches more closely what has actually been done. I also think some of the mechanistic interpretation should be softened slightly.

Recommendation: further revision.

Hide

RR by Anonymous Referee #1 (05 Apr 2026)

ED: Reconsider after major revisions (06 Apr 2026) by Koji Suzuki

Dear Authors,

We have received two reviews from experts who evaluated your manuscript twice previously. While Reviewer #2 was mostly satisfied with your revisions, Reviewer #1 still considers that several key issues, including data reproducibility, treatment, and interpretation, remain unresolved (see below). We therefore encourage you to submit a further revised version of your manuscript with point-by-point responses to the reviewers' comments.

Thank you again for your effort to improve the manuscript.

Kind regards,

Koji Suuzki
Associate Editor

---
Reviewer #1

This revised manuscript is improved compared with the previous version. The topic is important, the large-scale patterns are interesting, and the authors have clearly made a serious effort to respond to the earlier comments. In particular, the additions on significance testing, uncertainty, and the discussion of frontal migration make the paper stronger than before.

That said, I still do not think all of the key concerns are fully resolved. My overall view is that the manuscript is getting close, but I still have several substantive concerns about reproducibility, uncertainty treatment, and the strength of some of the interpretations.

The first point is that I could not find an explicit statement of which exact GOBAI-O2 version was used. The manuscript cites the dataset and describes the product generally, but I did not see a clear release/version number in the Methods or Data Availability sections. I think this is important for reproducibility, especially because different GOBAI-O2 versions can give noticeably different regional trend patterns (some versions (pre-2.3) even disagree on the sign of trend in oxygen change!). Since the conclusions here depend on relatively subtle spatial oxygen trends, the exact dataset version should be stated explicitly. Ideally the authors should also comment on whether the main results are sensitive to dataset version.

Second, the uncertainty treatment is improved, but I still think the wording is stronger than what is actually demonstrated. The new robustness metric comparing 20-year trend magnitude with uncertainty is useful, and it definitely helps. However, I do not think this is the same thing as formally propagating uncertainty into the trend estimates. As written, the manuscript sometimes gives the impression that this has been done more rigorously than it actually has. I think the current approach is acceptable as a heuristic or screening metric, but the language should be revised so that this is presented more carefully. In particular, the paper should avoid moving too quickly from “statistically significant” to “physically robust,” since those are not equivalent in a reconstructed ML-based product.

Third, some of the conclusions still read as more definitive than the evidence allows. The descriptive patterns themselves are interesting and likely robust at basin scale, but several of the mechanistic interpretations still seem to rely mainly on consistency arguments using OFES, GODAS, and previous literature, rather than direct quantitative attribution. I think that is fine, but the paper should distinguish more clearly between observed trend patterns, interpretations supported by consistency with other datasets, and mechanisms that remain hypotheses. A more cautious tone in some parts of the discussion would improve the paper.

Related to this, the new frontal-migration analysis is helpful and makes that part of the argument more convincing than before. However, the agreement across datasets is not completely uniform, and I do not think the results support an equally strong statement everywhere. I would encourage the authors to acknowledge this more directly and emphasize that the evidence supports a broad tendency rather than quantitatively consistent behavior at all locations and depths.

The discussion of possible sensor bias is also better than before, but it still remains fairly qualitative. I appreciate that the authors now acknowledge this limitation more clearly, and I do not necessarily think a full new sensitivity analysis is required if that is genuinely not feasible. Still, I think the manuscript should remain cautious when interpreting trends that may be influenced by unresolved observational or reconstruction uncertainties.

Overall, I think the manuscript is substantially improved and may become publishable, but I do not think it is fully there yet. My strongest requests are that the authors explicitly state the exact GOBAI-O2 version used and that they revise the wording around uncertainty propagation and robustness so that it matches more closely what has actually been done. I also think some of the mechanistic interpretation should be softened slightly.

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Reviewer #2

The manuscript has been improved with quantitative assessment of data uncertainty. However, there are still a few points that need to be revised.

Figures 1v-1bb: Change the color range from 0–1 to 0–2 (or similar) for easier comparison with Figures 3 and 4.

Figures 3d and 3h: Improve contour labeling to ensure values are easily identifiable.

Figures 4d and 4h: According to the definition, robustness is a non-negative metric. Therefore, the presence of the 0 contour line is inconsistent and requires clarification.

Lines 281 and 545, "climatology-based": Clarify this term. Climatology usually implies a long-term mean, and Ito et al. (2027) analyzed O2 anomalies.

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AR by Miho Ishizu on behalf of the Authors (13 Apr 2026) Author's response Author's tracked changes Manuscript

ED: Referee Nomination & Report Request started (13 Apr 2026) by Koji Suzuki

RR by Anonymous Referee #2 (21 Apr 2026)

Suggestions for revision or reasons for rejection

Thank you for the careful and thorough revision of the manuscript. I appreciate the substantial effort that has gone into addressing the previous comments. The additions regarding uncertainty, the clarification of the robustness metric, and the generally more cautious interpretation of the results have significantly improved the manuscript.

In its current form, the paper reads clearly and is much better aligned with the level of evidence provided. I do not have further substantive comments.

I would, however, like to point out one important issue that should be corrected before publication. The manuscript states that GOBAI-O₂ Version 4.4 was used. Based on the data archive (Index of /data/oceans/archive/arc0207/0259304/4.4/data/0-data), it appears that the analysis may in fact be based on Version 2.2 (it's the version of the .nc file that matters, not of the release). This distinction is critical and should be clarified explicitly. Versions prior to 2.3 rely on a different methodological framework, and the differences between pre-2.3 and later versions can lead to substantial changes in inferred oxygen trends. In some regions, these differences are large enough to affect even the sign of the trend (for example, in the Bay of Bengal, or in the Cape Verde region). Given that the conclusions of the manuscript depend on relatively subtle spatial patterns in oxygen trends, it is essential that the exact dataset version is correctly identified and reported.

I therefore strongly recommend verifying the dataset version used in the analysis and correcting the manuscript accordingly. If Version 2.2 was indeed used, it would also be appropriate to briefly comment on the implications of using a pre-2.3 product given the change in methods for GOBAI-O2.

Aside from this important clarification, I am satisfied with the revisions and have no further comments.

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RR by Anonymous Referee #1 (02 May 2026)

ED: Reconsider after major revisions (04 May 2026) by Koji Suzuki

Dear Authors,

We have received the following reviews of your revised manuscript. Therefore, we would appreciate it if you could prepare a revised version considering the reviewers' comments and submit it to us along with your point-by-point responses. Thank you.

Kind regards,

Koji Suzuki
Associate Editor

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Reviewer #1
Thank you for the careful and thorough revision of the manuscript. I appreciate the substantial effort that has gone into addressing the previous comments. The additions regarding uncertainty, the clarification of the robustness metric, and the generally more cautious interpretation of the results have significantly improved the manuscript.

In its current form, the paper reads clearly and is much better aligned with the level of evidence provided. I do not have further substantive comments.

I would, however, like to point out one important issue that should be corrected before publication. The manuscript states that GOBAI-O₂ Version 4.4 was used. Based on the data archive (Index of /data/oceans/archive/arc0207/0259304/4.4/data/0-data), it appears that the analysis may in fact be based on Version 2.2 (it's the version of the .nc file that matters, not of the release). This distinction is critical and should be clarified explicitly. Versions prior to 2.3 rely on a different methodological framework, and the differences between pre-2.3 and later versions can lead to substantial changes in inferred oxygen trends. In some regions, these differences are large enough to affect even the sign of the trend (for example, in the Bay of Bengal, or in the Cape Verde region). Given that the conclusions of the manuscript depend on relatively subtle spatial patterns in oxygen trends, it is essential that the exact dataset version is correctly identified and reported.

I therefore strongly recommend verifying the dataset version used in the analysis and correcting the manuscript accordingly. If Version 2.2 was indeed used, it would also be appropriate to briefly comment on the implications of using a pre-2.3 product given the change in methods for GOBAI-O2.

Aside from this important clarification, I am satisfied with the revisions and have no further comments.

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Reviewr #2
Figures 3d and 3h: The caption states the contour interval for thin lines is 0.2, but in the figure, it appears to be 0.5. Please correct this discrepancy.

Figures 3 and 4: The unit of uncertainty is umol/kg in Figure 3, but umol/kg/yr in Figure 4. Is one of them incorrect? Please check them again.

Figures 3 and 4: The latitudinal range for the 137E section is 0 - ~30N in Figure 3, but ~10S - ~55N in Figure 4. It would be better to use the same range for consistency and easier comparison.

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AR by Miho Ishizu on behalf of the Authors (17 May 2026) Author's response Author's tracked changes Manuscript

ED: Referee Nomination & Report Request started (19 May 2026) by Koji Suzuki

RR by Anonymous Referee #2 (19 May 2026)

ED: Publish as is (26 May 2026) by Koji Suzuki

AR by Miho Ishizu on behalf of the Authors (01 Jun 2026) Manuscript

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22 Jun 2026

Revealing hidden oxygen variability in the North Pacific: a two-decade analysis using GOBAI-O₂

Miho Ishizu and Tomomichi Ogata

Biogeosciences, 23, 4037–4056, https://doi.org/10.5194/bg-23-4037-2026,https://doi.org/10.5194/bg-23-4037-2026, 2026

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We used GOBAI-O₂, a new machine-learning-based dataset derived from Argo floats and shipboard observations, to examine 20-year linear trends in potential temperature, salinity, and dissolved oxygen in the North Pacific. Our results agree with prior studies and newly reveal spatially heterogeneous oxygen trends, including local increases. These are linked to physical processes such as the southward retreat of the oxygen minimum layer and a northward-shifting circulation front.


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Linear trends of temperature, salinity, and oxygen in the North Pacific based on GOBAI-O2 over 2 decades and their controlling factors

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Linear trends of temperature, salinity, and oxygen in the North Pacific based on GOBAI-O₂ over 2 decades and their controlling factors