The paper by Ditkowsky et al. investigates the response of the Indian Ocean OMZ to future climate change. To this end, authors analyze an ensemble of Earth system model high-emissions scenario simulations from the Coupled Model Intercomparison Project phase 6 (CMIP6). They show that the Indian Ocean experiences a broad oxygenation in the western tropical region driven by a decrease in waters supplied by the Indonesian Throughflow in favor of waters supplied from the south, typically richer in oxygen. This results in a prominent contraction of the OMZ there. However, the warming of the marginal seas, particularly that of the Persian Gulf, is suggested to cause a localized deoxygenation in the northern Arabian Sea, leading the OMZ to expand there.  

General comments:

I enjoyed reading this manuscript that I found generally well structured and well written. The question of future changes in OMZs in general and in the Indian Ocean in particular is critical and of very high relevance to the community, as the previously reported trends remain unclear and tainted with important uncertainties. The authors did an excellent job at summarizing and analyzing the O2 changes simulated by the CMIP6 models in the Indian Ocean for different classes of O2. However, I have a few concerns relative to the presentation and the interpretation of the results that need to be addressed before the manuscript can be accepted for publication.

My main concern is that when it comes to explaining the projected changes in O2, the authors did not explore all possible factors and focused nearly exclusively on the changes in water mixing ratios. While I believe that this likely plays a critical role in the transport and redistribution of O2 as suggested by the authors, other mechanisms that could also play a role were not considered. This includes for instance: 1) the changes (reduction) in biological export and respiration, 2) changes in winter stratification and convection (that could potentially be important in the deoxygenation signal that emerges in the northern Arabian Sea, alongside the effect of marginal seas warming). 

My second concern is that most of the analysis is focused on the presentation of multi-model means (MMM). While this is useful to derive average patterns among models, it masks out the behavior of individual models and hides important differences that can be useful to document and understand. For instance, do models that represent complex biogeochemical feedbacks behave differently from those that don’t when it comes to future O2 changes? Are there models where the oxygenation/deoxygenation patterns (or the drivers of those changes) are qualitatively different from the MMM despite having a decent representation of present-day O2 conditions in the Indian Ocean? And if so, why? It would be interesting to explore individual model responses in addition to the MMM to gain a broader understanding of the uncertainties around projections. The individual models can be shown in the main paper (for example for curve plots) or in the supplementary information (for maps or vertical sections) and discussed in the main paper.

Finally, OMZs are traditionally connected to hypoxia but some thresholds used in this study to characterize the OMZ (e.g. 150 mmol/m3) are much higher than the values typically used to define the OMZs of the Indian Ocean. As a result, regions that are far from the Arabian Sea and Bay of Bengal are also considered a part of the OMZ (e.g., the equatorial Indian Ocean and regions as south as 20S in the eastern Indian Ocean). Furthermore, some of the analyses (Figs 2, 3, 11) refer to the OMZ volume but show O2 concentrations as high as 225 mmol/m3. 

I suggest rephrasing the presentation to make it clearer that O2 changes in the entire upper ocean are analyzed and not just the OMZ. 

Specific comments:

Lines 9-10:

Is O2 at 180 considered part of the OMZ?

Lines 92-101 (model selection):

The figures in the Supp Info provide a visual evaluation with a horizontal map and a vertical section in the AS. A more quantitative approach that considers for instance the volumes of key O2 classes (e.g. below 20, or 60) would provide a more objective basis to retain or reject models. For instance, the UKESM model may not necessarily be better at simulating the OMZ (very thin and shallow) relative to the CNRM model (that was excluded). 

A related question is how sensitive the results are to this model selection. More generally, I think using multi-model means (MMM) can mask out systematic biases in models (for instance all models underestimate (overestimate) the intensity of the Arabian Sea (Bay of Bengal) OMZ and is not ideal  to understand the sources of discrepancies between the multiple models and hence the uncertainties around future projections. 

This point needs to be at least discussed. One way to reduce an excessive reliance on MMM is to present the results from individual models whenever possible.

Lines 99-101:

It would be good to mention other sources of biases such as the representation of the Persian Gulf, the remineralization depth, the ill-represented mesoscale eddy activity, the misrepresented upwelling and productivity in these coarse resolution models.

Lines 104-105:

Maybe justify the use of a single member instead of the ensemble average (given the importance of forced vs. internal variability)?

Line 116:

You mean the 85-year trend?

Lines 132-134:

Since the OMZ is located below 100m, why include the surface (0-100) layer? I suggest excluding the surface layer in the analysis of O2 classes’ volumes.

Lines 132-138:

It is not clear which OMZ definition this paper uses. OMZs are traditionally connected to hypoxia, but some thresholds used here (150 mmol/m3) are way above hypoxia, and result in nearly  ⅔ of the Indian Ocean being filled with OMZ waters (down to 20S). This is very unusual as the Indian Ocean OMZs are traditionally restricted to the Arabian Sea and the Bay of Bengal in the northern Indian Ocean. It is true that some previous studies suggested that some commercial fish species can be stressed at O2 levels around 150 or below. However, all these works are based on studies of the Atlantic and Pacific Oceans and hence may not necessarily be relevant  to the Indian Ocean species. The use of this threshold in the Indian Ocean needs to be justified and its implications discussed.

Line 155:

Why is only the transport through the Timor passage considered? Why not the total ITF? 

Lines 184-186:

Why not take a mean over the last 20 years or so to represent the climatology for the end of the century period instead? justify. 

Line 204:

Full stop before “Second”.

Figure 2:

Can you redo this analysis after excluding the top 100m? Since the OMZ is located in the thermocline, it makes more sense to exclude the surface layer.

Besides, this sort of analysis mixes up the OMZ in the Arabian Sea (AS) and in the Bay of Bengal (BoB). As most models overestimate O2 in the AS and underestimate it in the BoB, integrating the OMZ volumes across the entire IO can artificially hide those biases as these errors tend to compensate. It would be good to redo the same figure separately for the AS and the BoB.

Line 227:

Can you show this analysis for individual models as well? not just the MMM? You can also show individual model based biases in a table for the key classes: O2<5, O2<20, O2<60, O2<150 mmol/m3.

Lines 232-233:

This is an important difference (and I suspect it is worse in the BoB). It would be good to show these biases in a table while separating the AS and the BoB.

Figure 3:

This should not be titled “OMZ volume changes” since O2 thresholds as high as 200 and above are used. You are sampling the entire upper ocean (0-1000m) not just the OMZ.

Indeed, a significant portion of the analysis is dedicated to relatively well oxygenated waters, well above hypoxia. Therefore, I suggest rephrasing the presentation to make it clearer that O2 changes in the entire upper ocean are analyzed and not just the OMZ. 

Lines 235-236:

Given the very high threshold (180 mmol/m3), can one still refer to this as a part of the OMZ? This is likely in the surface layer and I suspect is essentially driven by solubility. Please rephrase.

Lines 239-245:

Maybe refer to the different classes of O2 as O2<20, O2<60, O2<150,...instead of OMZ20, OMZ60, and OMZ150.

Lines 294-295:

Why not diagnosing trends in O2 consumption or remineralization? if the remineralization fluxes are not available, trends in export fluxes at 100 and 500m can be considered. Furthermore, the drivers of O2 interannual variability may differ from those of long-term O2 changes. Therefore, I am not fully convinced that the correlation really demonstrates the sources of the long-term oxygenation/deoxygenation. The biology can clearly play a role as models show a robust and important decline in the tropical Indian Ocean productivity by the end of the century. In any case, the two factors (ventilation and respiration) need to be equally explored. 

Lines 303-304:

Same remark. Why is the role of biology not explored or discussed?

Lines 311-314:

How about the O2 content of these water masses? a change in the O2 content of these water masses can nullify those changes in the mixing ratios. This critical point needs to be explored. The ventilation depends both on the circulation/mixing as well as the O2 content of the water masses (e.g., pre-formed O2). The transport of O2 needs to be diagnosed.

Lines 329-331:

This assumes that these O2 concentrations remain constant under future climate change, which I am not sure is true.

Lines 374-379:

Need to cite the recent work by Vallivattahhillam et al. (2023, Frontiers in Mar. Sc.) who also show a robust shrinking of the OMZ in the Arabian Sea based on the analysis of a set of CMIP5 models as well as downscaled regional model simulations. 

Figure 11:

Again, can we talk about OMZ when O2 concentration is close to saturation (for O2 close to 200 mmol/m3)? This essentially shows the cumulative changes in O2 frequency distribution.

Line 430:

I suggest citing two recent studies of the Arabian Sea OMZ that discussed some sources of uncertainties in future model OMZ projections: Vallivattahhillam et al.  (2023, Frontiers in Mar. Sci.) and Lachkar et al., (2023, Frontiers in Mar. Sci.). 

Vallivattahhillam et al. (2023) explored the sensitivity of future O2 projections to model representation of present-day conditions and highlighted the importance of correcting biases in global models representation of present-day OMZs. This appears to considerably reduce the discrepancies among models and makes the OMZ shrinking signal emerge more robustly.

Lachkar et al (2023) discussed a few factors (e.g., timescales of the different mechanisms) that may explain the difference between recent trends dominated by deoxygenation in the AS and the projected future changes dominated by oxygenation.

Supplementary Information:

Fig S1: Maybe show the difference model-minus-obs to better visualize individual model biases.

Fig S5: You mean Figure 1 in the main paper?

Fig S6: Are these trends vertically averaged as stated in the caption, or are they trends in the vertically averaged O2 concentrations? 

I very much enjoyed reading this comprehensively written and well structured manuscript by Sam Ditkovsky et al. What is presented is an analysis of the general ventilation pathways in the Indian Ocean and which are “tailored” to the specifics of oxygen, including OMZ regions, putting an emphasis on oxygen concentration thresholds. On the whole, I agree with the approaches, and I like the analysis presented. However, some things are unclear to me and I would like the authors to comment on them. The analysis focusses on section/surfaces – what has been ignored in the study, but I assume is highly relevant when it comes to understanding the drivers of oxygen variability, is the physics of ventilation in particular in the southern source regions. In these regions a coexistence of stratified Central Waters (Ekman pumping driven subduction) with lateral flux dominated Mode waters exists (e.g. apparent in the global assessment of Hanawa & Talley, 2001, or specifically addressed in the Karstensen & Tomczak 1998 paper or). Given the specific role of Mode Waters in thermocline ventilation (in the Indian Ocean formed only in the southern hemisphere). I assume not addressing Mode Water and not contrasting them to Central Water may prompt wrong conclusions on the drivers/sources of variability – also for the Indian Ocean as a whole (because these water masses are a major source) . Can you show that this distinction between Mode and Central water does not matter? Within this context I wonder if analyzing the “southern source” by integrating over a vertical section at 30S only is maybe too simple? – for example the winter outcrop (determines area of permanent subduction) is not strictly zonal and thus the full ventilation signal is not accounted for by making use of a zonal section. This may get worst in ESM’s that, with climate trends may show shifts in outcrop density change/trend over time? Have you considered that? Also, I wonder if the AOU/ideal age comparison isn’t a bit too simple to deduce biology from it? (Line 150 etc.).

This is because AOU is a property that reflects in my view three processes:

1) respiration (biology, depth and eventually region depending)

2) mixing of water properties in the interior

3) mixing of the imprint of the saturation value in the respective outcrop region of the various water masses

To explain what I mean - let’s ignore 1) and just look at combinations of 2) & 3). For simplicity assume two water masses are 3 years old and have similar TS in their formation region – a mixing of the two would be invisible and the ideal age equals the oxygen propagation time (3 years). For this case all assumption are OK. However, if two water masses still are 3 years old but start with different TS and thus concentrations (saturation) in their formation region a mixing signal is seen in AOU but no signal in “ideal age” (this will be still 3 years). This change in AOU now is interpreted as a residual and thus “biology” (1)) despite the fact that in this thinking experiment no biology (1)) was considered. Given the complexity of water masses in the Indian Ocean and the wide range of oxygen concentrations and therefore oxygen gradients and therefore oxygen fluxes, the above process I assume may be significant an messes up the correlation and deduction of biology and driver of variability?

Line 172: note that for an “Optimum” Multiparameter Analysis, at least one parameter more than the number of source water types is needed. This is because the term “Optimum” refers to the “non-negative” constrain which takes away on degree of freedom (see Lawson and Henson algorithm mentioned in Tomczak & Large). Is that a problem for your case? (>3 source water types?

Line 224: is it known why ESMs simulate higher oxygen levels? Could it be that shallow subduction occurs in the Arabian Sea in the models? (the Ekman pumping during monsoon would support that). Figure 5: you indicate the expansion of water masses by dashed line. How can I envision this expansion? Say AAIW is defined by the salinity minimum – what does expansion mean? A density range? The 95% contour of AAIW content?

Line 327-329 or Line 353-355: you report about the increase/decrease of water masses overtime. Operating with changing source water mass characteristics is a challenge in water mass mixing analysis – simply because, from a mixing model point of view, each changing source water is introducing an additional water mass to the ocean interior while the water with the former characteristics still exist and contribute to the mixing. How do you deal with that? (maybe I overlooked it but do you list the source waters somewhere?)

Line 420: An additional fact on the Atlantic OMZ ventilation is the significant source of South Atlantic source waters on ventilating the North Atlantic OMZ (e.g. evident in the TS properties but also from subduction estimates). You may want to also consider this in your “first glance on Atlantic” discussions.