The manuscript provides a comprehensive review and discussion point about how the biogeochemical coupled hydrodynamic could; be used in an Ocean Alkalinization experiment s.  

The manuscript lays out all the processes and questions any project about OAE should explore. It provides a Guide to Best Practices.

I found the manuscript well written, with only a few points of discussion that could be added, listed below.

A few dotted points related to the manuscript:

• Any OAE experiment will require the modelling exercise to be complete as soon as the experiment is underway (near real-time) or, even better, in a forecast mode. There are many operational systems available at the global scale but few at the scale of OAE (regional to sub-regional). This is an essential point as the availability of forcing data initial conditions are crucial elements to any modelling system.
• In Section 2.2.3. I would add the resuspension due to waves as a significant process in representing the sediment-water exchange. While it only applies to particles, resuspension can be crucial in enhancing carbonate particle dissolution.  (Eyre, B. D., Cyronak, T., Drupp, P., De Carlo, E. H., Sachs, J., & Andersson, A. J. (2018). Coral reefs will transition to net dissolving before end of century. Science, 359(6378), 908–911. https://doi.org/10.1126/science.aao1118)
• In section 2.3, I would add a point about the development of unstructured model mesh. Using unstructured mesh allows increasing resolution in a specific area while retaining the ability to have a lower resolution elsewhere to capture larger-scale processes. It can act as an alternative strategy to multiple model nests.
• The discussion about the use of DA in OAE could be simplified.  While there is room to develop news DA technic to assist OAE, I don’t see DA as a major player in OAE.
• On the other hand, the author could discuss the use of DA in the optimisation phase of any OAE, for example, some of the DA machinery could be used to optimise the amount of Alkalinity being released to maximise the impact area.

I feel honoured to be asked to review the work of this distinguished group, and I hope that some at least of the following comments and suggestions—coming as they do from a non-oceanographer—will be helpful.

1. Line 51: I would argue that MRV is primarily a deployment rather than a research challenge, and that “skillful and fit-for-purpose models” will be essential in meeting this challenge, rather than merely “valuable”.
2. Line 66: It would be helpful to expand on what is “sufficient”, or how one would go about establishing that.
3. Line 77: Is the conflation of “scenarios” and “counterfactuals” standard nomenclature in this field? I am used to a different definition of scenarios, as not being limited to counterfactual cases. Where do simple sensitivities—essential in uncertainty analysis—fit into these four general types?
4. Line 84-85: “especially when considering” seems superfluous, as successful model implementation will be even more of a challenge for eventual OAE deployment. Also, modelling of OAE field trials involving small alkalinity additions may still involve large spatial and temporal scales, depending on local circulation and given the long gas exchange timescale.
5. Line 115: Suggest to replace “mCDR” with “CDR”, since terrestrial methods will also affect Earth system feedbacks.
6. Line 176: Might be useful to mention Lagrangian methods as also being relevant to modelling the physical dynamics of particles.
7. Lines 246: “significant” is superfluous here, as even small-scale field trials are subject to the slow air-sea gas exchange. See note 4.
8. Line 248: I think it is incorrect to state that transportation distance is not relevant in the case of alkalinity “added to seawater that is oversaturated in CO₂”. Such waters can also be transported large distances before equilibration (same gas exchange timescale applies!), and the seawater conditions at the point of equilibration (not at the point of alkalinity addition) are what determine OAE efficiency.
9. Line 295: “wastewater treatment plants” could also be mentioned in lines 290-293 in relation to reactive mineral addition (Planetary's approach).
10. Line 313: “precludes” is perhaps too strong. Regional models may be sufficient if they fully contain the area of ocean circulation prior to subduction.
11. Lines 364-367: I do not think that local variations in relation to carbonate chemistry equilibrium coefficients impact eventual OAE efficiency. The relevant equilibrium coefficients are those at the point of air-sea gas equilibration or, more strictly, at the point of eventual subduction of the DIC enhanced waters, in the case that there are significant changes in seawater conditions between equilibration and subduction.
12. Line 383: Challenging indeed, but perhaps not required for deployment modelling since, as suggested in line 542, there are likely reduced-complexity approaches that will be sufficient.
13. Line 463: I guess this should be “and decreasing pH”?
14. Line 496: Does this really stand out as “critical” among the many areas needing improvement?
15. Lines 535-536: The concern regarding prescribed atmospheric pCO₂ is really only relevant in the case of conceptual global studies. For actual OAE deployments, hindcast modelling with the actual pCO₂ history will be required for the quantification of removals.
16. Lines 575-577: Re. “ultra-high-resolution modeling tools” if might be of interest to note (as a “personal communication”?) that Planetary is developing a Lagrangian particle tracking approach to determine the “alkalinity release field” generated by the advection, sinking, and dissolution of reactive mineral particles with a generic size distribution, which will then be imposed on a regional scale Eulerian model.
17. Lines 580-581: “The mean state …”. See points 4 and 11 above.
18. Lines 650-652: “different points …” seems a bit too indeterminate, given that some points in space and time are far more relevant than others! (points 4 and 11 again).
19. Line 910: While the pCO₂-pH pair results in higher parameter uncertainties, I don’t think it is correct to characterize this as a “high” uncertainty in the overall context. (See e.g. CarbonPlan’s Verification Framework, which characterizes carbonate system uncertainty as Low (1 to 5% impact) https://carbonplan.org/research/cdr-verification/ocean-alkalinity-enhancement-mineral  Component 2; Mineral dissolution).
20. Line 929: I think ensemble-based methodologies, in general, provide a useful framework for uncertainty analysis, whether incorporating DA or not.

Typographical corrections

1. Line 67: An array “is”, rather than “are”. Alternatively, “Many methods … are …”
2. Line 104: Replace “.” by “,” after “2.2”.
3. Line 314: Suggest to replace “alone and” with “alone, which”.
4. Line 414: Replace “104” with “10⁴”.
5. Figure 4: Bottom right text, “desirable”.
6. Line 614: “pCO₂” has been used multiple times already and does not need to be defined here.
7. Line 713: Replace “properties. But” with “properties, but”
8. Line 906: Replace “an” with “and”.
9. Line 932: Replace “to made” with “to be made”.