Liu et al present a study of ocean alkalinity addition (OAE) in the North Sea using a regional ocean model. They vary the location of OAE deployment and explore the impacts on alkalinity transport, ocean carbon uptake and long-term carbon storage. This was an enjoyable read and is a really nice addition to the growing OAE literature that provides a number of interesting and often counterintuitive findings which are robustly explored. These findings include:

1. The North Sea is a relatively efficient OAE deployment region despite its shallow depth and short flushing time.
2. Small differences in the location of North Sea OAE deployment can lead to considerable differences in alkalinity dispersion, ocean carbon uptake enhancement and long-term carbon storage.
3. Loss of alkalinity to the deep ocean does not necessarily equate to inefficient CDR as although the total carbon flux enhancement is reduced, a greater proportion of this carbon may be transported below the permanent pycnocline.

The manuscript is clear and well-written and I recommend publication subject to a few minor corrections. My main suggestion is that the authors provide a little additional detail on the validation of simulated carbonate chemistry (even though this is published in greater detail elsewhere). Alongside this, it would be good to provide some detail on the maximum attainable CDR efficiency possible in North Sea waters based on mean present-day carbonate chemistry conditions.

Minor comments

L90 “efficiently transported” I recommend phrasing this differently.

L101 Deployment areas appear stippled not “hatched”.

L161. I see that carbonate chemistry evaluation is published elsewhere. Nonetheless I think some comment on the ability of the model to simulate DIC/TA/pCO2 in the North Sea is required here.

L176. Can you clarify here whether there are sediment fluxes of alk and DIC and whether these fluxes are influenced by benthic carbonate chemistry and OAE?

L301. Typo “of the” tracer

L318-340 Some context on the maximum attainable uptake efficiency in the North Sea would be useful here. Is this around 0.85 given the mean simulated surface ocean DIC and alkalinity fields?

Figure 9. It’s difficult to distinguish between the thick and thin lines in panel a. I suggest using another line type (e.g. dotted or a different dash type).

L386. Recommend not using “captures”

L389-390. Are these values still increasing at the end of the simulations? Does this explain why the deep DIC inventory plateaus within the model domain?

L454-456. Can the authors explain this finding here as they do in the supplementary material? How do they know full equilibration has occurred? I recommend phasing this as CO2 system equilibration not alkalinity equilibration.

A clear and well-reasoned regional model study of ocean alkalinty enhancement in the North Sea. The study focuses on 3 "likely" hypothetical application scenarios with regards to potential EU or national implementations, which have largely been absent from previous global simulations.



This reviewer has 2 questions that they would hope the authors will consider and a very minor comment.

1. Given that the study is already constrained to the North Sea region, uses a model which has been published at finer scale and there exist global Earth System Model simulations at 1 km scale; why did the authors choose to perform simulations at ~4.5 km scales? The reviewer appreciates that the authors do discuss the necessity of further finer scale studies in future.

2. Could the authors please clarify Figure 13? This reviewer is unclear how the plotted lines are represented in axes of Time, Area (km²) and Amount of added alkalinity (e^-3 Gmol) at the same time.



This reviewer wouold also like to suggest to the authors, with reference to line 11, that the effects of OAE are dependent upon the physical and biogeochemical conditions of a system.