General Comments

The authors provide an informative and insightful analysis of ocean acidification extreme events (OAX) in the North Pacific.  They demonstrate the skill of their model in simulating observed OA trends, and they have addressed its representation of mean conditions and variability in a previous study (Desmet et al, 2022). They highlight the regional heterogeneity in frequency and intensity of extreme conditions in a changing climate and point out the processes contributing to regional differences, such as the magnitude of regional variability relative to the long-term trend and the influence of off-shore propagating eddies on property distributions. Additionally, they attribute changing OAX conditions to CO2 increase by comparing to a control simulation based on 1979 conditions. Their analysis of the role of mesoscale eddies in driving regional OAX characteristics is particularly interesting. 

Specific Comments

1. It appears that the model-observation comparisons are made only with surface data. Since an analysis of conditions at depths of up to 250 m is one of the interesting aspects of this study, could the authors provide some evaluation/discussion of model skill at these depths?
2. Please include some discussion of the altered southern boundary condition in the control run in the context of attribution of OAX events to the rise in atmospheric CO₂.   How large is the impact of the southern boundary DIC concentration relative to gas exchange within the domain? Is the removed trend in DIC fully attributable to atmospheric CO₂ increase or could there be a contribution associated with interannual variability?  
3. In the context of relative threshold, fixed baseline stressor detection, the authors highlight the importance of the ratio of the long-term trend to local variability in determining OAX increase. Could they also shed light on whether temporal changes in the variability contribute? For instance, does the magnitude of variability increase in some regions? This is addressed to an extent by Figure B4, but I find the histograms difficult to interpret due to their small size. 

Technical Corrections

line 49: please add citation.

Figure 1: Please consider rephrasing “[H+] intensity” to reflect that it is the intensity of high [H+] events. Perhaps something like “[H+] event intensity”

line 231: “this horizon shoals by 50 m on average”. Is this the average over the whole domain or the coastal areas?

line 532: Are the units on sigma degrees latitude?

REFEREE REPORT(S):

Referee: 1

GENERAL COMMENTS

The manuscript titled “Spatiotemporal heterogeneity in the increase of ocean acidity extremes in the Northeast Pacific” undertook an assessment and quantification of ocean acidity extreme events (OAXs) in the Northeast Pacific region. This was accomplished using a high-resolution regional Earth System Model (ROMS-BEC) building upon prior research (Desmet et al. 2022). The authors comprehensively examined various properties of OAXs in the Northeast Pacific Ocean including their intensity, frequency, duration, and heterogeneities in time and space. By employing the return periods and time of emergence (ToE) framework, they substantially captured in the occurrences of OAXs in depths over the recent 30 years.

The paper is logically organized, and the figure are well-presented. The investigation into OAXs in the Northeast Pacific is particularly interesting and holds substantial value due to utilization of a high-resolution model validated against observations. Nonetheless, there remain certain questions and comments that require clarification prior to proceeding further consideration of publication.

MAJOR COMMENTS

1. The authors underscored the significance of mesoscale eddies in shaping the spatial heterogeneities of OAXs throughout the manuscript. This phenomenon was enabled by the employment of the high-resolution model (ROMS-BEC model), which resolves the mesoscale eddies and associated processes. Would the distinctive pattern of maximum intensity persist under coarser resolutions? To elucidate the reason for the spatial heterogeneities, I propose conducting the additional simulations using the same model with a coarse resolution. While a comparison with Burger’s findings in Earth system model has been made, the models are different, and this evidence is indirect. By comparing outcomes from the coarse resolution experiment using the same simulation parameters, you could substantially bolster the argument for the role of mesoscale eddies in driving OAXs.

1. While authors use various properties of OAXs, some of these properties require explicit definitions prior to their usage in the manuscript such as volume fraction of OAX, contribution in Fig.2, near-permanent OAX in Section 3.5, 4.2. Additionally, it’s unclear whether the subsurface layers encompass both 100m and 200m or solely the 200m depth. While these undefined properties are conceptually understandable, but it is needed to provide more precise definitions in an academic context. These specific properties would be recommended within the methods or results sections. Additionally, because the manuscript includes substantial number of acronyms employed throughout the manuscript, it could be beneficial to present a comprehensive table listing these acronyms for clarity.

1. The manuscript employs substantial numbers to elucidate OAX properties. However, the manuscript is inconvenient for matching the numbers with the figures. Consequently, the manuscript is difficult to follow, and the messages of research can be hindered. For instance, the discussion of Time of Emergence (ToE) is described as 16 years, 25years, 30-32years (L400). However, the ToE years are represented by the year ranging from 1984 to 2020 in figure 7. Authors need to make consistency by providing supplementary for matching ToE explained in the discussion. Similarly in Section 3.2, matching the numbers with figure 3 is also difficult to follow. This issue of mismatching or uncomfortable explanations are abundant through the manuscript. So, the manuscript is needed to enhance its comprehensibility.

1. The abstract, while comprehensive, stretches to approximately 550 words, which surpasses the typical length of around 300 words. To effectively encapsulate the core research messages, I recommend shortening the abstract to a more concise form.

MINOR COMMENTS

L152: What about the mean biases in the spatial patterns of pH and ? Comparing these biases with data from the Global Ocean Data Analysis Project (GLODAP) could provide the better information for model validation.

L210: The authors mention the strong year-to-year variability and abrupt transition in the subsurface layer. However, there is no explanation of mechanisms for the strong variability and abrupt transition.

L315: It would be helpful to specify which figure corresponds to the statement being made.

L326: Also, there is no description of explanation in strongest step increases in subsurface of the CNP, not merely mentioning the occurrence of the strongest step.

L357-361: It is advisable to compare the results of Burger et al. 2020 and provide supplementary with similar analysis by using GFDL model results or other Earth system models. Alternatively, if the explanation relies on a specific figure, please indicate which figure elucidates the increase in the number of days as per Burger et al. 2020.

L388: It would be helpful to specify which figure corresponds to the statement being made.

L400, L411: Provide additional details regarding the marine environment, such as temperature, salinity, or other influential factors.

L412: How to suggest the emergence of near-permanent OAXs from this figure? A more comprehensive explanation is needed to clarify this point.