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Review 2

The paper makes use of a sophisticated sea-ice model (CICE-Icepack) to investigate the processes affecting ice mass gain/loss in the marginal ice zone (MIZ) compared to the pack ice. Two simulations with different atmospheric forcing are performed: one using the NCEP reanalysis, available from 1979 to 2020 and the other using the HadGEM2-ES model, available from 1980 to 2050. Such a framework allows for validations of the model with observations in the 1980s, where the ice cover is dominated by the presence of thick multiyear ice (low-MIZ) and in the 2010s when the ice cover is much thinner and younger (high-MIZ). Then, the model is used to assess the processes affecting the ice mass balance in the 2040s, where the sea-ice cover is projected to be seasonally ice-free (all-MIZ). While the research topic is important, the methods are sound and the results are interesting, the text and the structure have significant flaws. Therefore, I suggest that major revisions should be conducted by the author and co-authors to improve the flow and the clarity of the text. 

General Comments 

There are several structural issues within the text that require attention. First, the introduction is mostly composed of method statements with some discussion statements. It should focus more on presenting the existing literature, which would help emphasize the paper's significance and delineating its original contributions. Thank you for your suggestions. An extra paragraph has been added to the introduction in the revised manuscript, specifically on processes and feedbacks that differ in the MIZ compared to the pack ice to help better motivate the work. The new text added:

“The strength of sea ice is strongly dependent on the SIC. For 80% SIC (the upper MIZ boundary), we can estimate (Hibler 1979) that ice strength is less than 2% of its maximum. In the MIZ internal stresses in the ice play only a small role and sea ice is essentially in free drift. The sea ice in the MIZ behaves distinctly to pack ice as it can be more easily advected. This has implications for those wanting to cross the Arctic: a larger Arctic MIZ would be easier to send ships across.

The larger concentration of smaller floes and lower sea ice concentration in the MIZ has a number of consequences for the sea ice interactions with the ocean and atmosphere. Lateral melting will be enhanced due to the increased perimeter to surface area ratio, creating open water more efficiently than top or basal melt, and potentially fuelling the ice-albedo feedback. The lower the ice concentration, the more the surface ocean is warmed due to the lower albedo of open ocean, further enhancing ice melt and leading to the positive ice-albedo feedback. The increased open water fraction can also mean an increase in wind mixing in the mixed layer and will affect Arctic Ocean spin up (e.g. (Martin et al 2016)). There is a wave-floe size feedback that means the smaller the floes, the larger the impact of the waves, so a positive feedback loop exists that can act to increase the action of waves on the sea ice floes and further increase the concentration of smaller floes. The location and volume of sea ice melt has implications for stratification and so how deeply solar heat is mixed down. More sea ice melt means the mixed layer is shoaled and solar heat is concentrated in the upper water column.

Meanwhile there are other important sea ice processes, such as top melting where it is less clear that we would expect there to be a contrast between the MIZ and the pack ice, for example in the formation of melt ponds. In the Arctic, the snow thickness is generally modest compared to that on Antarctic sea ice, and the location of top melting and the formation of surface melt ponds is primarily driven by atmospheric conditions. Projections suggest that the MIZ will increasingly dominate the Arctic sea ice cover, especially in summer. It seems likely, therefore, that MIZ-focused processes will play an increasing role in controlling the mass budget of Arctic sea ice.”

Second, the method section appears to duplicate information already mentioned in the introduction. A data section, presenting the two observational datasets (i.e., Nasa Bootstrap and the Nasa Team) is required as they are an essential part of the results.

We have reorganised the Method Section, which included additional sections in the revised manuscript on the Atmospheric forcing data (2.2), Model Validation Data (2.3, this includes observational data sets and PIOMAS) and a section on the Analysis Method (2.6).

Third, more effort is needed to explain the results clearly. Specifically, the figures are almost never referenced when commenting about a result which makes it hard to follow. An excess number of percentages are used to compare different decades, atmospheric forcings, and observation datasets, making it arduous to follow the narrative.

We have added more Figure references throughout and have removed some of the percentages and numbers from the text.

Finally, many clarifications should be moved to the figure captions to enhance clarity.

We have removed a number of clarifications throughout the manuscript.

Last, the conclusion also appears disorganized primarily due to an excessive emphasis on stating percentage changes, which detracts from highlighting the main study results. 

We have removed a number of percentages from the conclusion.

2- Is there any wave forcing in the model? If so, a detailed description of wave forcing used in the simulations should be mentioned in the method section as waves plays a critical role in the processes affecting the FSTD - via wave-induced fracture leading to enhanced lateral melt and from the wave-dependent new-ice formation proposed by Roach et al. 2019. In fact, the FSTD model developed by Roach et al. 2018, 2019 should not be used in the absence of a wave field as mentioned in the model documentation. If there is no wave forcing, the authors should seriously reconsider the analysis or/and justify how the absence of a wave field affects their results. 

This has been added to the model description section. From lines 96-107 in the revised manuscript:

“We used the same wave forcing set up as Bateson et al., (2022), note that this is a different set up to Roach et al., (2019) where a separate wave model is coupled to the sea ice model to calculate the wave properties in the grid cells that contain sea ice. Instead, we use an extrapolation method as used and documented in (Roach et al., 2018), where ERA-I wave forcing (Dee et al., 2011) is used to calculate the necessary in-ice wave properties. The wave forcing consists of the significant wave height and peak wave period for the ocean surface waves. These fields are updated every 6 hours in the grid cells that contain less than 1% sea ice. Crucially for this study, despite not having a coupled wave model our set up still enables wave induced fracture causing enhanced lateral melting and wave-dependent new ice formation, as outlined in Roach et al., (2019). After 2017 we repeat the wave forcing, which does mean there is no trend in the wave forcing. Sensitivity studies varying the wave forcing using this model have demonstrated limited sensitivity to the wave forcing (Bateson. 2021) and comparisons to future 2056-2060 climatology from a global RCP8.5 wave simulation shows no significant change in significant wave height or interannual variability in significant wave height (Bateson. 2021). Although the wave forcing fields do not have any trends, the propagation of the waves into the ice field does respond to the changes in the ice cover over time. The simulations were initialised with a 6 year spin up period, this is a similar length to previous studies using the same model setup (Rolph et al., 2020; Bateson et al., 2022). As we are using a standalone sea ice model, the amount of spin-up required is much shorter than a climate simulation, or a coupled sea ice-ocean model.”

Specific comments 

1) Lines 2-3, 4-5 and 7-8: Theses sentences are repetitive : " [...] the MIZ has been widening and making up an increasing percentage of the summer sea-ice", " The MIZ is projected to become a larger percentage of the summer ice cover, as the Arctic transitions to ice free summers"  and " [...] as the summer MIZ fraction and amplitude of the seasonal sea ice growth/melt cycle increases over decadal timescales ". This is not appropriate in an abstract. 

A has been changed to SIC throughout. Sentence changed to “and examine how these processes change as the summer MIZ fraction increases over time.”

2) Line 4: Use SIC instead of A for sea-ice concentration. If you are to define an acronym, use it consistently through the manuscript. A has been changed to SIC.

3) Line 9: The name of the model used (CICE) should be mentioned in the abstract. The name CICE has been added, see line 8.

4) Line 12: The notation that you start using to compare NCEP and HadGEM2-ES atmosphere-forced simulation is NCEP (HadGEM2-ES) (e.g., "from 14 % (20 %)"). Please use this notation consistently through the abstract.   This has been amended, see line 18.

5) Line 18: Why redefine MIZ? We have removed this from text.

6) Line 18: Saying that "the ice cover becomes more marginal" is incorrect. Use thinner, younger, mobile or "MIZ-like".  This has been amended, see lines 17-18 “As more of the summer ice cover becomes MIZ”.

7) Line 22: Remove "traditionally". This is its definition. You could cite an older paper.  Removed, see line 22.

8) Line 24: Cite from the oldest to the newest. This comment is valid for the whole manuscript.  Corrected see line 24.

9) Line 26: What kind of fragmentation are you referring to here? Wave-induced? "[...] higher concentration of smaller floe" I would use "number" or "fraction", to avoid confusion with sea-ice concentration.  We are referring to the summer break up of the sea ice cover including wave induced fragmentation and brittle fracture. “Concentration” has been changed to “fraction”, see line 26.

10) Lines 27-29: You say: "As the sea ice cover shrinks [...] the MIZ contracts" followed by "As summer Arctic Sea ice has retreated [...] the MIZ fraction [...] has been increasing". These two sentences appear to say the exact opposite. This needs to be clarified. We have changed this to: “As summer Arctic sea ice has retreated over the past 40 years the fraction of the summer sea ice cover that is MIZ has increased (Rolph et al. 2020).” See lines 27-29

11) Lines 32-44: This is the closest from an introduction that we get. You need to expand this section. Explain clearly what has been done in previous studies. What method did they use? What were their results? Then finish the introduction by clearly stating which gap in the literature you are trying to fill.  We have added some lines about processes in the MIZ, to introduce and motivate why we are studying the mass budget in the MIZ, see lines 32-49 (also see answer to the first of the general comments).

12) Lines 47-53: This is all method section, there is no need to describe in depth all the components of the model here. We have now created an “Analysis Method” section and moved it there, see Section 2.6 in the revised manuscript.

13) Lines 53-60: What is this? Why use the future tense? Is this a general statement or are you discussing the results of your model? If so, this goes into the result/discussion section.  It was a general statement, but has now been moved to a Discussion section, see Section 4.

14) Lines 61-74: Again, this is part of the method.  We have moved this to a new “Analysis Method” subsection within the Method section, see Section 2.6

15) Line 85: This is a joint floe size and thickness distribution model (FSTD). Not an FSD. This is valid for the whole manuscript. FSD has been changed to FSTD.

16) Line 85-90: This should go to the introduction. We should have more background information about the previous study that used this model, so we have a better understanding of why this model is adequate at this point. We have rewritten the information about the model used in the Introduction section to:

“In this study we present an analysis (the first to our knowledge) of the relative contribution of sea ice processes controlling the mass balance in the pack ice and MIZ, and how this may change in the near future in a warming Arctic. This motivates the use of a sea ice model with a higher physical fidelity than used by climate models that is able to capture the distinction of MIZ processes. We use the dynamic-thermodynamic model CICE coupled to a mixed layer model (Petty et al., 2014), the version we use is described in more detail in Section 2.1. The model has been used in a number of previous modelling studies including Schroeder et al., (2019), Rolph et al., (2020) and Bateson et al. (2022).”

And the Model Set up section (2.1) has been rewritten to include more details on the wave model and refinements to the version of CICE being used. We think this in combination with a description of key physical processes (see response to General comments) has made it clearer why the model we use is a suitable choice for the requirements of our study.

17) Line 91: "MYO-WP4-PUM-GLOBALREANALYSIS-PHYS-001-004" What is that name? There must be a simpler, more understandable way to say that?  This has been changed to “MyOcean global ocean physical reanalysis product” in the revised manuscript, see line 91.

18) Line 94-100: This is a repetition of the previous paragraph in many ways. Is it all necessary? You could just mention the most important components for the study, and perhaps put a namelist file in supplementary material to avoid making such a long enumeration of all the model components that breaks the flow of the paper.  

We have kept the model description where it is, as this is a common approach used by a number of studies that have used a similar model set up eg Rolph et al. (2020) and Bateson et al. (2022). The problem with having the information in a namelist file is that it is only helpful to people who use the CICE model, whereas having a short paragraph with the main model settings used could be helpful to a larger range of readers. What we have included is already a relatively concise description of the most important settings.

19) Lines 101-105: This needs to be more detailed as it is the heart of your method. Why use those two specific atmospheric forcing? What question are you specifically trying to answer with that? We have moved this to a new subsection “Atmospheric forcing data” where the two forcing sets are described in more detail. See added Section 2.2 in the revised manuscript.

20) Lines 106-109: These sentences are redundant, especially if you are ending the introduction with an enumeration of content of the paper.  These lines have been removed.

21) Line 112: "[...] initialised with a 6-year spin-up". This is part of the method. Also is 6 years of spin-up enough? Please add a citation that supports this. 

This has been moved to the model set up section, see text from lines 107-110: “The simulations were initialised with a 6 year spin up period, this is a similar length to previous studies using the same model setup (Rolph et al 2020, Bateson et al 2022) As we are using a standalone sea ice model, the amount of spin-up required is much shorter than a climate simulation, or a coupled sea ice-ocean model.”

22) Line 112-113: The content of the caption should not be repeated in the text. This comment applies to all figures.  We have tried to minimise this apart from where we think it makes the text clearer.

23) Lines 114-116: Here you are talking about the method used to analyze the data (i.e., using the last 5 years of each decade). I would move that to the method.  This has been moved to an added “Analysis Method” section. See section 2.6.

24) Table 1: I do not really like the use of a table here. I would prefer a plot of the average monthly SIE and MIZ extent, with the 5 years minimum and maximum as a shading. If you keep the table, add a top row with only the title 1980s, 2010s and 2040s. 

We have removed this table and replaced it with a figure of the average annual cycle (average monthly values) for each period of sea ice extent and MIZ extent. See Figure 4 in the new manuscript. The new figure includes the observations (requested by reviewer 1) and shows the differences between the study periods in each product/simulation.

25) Line 121: What are those data (NASA Team, NASA bootstrap) ? They are key for the validation of your model. They need a separate Data section. Same for PIOMAS. We have added a model validation data section as suggested, see Section 2.3 in the revised manuscript.

26) Line 121-135: Refer to the figure that you are talking about more often. Not only once, at the beginning of the paragraph, but each time you are pointing at something new in the figure. This will make it easier to follow. This is valid for the whole result section.  We have now added more Figure number references throughout.

27) Lines 117-118: The narrative of your result is extremely hard to follow. This is because you are meticulously comparing all the values between 2 different simulations with observation using different atmospheric forcing (NCEP and HADGEM2-ES), in 3 different periods (the 1980s, 2010s, and 2040s) and in 3 different regions ("always MIZ", "become MIZ" and "always pack ice"). For a general reader, it is extremely confusing to read. I would try to avoid mentioning the exact values when you are making comparisons. For example, in this case instead of saying "The maximum sea ice extent [...] shows a modest decline from 1.22e7 km2 in the 1980s to 1.15e7 km2" in the 2040s" I would just say something like "The difference between the maximum sea-ice extent in the 2040s compared to the 1980s is modest in the HadGEM simulation (Table 1)." Then the reader that is really interested in the exact values can investigate the table or the figure. This is a general comment that should be applied to all the result sections. The author should unearth the “story” in their simulations and present that instead. 

We have rewritten the results section to try to make it easier to follow and removed exact figures in the text in the revised manuscript.

28) Line 137: Why only North of 66.5 N? There are large MIZ regions south of that (e.g., Labrador Coast, Bering Sea, etc.) This was a mistake in the text, we have removed this text – there had been a figure of the averaged reanalysis atmospheric fields in an earlier draft that was removed where this latitude had been used.

29) Figure 1: Put units in millions of km2 instead of having a 1e13 factor. This is valid for all plots. Why are there vertical bars in the NASA team and Bootstrap SIE data? Reduce the number of x ticks. Increase the size of x and y labels for all figures. 

Units and labels have been adjusted, see Figure 3 (figure has been renumbered).

30) Figure 2: add a) and b) for each panel and refer to each of the specific panels in the text. This is noisy, plot a running average? We have made the plots wider, and only included the 1979-2020 period to make it a bit clearer and added Figure references.

31) Table 2: Again, I do not really like the use of a table here. I would prefer an histogram. This would facilitate visual comparison for the reader.  The table has been replaced by Figure 7 in the revised manuscript, as suggested.

32) Figure 4: I would add larger title on top of each column (e.g., on the top of the first column put "NCEP" and the left of the first row put "Always pack". No need to keep a long subtitle for each panel. Remove the y ticks label in each of the subplot and keep only the one in the first column. Merge the third column with the second column (i.e., add 2040s with comparison with both the 1980s and 2010s). These comments are valid for figures 5 and 6.  

We have adjusted the labels on Figures 5-7. We’re not keen on combining the second and third columns as the regions are different for the two comparisons (1980s vs 2010s and 2010s vs 2040s), the two 2010s will be different as the areas used are different, so there would be four bars in the second column if we do this.

33) Lines 251-281: Same comments as in the results there is no need to say all the percentages. This will help to highlight what are the main results of this study. We need to clearly see how they are original compared to what was previously done. What could be done in future work?  

A number of percentages and exact values have been removed in the results and conclusion sections. We have added a paragraph to the end of the Conclusions that makes some comments on broader insights from this study: “Our analysis demonstrates a different balance of processes control the volume budget of the MIZ versus the pack ice. They are understandable in terms of the physical processes that dependent on the ice concentration, such as wave-ice interaction and lateral melt, which we are able to account for in our relatively physics rich sea ice model. We suggest that representation of such processes, in models such as climate models, requires more attention as a greater fraction of the sea ice cover becomes MIZ.”

As I mentioned earlier, I believe that this is truly interesting and useful work.  However, it is not yet a finished product that is ready for submission to a scientific journal.  Here, the specific comments will help to improve the manuscript, but are certainly not the only changes that are required for the future submission. I believe that more efforts, especially from the co-authors, are required for the organization of the material and for a clearer writing of the results.