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Interactive comment on 

"Arctic regional changes revealed by clustering of sea-ice observations” 

by Amélie Simon, Pierre Tandeo, Florian Sévellec and Camille Lique.

General Comments

This paper introduces a novel method for characterizing the seasonality of Arctic sea ice. The k-means clustering method, which has been successfully applied in other contexts, serves as the foundation for this approach. This study enhances the method by incorporating the Mahalanobis distance, providing a more robust physical representation of sea ice seasonal cycles. By applying this refined technique to sea ice concentration data spanning 1979 to 2023, the authors identify four distinct clusters that effectively describe the seasonality of Arctic sea ice.

Building on these results, the authors analyze the probability of individual seasonal cycles within the dataset belonging to each of these four clusters. They also introduce new diagnostic tools to describe the temporal evolution of sea ice seasonal cycles, as well as to pinpoint the moments when a seasonal cycle transitions from one cluster to another.

This study offers an innovative approach that complements previous research on sea ice seasonality. I commend the authors for the thoroughness of their analysis and the clarity of their well-structured manuscript. Overall, I believe this work represents a significant contribution to the field and, with the revisions suggested below, has the potential for a strong impact upon publication.

While the bibliography section requires attention before submission (as detailed later), most of the comments below are intended to enhance the paper's clarity, precision, and relevance. Authors are encouraged to consider these suggestions and apply them at their discretion.

Major Comments

The only major comment regarding this paper concerns the paragraph between lines 325 and 339. In my opinion, this paragraph requires further clarification or a more detailed presentation of the results to be fully convincing. The results presented in this section and reiterated in the conclusion, showing that ice conditions during the summer (or winter) are strongly correlated with the onset of melting (or freezing), are highly interesting for understanding the mechanisms behind the Arctic sea ice seasonal cycle. However, I find that relying solely on clusters to describe these results is limiting. 

If I understand correctly, the seasonal cycles of each grid point tend to group around one (or several, considering the results in Section 3.2) cluster. Therefore, I am not fully convinced that the seasonal cycles associated with a particular cluster (as shown in Figure 4b) behave exactly as summarized in this paragraph.

I believe it would be helpful to visually demonstrate this with supporting evidence to strengthen the argument. For example, you could present the interquartile range around each cluster on Figure 4a, using the data already employed to generate Figure 4b. Alternatively, if you prefer not to overload Figure 4a, you could clearly define the melting onset and freezing onset dates here (using concentration thresholds already applied in other studies) and provide statistics of these diagnostics in each cluster.  In my opinion, these revisions would significantly enhance the impact and clarity of these results.

Specific Comments

Lines 137-139 and lines 152-157:  I feel, with both sentences, that the authors try to highlight the novelty of their method compared to previous studies​, but these sentences appear before the work itself is introduced.  I am not sure of the relevance of these sentences at this point. This creates some confusion during a first reading. I suggest either removing these sentences or moving them to a later paragraph, ideally after the authors have introduced their work more clearly.

Lines 200-202: This statement could benefit from additional evidence (by including figures or statistics in the supplementary material, for example). 

Lines 216-217: It could help the reader if you briefly explain what you mean by “non-zero seasonal cycle.”

Lines 353: “consistent and continuous patterns”, consistent according to what? 

Lines 395-397: I find it difficult to discern "the edge of the 0.3 probability" in Figure 5 due to the continuous colorbar. Adding a contour line to indicate this boundary could make it clearer.

Figure 5: I find it difficult to discern the probability differences between clusters when the colorbar changes for each cluster. While I understand your choice to maintain consistency with other figures, for this particular figure, I suggest using a single colorbar for all four clusters to enhance readability.

Paragraph 464-476: The reference to Figure 7 is missing here and should be explicitly mentioned for clarity.

Line 468: “ the trend for the other two clusters are statistically significant”. Does it mean that the trends for the partial and full winter-freezing clusters are not significant? 

Lines 476: “and to a smaller extent, of the full winter-freezing cluster”. I am confused by this statement. Earlier (line 466), it was mentioned that the trend for the full winter-freezing cluster was nearly constant, and line 468 suggests that the trend is not significant.  This part seems to lack precision to be clearly understood. 

Lines 502-503: "while the partial and full winter-freezing clusters remain relatively stable." I find it unclear in Figure 8a that the total area covered by the partial and, in particular, the full-winter cluster is stable. It might be helpful to add the trend for each cluster, as was done in Figure 7, to make this clearer.

Lines 505-509: I find it difficult to discern the nuances between these two sentences. Perhaps the last sentence could be omitted, as it might not add significant value.

Lines 518-519: “Sensitivity tests have been performed on this definition, and the results do not change when we apply small definition changes (i.e., 9 to 11 years minimum length of the same cluster with zero to 2 years of tolerance).” It would be valuable to include the results of these sensitivity tests, perhaps in the supplementary material, to provide additional context and support for this statement.

Lines 522-525: A reference to Figure 9 would enhance clarity here, as the figure significantly helps in understanding the definition of these new diagnostics.

Lines 578-582: I am uncertain about the relevance of this paragraph, as all the information presented here seems to be already covered in the previous paragraph.

References section : 

I noticed that several references cited in the paper are missing from the reference list. I’ve compiled a list of the missing references I found, but I strongly recommend that the author carefully review this section, as there may be other errors that I may have missed. Additionally, the format of the references is not consistent throughout the section. For example, the publication date is sometimes listed immediately after the authors' names, and in other cases, it appears at the end of the reference. To ensure consistency, I suggest following the EGU standardized citation format. 

Missing reference in the references section : 

Ardyna and Arrigo, 2020: line 83

Eisenman, 2010: line 439 ; 693

Eyring et al., 2021: line 77

Lebrun et al., 2019: line 125 ; 137 ; 631 ; 648

Meier et al., 2007: line 146

Markus et al., 2009: line 127 ; 629

Maze et al., 2017: line 161

Parkinson and Cavalieri, 2008: line 678

Parkinson et al., 1987:  line 304

Parkinson et al., 1999: line 677

Parkinson, 2014: line 127

Peixoto and Oort, 1992: line 683

Peng and Meier, 2018: line 146

Regan et al., 2023: line 119

Lines 803-805:  Houghton and Wilson 2020 should appears before Huntington et al., 2017