Review of "On the control of the position of the winter sea ice edge by the Antarctic Circumpolar Current" by Goosse et al.

This manuscript investigates the influence of the ACC fronts on the climatological mean position of the Antarctic winter sea ice edge. Using established frontal definitions (Orsi et al., 1995; Park et al., 2019) and observational/reanalysis datasets for sea ice, atmospheric, and oceanic variables, the authors find strong correlations (> 0.85) between the latitudes of all major ACC fronts and the winter sea ice edge. The Polar Front (PF) is identified as the most consistent indicator. The study proposes two primary mechanisms for this control: 1) poleward heat transport by mesoscale eddies generated downstream of topographic barriers, and 2) atmospheric warming above warmer surface waters near the PF, with this heat subsequently transported poleward towards the ice, particularly with southward-directed winds. The authors conclude that bathymetry, by shaping the PF's path, strongly constrains the winter sea ice edge.

General Comments:

A very well-written, clearly structured, and valuable contribution to understanding the controls on Antarctic sea ice extent. The study addresses an important and under-explored link in a circumpolar manner. The use of multiple frontal definitions and a relatively simple yet effective methodology lends robustness to the main conclusions. The identified mechanisms are physically plausible and supported by the presented evidence and previous studies. The figures are generally clear and effectively support the text, with Figure 9 providing an excellent summary.

My concern is the relative lack of discussion on the role of subpolar gyres and the Antarctic Divergence. These features are intrinsically linked to the ACC, upwelling of CDW, and spread of WW, and thus might be highly relevant to SIE positioning. The central narrative that the "ACC/PF controls the winter sea ice edge," while supported by the presented correlations, might potentially be a trivialization or, at least, could benefit from a more nuanced discussion of these interconnected Southern Ocean dynamics.

Specific Comments:

1. The manuscript does mention subpolar gyres (L338-340: "Consistent with the development of the subpolar gyres to the south of the ACC..."). However, their role feels somewhat secondary to the direct influence of the fronts. Subpolar gyres are major conduits for heat towards the Antarctic continent and potentially influence subsurface ocean heat content and sea ice formation/melt. I think the authors can elaborate on how the ACC fronts interact with or shape these gyres, and how gyre dynamics themselves contribute to the SIE position. Is the gyre influence primarily a consequence of the ACC's path (as implied), or do they exert a more independent, synergistic control on the SIE alongside the fronts? Ultimately, the authors may want to present how the ACC is more important than gyres in locating the SIE. The discussion on EKE hotspots (L348-353) or somewhere around could be a place to better integrate gyre dynamics, as these are often associated with gyre boundaries or instabilities. As a consequence of such discussion, can we still say “the ACC is controlling the winter sea ice edge“?
2. The Antarctic Divergence is a circumpolar feature characterized by the zero zonal wind and the associated surface Ekman upwelling. This potentially impacts mixed layer depth and temperature, which might be crucial for sea ice formation and the position of the SIE. I hope the manuscript should explicitly discuss the potential role of the Antarctic Divergence. How does its mean position relate to the ACC fronts and the SIE? Could variations in upwelling along the Divergence explain some of the regional variability in the SIE or the distance between the PF and the SIE? How is it related to the surface meridional winds mentioned in the manuscript? The influence is not just about heat transported from the PF, but also about heat supplied from ocean closer to the ice edge via wind-driven divergence of ice floes.
3. While the correlations are strong, the term "control" sounds like a very direct and dominant causal mechanism. The ACC fronts (and gyres) are themselves largely controlled by bathymetry. The paper argues the fronts are key mediators of this bathymetric influence on the SIE. This is plausible. While the authors do use "constrain" and "influence," consider if the overarching message of "control" is fully supported for all aspects, or if wording like "influence" or "constrain" could be more accurate in some contexts (especially when considering currently unaddressed roles of gyres and the Divergence).
4. The two proposed mechanisms (eddy heat transport and atmospheric heat transport mediated by SSTs near the PF) are The link between southward-directed winds and atmospheric heat transport (L391-396) is interesting. However, the correlation between meridional winds and the PF-SIE distance is positive (Table 2, Fig 4), suggesting stronger southerlies are associated with a larger PF-SIE distance. The text (L312-318) acknowledges this and argues against wind-driven sea ice transport being the primary factor for this correlation. The subsequent argument for winds influencing atmospheric heat transport (L391-396) needs to be carefully reconciled with this earlier point to avoid reader confusion. Perhaps the argument is that despite stronger southerlies pushing ice north (which would intuitively decrease the PF-SIE distance if the PF were a fixed barrier), the atmospheric heat transport effect in regions with southward winds (from PF to SIE) is more dominant in setting the SIE further south (thus increasing the PF-SIE distance if the PF is far north). I think this needs very clear articulation.
5. To better understand the spatial extent of the ACC front-SIE relationship, I hope the authors to consider estimating a characteristic horizontal length scale of the observed high correlations. Analyses like lagged spatial correlation or spectral methods could achieve this.
6. Although the focus on the climatological mean is a valid simplification, it would be beneficial to briefly acknowledge in the discussion that interannual variability of winter sea ice edge, regarding the recent sea ice extremes, even if it's beyond the scope of this paper.
7. L290: “the further north the fronts are, the larger the distance between the fronts and the winter ice edge.” – Intriguing. I wonder why this is the case, and the authors might also want to explain more about it (perhaps in terms of the meridional gradient of ocean temperature and/or EKE).
8. The Results section is quite extensive. Please consider dividing it into thematic subsections for readability.
9. Fig 2: Adding the winter sea ice edge would be useful for interpretation. Please also clarify which definition is adopted for the frontal positions (perhaps Orsi or Park) in the caption.
10. Fig 4: Please unify the y-axis ticks for the wind velocity.
11. Is Fig 8 supposed to be referred in somewhere around L409-420?
12. Fig 9: Might be more effective and easier to interpret if presented in a normal plan view (top-down map perspective) rather than the current tilted view.

I believe that addressing these points will strengthen the manuscript and offer a more balanced perspective on the complex oceanographic controls influencing the Antarctic winter sea ice edge. This work is otherwise of high quality and is well-suited for publication in The Cryosphere. I sincerely thank the authors for their valuable contribution and look forward to their response.

Best regards,

Dr Kaihe Yamazaki

ARC Australian Centre for Excellence in Antarctic Science (ACEAS)

Institute for Marine and Antarctic Studie, University of Tasmania