General comments

This study defines a new index for characterizing the marine coastal winds around the coast Antarctica. Compared to the minimum zonal wind boundary (MZWB), the new defined Antarctic coastal wind boundary (ACWB) better reflects the influence of coastal topography and the katabatic winds that pour off the Antarctica continent while still being influence by the large-scale circulation patterns. Then they calculate future changes in the ACWB using CMIP6 simulations which show differing positional changes in the lower latitude westerly jet and the near-coastal winds. The methodology for creating the ACWB appears very sound and is described clearly. The authors demonstrate a solid understanding of Antarctic boundary layer and synoptic-scale meteorological processes which raises confidence in their analysis. I believe this manuscript will be an important contribution to the Antarctic meteorological community and is suitable for publication after some minor comments are addressed.

-Jonathan Wille

Specific comments

1. Methods: While I appreciate the narrative logic of defining the Antarctic coastal wind boundary (ACWB) in Section 4, I feel like it would be clearer for the reader if it was defined in the Methods section (Section 3). Seeing it defined just after or before the minimum zonal wind boundary (MZWB) would make it easier to differentiate the two when they are discussed together later in the text.
2. Section 4.4: The highlight for the Ross Sea is compelling, but it could be interesting to see the analysis extended over the Ross Ice Shelf as well. When looking at Figure 2, several interesting features appear over the ice shelf. You see the Ross Ice Shelf Airstream along the Transantarctic Mountains and the barrier wind corner jet near the Prince Olav Mountains. If you want, you can incorporate the difference in actual and large-scale directional consistency as shown in Figure 2b to show how your analysis also highlights the influence of polar jet stream/westerlies in controlling these inland Antarctic features. (Nigro et al. 2012; Seefeldt and Cassano 2012).
3. If it is not computationally expensive, it could be interesting to see how future climate projections in the position of the MZWB are different than the ACWB. This may potentially highlight the different future climate interpretations that come from utilizing the ACWB instead of the MZWB.

Minor comments

Line 10: If there is space, perhaps include a broader implication about what a reduction in the extent of the circumpolar trough means for the Antarctic climate

Line 122: Do you mean 10 m above the surface? Please clarify.

Fig. 2: For clarity, I suggest replacing “the actual minus large-scale directional constancy” with “the difference between the actual and large-scale directional constancy”

Line 153: Add, “Beyond the MZWB, …” before the sentence “The directional constancy increases further equatorward into the region dominated by westerly flow.”

Fig. 3a and 3b: Is it possible to slightly extend the northern bound of the figures to show more of the ocean and thus the westerly jet?

Line 188: Assuming you are referring to the westerly winds north of the low consistency band, then you should say at “lower latitudes” than “higher latitudes”. Higher latitudes refer to getting closer to the Poles.

Line 227-239: Please mention if these correlations are statistically significant.

Figure 7: Please include a JLI and ACWB label for the two boxes.

Line 328-329: Could you possibly infer/provide an educated guess as to what could be the effects on the Antarctic climate from this projected mismatch in poleward contradiction between the westerly jet and the ACWB?