Review comments for "Drivers and rarity of the strong 1940s westerly wind event over the Amundsen Sea, West Antarctica" by O'Connor et al. (tc-2023-16).

This study uses paleoclimate reconstructions to investigate atmospheric events in 1940s, which is supposed to be a trigger for the West Antarctic ice sheet retreat through wind-driven oceanic ice-shelf melting. The results demonstrate that the 1938-1942 anticyclonic anomalies and the associated westerly wind anomalies in the West Antarctic region can not be explained only by the El Niño event in 1940-1942, and thus other factors contributed to the atmospheric event. Furthermore, this study quantifies how rare this phenomenon is by comparing the characteristics (magnitude and persistent length) to results from several climate model simulations. In atmospheric reanalysis datasets that are widely available today (e.g., ERA5), atmospheric fields prior to 1979 are not strongly constrained by the data, and therefore, there are large uncertainties. However, it is important to know the long-term atmospheric variability for understanding climate components such as the Antarctic ice sheet and ocean, which have a long memory. Therefore, it is vital to understand the atmospheric conditions in the first half of the 20th century, and the atmospheric reconstructions by paleoclimate data in this study are very useful for this purpose. Thus, I believe that the purpose of this study is to the scope of The Cryosphere. However, I have some suggestions about the presentation and conclusions (see Major comments). I hope my comments will be helpful for the authors to improve the manuscript.

Major comments

1. Motivation and Conclusion.

The manuscript starts with a motivation of understanding the retreat of the West Antarctic Ice Sheet in the 1940s due to the increased heat transport of Circumpolar Deep Water onto the continental shelf regions driven by the westerly wind anomalies in the 1940s (e.g. L10-14, and L26-43). However, the conclusion ends with the following sentences “Our results suggest that the 1940s event was probably not unprecedented in the Holocene. However, if the event were superimposed on favorable oceanic or glaciological conditions, or followed by anthropogenically forced trends, the event may have played a role in initiating ice loss. Ocean simulations forced by realistic climate histories, and continued direct observations in the field, are needed to better constrain the mechanisms responsible for glacier retreat in West Antarctica.”, meaning that the westerly wind anomaly associated with the anticyclonic pressure anomalies is not a main driving force for the enhanced ice-shelf basal melting and the subsequent ice-sheet retreat. It was very confusing for me. Of course, there is no problem in describing the relations of the atmospheric change with ice sheets and oceans in the introduction and discussion, but motivating with understanding ice-sheet retreat and the causal ocean change does not seem to be suitable for this study. In fact, there is little analysis or discussion of ice sheets and oceans. Perhaps it would be better to structure the manuscript to focus more on items of atmospheric science.

2. The expression about Holocene.

This study uses ensemble results from pre-industrial and 20th-century experiments to assess the rarity of the target atmospheric variability in the 1940s. In the manuscript, the authors discuss how many times such the atmospheric event occurs in 10,000 years, a comparable length to the Holocene. I think it’s very misleading to refer to them as the Holocene probability. The Holocene has a similar land-ocean distribution to the present, but the other forcing of solar radiation and the freshwater cycle through ice sheets was very different from the present (e.g, “Holocene climate variability” by Mayewski et al. 2004). Throughout the manuscript, the phrases of the Holocene should be rephrased/removed if the characteristics of the Holocene are not taken into account in the climate simulations. 

3. Sentences about the rarity (Section 4)

What is the threshold between common and uncommon. Although the probability of occurrence is examined in Section 4, it seems strange that the results (“common” and “uncommon”) can be changed simply by changing the length of the period.

 

4. The other variability (local variability or response to tropical forcing outside the Pacific, L296-297).

After reading the manuscript, I ended up not knowing what “the other variability” was. It seems essential to find out “the other variability” in the paleoclimate reconstructions (local response or Tropical Atlantic wave train in Li et al. 2021?). It seems to me that showing the extent of influence of the ice core data in the paleoclimate reconstruction would be helpful in understanding the variability and the pattern.

5. Readability (assumed readers)

While this manuscript may be understandable to researchers focusing on West Antarctica, I found it difficult to read for a broad general audience in The Cryosphere (including me). It would be easier to read if there were two large panels (as Fig 1) showing the mean atmospheric fields and the anomaly fields in 1940s. I hope that the locations of ice core and coral records are also plotted.

Specific comments

6. Appendices

Since the appendices have only one paragraph, I suggest the author to include them in the main text.

7. Fig. 2

All the ensembles hardly cross each other and remain parallel. What determines this variance? I think the scrambled reconstruction has the same variance. Is it correct?

8. Fig. 3

Panels in different columns use different projections and spatial domains. Could you please use the same spatial domain at least for SLP and Us? Furthermore, showing bathymetric features (e.g., 1000- and 3000-m depth contour as the representative of shelf break position) is helpful.

O’Connor et al use reconstructed winds and sea level pressure over the Amundsen Sea to quantify the rarity of westerly wind anomalies that occurred between the late 1930s and the early 1940s. These winds anomalies have been associated with anomalous ocean heat transport toward Amundsen Sea glaciers, possibly favouring the glacial retreat in the 1940s inferred from sediments. The authors use proxy-based reconstructions that have been shown to well reproduce present atmospheric variability. I found the analysis accurate and the results very interesting as they provide 1) new information on the mechanisms driving atmospheric variability in the Amundsen Sea and 2) useful insight on the role of atmospheric variability in changes of outlet glaciers in the Amundsen Sea. In particular, the authors find that this event was rare on centennial time scale, but common on millennial time scale. This suggests that multiple drivers beyond anomalous local winds in the 1940s acted to initiate the retreat of glaciers in the Amundsen Sea.  I have a couple of major comments below and few minor suggestions.

Major Comments

• Line 185-200: you refer here to internal climate variability. But is this internal climate variability reflecting the “modern climate variability” or does it include evolving climate variability over several thousands of years? e.g. is ENSO variability the same over the past 10000 years?
• Section 4: Is the rarity of the event relative to the last 10000 years or to a “repetition” of the present state? I think this should be clarified because this might have implications on connections with the stability of the WAIS over the past 10K years and the initiation of the retreat in the last century.

Minor comments

• Line 30. I like this introductory paragraph and find useful to put things into context. However, I feel the wording “There is evidence that these glaciers have been relatively stable for the last ~10,000 years (Larter et al., 2014), which implies that a change in ocean circulation, and a corresponding increase in heat delivery to the ice shelves, must have occurred to initiate the current stage of retreat” to be a bit too strong here. Could a slow change in ocean forcing and/or surface mass balance be part of the story?
• Are winds in the Amundsen Sea important only for ice shelf basal melting? What about carbon uptake, ecosystems, sea ice etc.? how a rare wind event would impact other components of the system? Might be worth adding a few lines, given that this manuscript does not focus on ice shelves.
• There are also other mechanisms (beyond shelf break winds) that have been proposed to potentially explain changes in ice shelf basal melting in the Amundsen Sea, including surface buoyancy forcing and remote forcing (Ross Gyre and melting of glaciers in the Western Peninsula). I would suggest to either mention that other mechanisms have been proposed and therefore this new study can provide new information on potential processes, or simplify and shorten the text highlighting that rare winds events can affect heat delivery to ice shelves and potentially their stability. Given the strong motivation and evidence based on sediments and ocean dynamical studies, the first option might be more appropriate.
• Line 130: “Magnitude” of what? Please specify.
• Line 215: “SAT” for surface air temperature?
• Line 465. I would provide a very short summary of the key results at the beginning of the Discussion.
• Line 500-505. The discussion here is very important as it highlights that many mechanisms are at play. As suggested before, some of these processes should be mentioned in the Introduction. Something that could be also discussed a bit more is the role of the IPO in all this. A recent study by Vance et al (https://doi.org/10.1038/s43247-022-00359-z ) suggests IPO anomalies in the 20^th century. Would this be important for the 1938-1942 event? Or for trend/interdecadal variability over the reconstructed period?
• Discussion and conclusion. Similar to the introduction, would an event like this affect other components of the system?

This study uses paleoclimate reconstructions to assess the drivers and rarity of the 1940s events that led to large ice shelf melting and glacier retreat in the ASE. These events were rare due to their large magnitude and duration. Local forcings in combination with a major ENSO led to the 1940s events, which are rare on centennial timescales but uncommon on millennial timescales. The paper is written in an elegant way, with clear messages and figures. The subject is of interest to glaciologists and oceanographers studying West Antarctica, and I recommend publication in The Cryosphere. Below are some comments and suggestions that might help to improve the manuscript.

Major:

1) The authors say they evaluated the rarity of the event in the Holocene, but their simulation goes back to ~2000 yr (lines 192-194). Although this is part of the Holocene period, it does not cover the full Holocene or the Holocene conditions. I need a better explanation of why they claim the analysis covers the Holocene. Or, the authors could change Holocene to a simple "10kyr period"?

2) Section 3.2: When I read "Drivers" (also in the title), I thought the authors would track down the origin of the rare 1940s event. But I have the feeling that they don't. Instead, their assumption is that other events in combination with ENSO could trigger and amplify the 1940s event. My questions remained: How this event was generated? What (local conditions) triggered this event? Saying that the 1940s event was due to local drivers and not totally ENSO-related does not properly address the "Drivers" of the 1940s event, in my opinion. I'd like to see more analysis on this to properly address what led to the 1940s event. I think this is a key question that could benefit this paper to be a greater contribution to the glaciological/ocean/atmospheric community. If the authors decided to not track down the causes of the 1940s event, which I understand can be a lot of work, I'd recommend avoiding using "Drivers". However, I think understanding the local conditions could be a great addition to the paper.

Minor: 

- Line 294: Figure 5 instead of Figure 4?

- Line 348: Figure 6e?

- Why the authors use 10kyr most of the time, but sometimes 10ka?

- In the authors' opinion, what is most relevant for the ice shelf melting that occurred in the ASE: duration or magnitude of the 1940s event?