Drivers and rarity of the strong 1940s westerly wind event over the Amundsen Sea, West Antarctica
Abstract. Glaciers in the Amundsen Sea Embayment of West Antarctica are rapidly retreating and contributing to sea level rise. Ice loss is occurring primarily via exposure to warm ocean water, which varies in response to local wind variability. There is evidence that retreat was initiated in the mid-20th century, but the perturbation that may have triggered retreat remains unknown. A leading hypothesis is that large pressure and wind anomalies in the 1940s drove exceptionally strong oceanic ice-shelf melting. However, the characteristics, drivers, and rarity of the atmospheric event remain poorly constrained. We investigate the 1940s atmospheric event using paleoclimate reconstructions and climate model simulations. The reconstructions show that large westerly wind anomalies occurred from ~1938–1942, a combined response to the very large El Niño event from 1940–1942 and other variability beginning years earlier. Climate model simulations provide evidence that events of similar magnitude and duration are unusual but may have occurred tens to hundreds of times throughout the Holocene. Our results suggest that the 1940s westerly event is unlikely to have been exceptional enough to be the sole explanation for the initiation of Amundsen Sea glacier retreat. Naturally arising variability in ocean conditions prior to the 1940s, or anthropogenically driven trends since the 1940s, may be needed to explain the onset of retreat in West Antarctica.
Gemma K. O'Connor et al.
Status: final response (author comments only)
RC1: 'Comment on tc-2023-16', Anonymous Referee #1, 23 Mar 2023
- AC1: 'Reply on RC1', Gemma OConnor, 19 May 2023
RC2: 'Comment on tc-2023-16', Anonymous Referee #2, 24 Mar 2023
- AC2: 'Reply on RC2', Gemma OConnor, 19 May 2023
RC3: 'Comment on tc-2023-16', Anonymous Referee #3, 27 Mar 2023
- AC3: 'Reply on RC3', Gemma OConnor, 19 May 2023
Gemma K. O'Connor et al.
Gemma K. O'Connor et al.
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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.
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.
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.