the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 13 Mar 2025
Review article: Weddell Sea polynya Formation, Cessation and Climatic Impacts
Abstract. Open-ocean polynyas, openings in the sea ice, reappeared extensively in 2016 and 2017 over the Maud Rise in the Weddell Sea after a 40-year hiatus, raising a series of unresolved questions about the atmosphere-ice-ocean interactions in the Antarctic region. These major polynyas significantly influence moisture and heat exchange between the atmosphere and ocean, impacting both regional and global climate dynamics, as well as ecosystem functioning and biogeochemical processes. Notably, they could play a crucial role in contributing to the formation of Antarctic Bottom Water and influencing global ocean circulation. In this Review, we synthesize current knowledge on the drivers and impacts of Weddell Sea polynyas. Recent occurrences have been linked to factors such as a strengthening Weddell Gyre, a negative Southern Annular Mode, extreme local atmospheric conditions (atmosphere river and cyclones), and subsurface ocean heat buildup which acts as a preconditioning factor. The associated deep ocean convection from these polynyas can enhance air-sea gas exchange and trigger earlier phytoplankton blooms due to the influx of iron and nutrients from the deep ocean. While advancements in observation and modeling techniques have significantly improved our understanding of polynyas, substantial uncertainties remain regarding their interaction with recent Antarctic sea ice loss, their sensitivity to ocean mixing schemes and their excessive size or frequency in climate simulations, and future projections. Therefore, future research should focus on developing comprehensive four-dimensional regional observatories and targeted, assimilated coupled models that accurately capture atmosphere-ice-ocean interactions across various timescales.
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RC1: 'Comment on egusphere-2025-999', Anonymous Referee #1, 21 Mar 2025
General comments
This study synthesized current knowledge on the drivers and impacts of Weddell Sea polynyas (WSPs), as the open-ocean polynyas that rarely exist in Antarctica. Given their importance in atmosphere-ocean energy and mass exchange, sea ice production, and formation of AABW, it is necessary to systematically review previous studies to draw pictures on the formation and cessation of the polynyas, their driver mechanisms, climate impacts, as well as current gaps in observations, modellings and knowledge. This study would benefit a deeper understanding of the roles of the open-ocean polynyas in the air-sea ice-ocean interactions in Antarctica, especially in the context of the fast-declining Antarctic sea ice in recent years.
The paper is well-written and organized. The research progresses and current knowledge of WSPs are summarized sufficiently. However, some revisions should be made before it is accepted for publishing:
- Section 3.4. It is suggested to add some quantitative summaries of the changes in biogeochemical parameters due to the appearance of WSPs, e.g., the oxygen concentration, carbon concentration, nutrients, etc.
- Section 6. The Conclusion needs to be further improved. In my view the conclusion needs to summarize the following four parts: 1) the drivers of the formation and cessation of the WSPs, 2) the impact of WSPs on energy and mass exchange between the coupled atmosphere-sea ice-ocean system, 3) future projections of WSPs in response to global warming, and 4) current uncertainties and gaps in observations, modellings, and understandings in WSPs. The current version is more like the part 4 only.
- Additionally, it is recommended to add a new section before Conclusion, discussing the current uncertainties and gaps in observations, modellings, and understandings in WSPs and future perspectives. It will be even better to list a table summarizing the differences (quantitatively or qualitatively) between study results on WSPs in terms of, e.g., formation and duration period, ice productions, heat release, climate impacts, projected occurrence frequencies, etc. This would help the readers quickly understand the progress and uncertainties in current WSPs studies.
Specific comments
P: page, L: line
P4, L110. The authors are suggested to add some descriptions of how previous studies identified and quantified the WSP or MRP events in 2016 and 2017 (e.g., by in-situ observations, satellite observations, or modellings).
P8, L243. Better briefly describe the patterns/features of the three Zonal Wave patterns 1-3.
P9, L286. The authors summarized the study of Ayres et al. (2024) that the atmospheric response to the 1974 WSP is “localized, vertically confined to the boundary layer, and short-lived”. Are there any quantitative statistics, e.g., the extent, area, height, and duration?
Citation: https://doi.org/10.5194/egusphere-2025-999-RC1 -
AC1: 'Reply on RC1', Lu Zhou, 22 May 2025
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-999/egusphere-2025-999-AC1-supplement.pdf
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RC2: 'Comment on egusphere-2025-999', Anonymous Referee #2, 11 Apr 2025
The Weddell Sea polynya, first observed in the mid-1970s through early satellite microwave imagery, is a highly intriguing phenomenon within the coupled atmosphere–sea ice–ocean system and has been the focus of several studies. However, it did not reappear for several decades, and due to a lack of observational data—particularly in situ measurements—research on the phenomenon remained limited. However, during the winters of 2016 and 2017, although on a smaller spatial and temporal scale, a region of open ocean re-emerged over the Maud Rise area, prompting renewed investigations from various perspectives.
This manuscript provides a comprehensive synthesis and integration of numerous previous studies on this fascinating natural phenomenon, resulting in a highly valuable review. To further enhance the quality of this review paper, I would like to provide the following comments.
P.1, L.1. “Abstract”: Polynyas are generally described as open water areas within sea ice regions. However, in reality—particularly for coastal polynyas during winter—they are more often characterized by areas of frazil ice or new/thin ice. I believe this distinction should be described more precisely to reflect the actual conditions.
P.2, L.32 “less frequent”: Compared to coastal polynyas that appear almost daily during winter in the same region, sensible heat polynyas occur far less frequently and are regarded as extremely rare phenomena. Therefore, the term “less frequent” may be too mild, and a stronger expression such as “rare” or “very infrequent” would more accurately reflect the true nature of their occurrence.
P.2, L.51 “the current unprecedented decline in Southern Ocean sea ice”: The rapid decline in sea-ice extent in the Southern Ocean has emerged as a particularly prominent topic in recent years. In this context, it would be valuable to also address the potential relationship between this decline and polynya occurrences, especially in connection with the future projections discussed later in the modeling results. Including such a discussion could provide a more comprehensive review and further strengthen the overall impact of the manuscript.
P.6, L.156 “Fig. 5”: Figure 5 is introduced prior to Figure 4, creating a mismatch between the figure numbering and the order in which they appear in the text. For clarity and to improve reader comprehension, it would be preferable to present the figures in the same sequence as they are referenced.
P.6, L.163-164 “Taylor Cap” and “warm-water Halo”: If possible, visually indicating these characteristic structures in Figure 3 would greatly enhance reader understanding. In addition, labeling “Maud Rise” on the figure would help readers more easily connect the figure with the corresponding discussion in the text, making the illustration more informative and user-friendly.
Paragraph beginning on P. 11, L. 320 – Comment: The melt–freeze cycle described in this paragraph appears to be a plausible process and likely plays an important role in the maintenance of the polynya. However, even with increased sea-ice production in such cases, the salinity of the underlying water may already be too low to allow the formation of sufficiently dense water necessary for Antarctic Bottom Water (AABW) formation. Including a brief discussion of this aspect could further enrich the overall argument.
P.13, L.413 “A minor component of the observed abyssal warming since the 1990s (Purkey and Johnson, 2010) could be linked to recovery from the mid-1970s WSP event (Zanowski et al., 2015).”: This statement is quite intriguing; however, the roughly 20-year time gap between the events makes the proposed causal relationship somewhat difficult for readers to intuitively grasp. Including a brief explanation of the mechanism by which such an effect could emerge after such a long interval would help strengthen the argument and improve clarity.
Citation: https://doi.org/10.5194/egusphere-2025-999-RC2 -
AC2: 'Reply on RC2', Lu Zhou, 22 May 2025
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-999/egusphere-2025-999-AC2-supplement.pdf
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AC2: 'Reply on RC2', Lu Zhou, 22 May 2025
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