Linking open-ocean polynyas and deep convection in the Southern Ocean across CMIP6 models

Lu, Shunzi; Marinov, Irina; Molodtsov, Sergey

doi:10.5194/egusphere-2026-3060

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https://doi.org/10.5194/egusphere-2026-3060

Preprints

10 Jul 2026

| 10 Jul 2026

Status: this preprint is open for discussion and under review for The Cryosphere (TC).

Linking open-ocean polynyas and deep convection in the Southern Ocean across CMIP6 models

Shunzi Lu, Irina Marinov, and Sergey Molodtsov

Abstract. Open-ocean polynyas (OOPs) and deep convection in the Southern Ocean are critical features of the global climate system, however, their representation and mutual dependence in climate models remain poorly understood. This study investigates the occurrence and coupling of OOPs and deep convection across 49 CMIP6 models using long-term pre-industrial control simulations. Our results reveal that while most models simulate both phenomena, their spatial and temporal co-occurrence varies substantially. Although deep convection is typically associated with surface salinification and heat loss, it does not always result in detectable polynyas. We identify two distinct regimes of OOPs across the ensemble: "deep OOPs", which are directly coupled to deep ocean convection, and "shallow OOPs", which form independently of deep mixing, likely driven by surface forcing or sea-ice divergence. The representation of these regimes is strongly influenced by the choice of ocean model component. These findings highlight the importance of process-based diagnostics in evaluating Southern Ocean overturning and suggest that the connection between surface polynyas and deep water formation is more complex than traditionally assumed in climate models.

Received: 05 Jun 2026 – Discussion started: 10 Jul 2026

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Shunzi Lu, Irina Marinov, and Sergey Molodtsov

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RC1:
'Comment on egusphere-2026-3060', Anonymous Referee #1, 28 Jul 2026 reply
Review of Linking open-ocean polynyas and deep convection in the Southern Ocean across CMIP6 models
This study examines Southern Ocean open-ocean polynyas (OOPs) across CMIP6 models. The study is extensive, analysing 49 models from the CMIP6 piControl ensemble using the final 500 years of data from each model. The authors use a combination of deep convection diagnostics, polynya detection methods, and a standard set of Southern Ocean variables, presenting frequency maps and correlations with sea ice and ocean variables.
The authors find that, while deep convection is associated with salinification and heat loss, it is not always associated with the occurrence of OOPs. This was a particularly interesting result. They identify two distinct types of OOPs within the models, referred to as deep and shallow polynyas, which are associated with different mixing processes and are highly model dependent.
Overall, the manuscript is very well written, with a clear experimental design and results presented in a logical and accessible manner. The science is of high quality, including appropriate statistical analyses, and fits well within the scope of The Cryosphere. The methodology is sound and well justified, acknowledging the wide range of approaches used in the literature for detecting both polynyas and deep convection, while adapting these methods to the availability of variables across CMIP6 models. Additionally, I particularly appreciated the comprehensive assessment of each model group.
The results will be of interest to researchers studying polynyas in either polar region, the mechanisms driving their formation, and their representation across climate models. The manuscript builds upon the currently limited CMIP6 sea ice literature and contributes to the growing body of work assessing polynya formation. With a few very minor corrections, I believe this manuscript will be suitable for publication in The Cryosphere. I therefore recommend minor revisions.
General comments
Several references require revision. In a number of places, citations appear somewhat arbitrary or overly specific. For example, (Bromwich and Kurtz, 1984; Thompson et al., 2020) are both valid references, but it is unclear why Thompson et al. (2020) is cited specifically when numerous other studies have examined the same topic. Consider either replacing these with a more representative citation, using e.g., or providing justification for selecting that particular study. This issue appears throughout the manuscript and should be addressed consistently.

On the deep convection, I think it would be interesting to look at the driving forces at the surface and with depth, and whether temperature or salinity change drives the convection, specifically when associated with the polynyas, in different regions and models in more detail. I appreciate that the authors briefly touch upon this in Figure 5, 6, 8 and the Appendix C, and you mention briefly on line 579 that the direction of the relationship between these variables and polynyas remain unclear and of course that salninification is a driving force, This may be extensive beyond the scope of the paper, however, maybe a small discussion point could be included.

Specific comments
Line 52: The 2016/17 Polynya should not be described as a full WSP. It is more accurately classified as a large MRP (e.g. Campbell et al., 2019; Cheon and Gordon, 2019).

Line 79: The statement "However, OOPs remain prevalent in CMIP6 simulations and are often overrepresented relative to observations (Mohrmann et al., 2021)." should be reconsidered. The comparison is somewhat problematic given the relatively short observational record compared with the much longer climate model simulations. OOP occurrence is highly variable over time, and models may exhibit extended periods with clustered events followed by long intervals without occurrence. As such, the assertion that OOPs are "overrepresented relative to observations" should either be supported with additional context.

Line 210: Repeated ‘Specifically’.

Line 230: I think that this is an important result. There is a lot of discussion around the quantitative definition of a polynya in the literature and showing that no single metric is consistent between models is important. In Figure 8 you even show that sea ice thickness varies vastly between models, which as you rightly mention, has been used as a polynya detection method in a number of studies.

Line 239: Could you specify what you mean my the OOP region? Do you mean the larger scale Weddell Sea/ Ross Sea etc, or do you mean directly under the masked polynya area in Figure 1?

Line 250: The wording "only 33 models show OOPs" is potentially misleading. Thirty-three models represent approximately 75% of the models that produce OOPs, which is a substantial proportion. Consider rephrasing to avoid implying that this is a small number.

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Citation: https://doi.org/10.5194/egusphere-2026-3060-RC1

Shunzi Lu, Irina Marinov, and Sergey Molodtsov

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Short summary

We studied how CMIP6 models represent Southern Ocean open water areas within sea ice, known as open-ocean polynyas, and the mixing of deep ocean waters, known as deep convection, and found that these two processes are not always connected, contrary to common assumptions. While some open-ocean polynyas are associated with episodes of convection, others form through different mechanisms. These results show that polynyas should not be used as a direct proxy for deep convection in climate models.


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