| 03 Mar 2026
Winter Arctic polynyas in CMIP6 models
Abstract. Winter Arctic polynyas, openings in the pack ice, play a crucial role for the climate from sea ice production to cloud formation and are hotspots for the ecosystem and human activity. Their area has significantly increased since satellite records began. Yet their representation has yet to be evaluated in any generation of global climate models, most likely because their automatic multi-model retrieval is challenging. We here use a newly-developed machine-learning based method and evaluate polynya activity against the satellite-derived one over 1979–2024 in the 18 models with daily sea ice concentration available. We find that models overestimate winter Arctic polynya area but underestimate its frequency, and limit their opening to the seasonally ice-covered regions. Polynya area is increasing in most models, but the bias of these trends are inconsistent. Although the model with the highest resolution has both the highest areas and frequencies, the sea ice model component is a more robust predictor of polynya activity, with most activity in the models whose thermodynamics scheme enhances ice growth/melt and least activity in models whose dynamics scheme dampens the influence of wind on ice. Accordingly, we found more polynyas in the models with a larger seasonal cycle, in particular those with a warmer autumn that would delay ice growth. Finally, we confirm preliminary findings that polynya activity does not seem to impact the representation of the water column; if anything, we find less-dense water at the bottom of the continental shelf following larger polynya activity, i.e. supposedly with dense water formation.
I have reviewed the manuscript titled „Winter Arctic polynyas in CMIP6 models“ by Heuzé et al..
Their study evaluates and compares how current (CMIP6) climate models represent polynyas. To my knowledge, such a study was missing for the Arctic (but exists for the Antarctic), despite the important role polynyas play in the Arctic climate system (e.g., sea ice production, biological hotspots/biodiversity, coupling of ocean to the atmosphere, etc.).
The authors use a newly developed polynya detection method (Wong et al., 2026) based on sea ice concentration (SIC) to compare yearly/winter mean polynya area, frequency and trends across models and against satellite observations. The writing is clear and the methodology is sound as the authors also discuss (the limits of) their detection method (e.g. testing different thresholds, the limitations in the marginal ice zone, the dependence of the results on grid resolution, etc.) as well as their statistical diagnostics (e.g. using cumulative polynya area instead of winter mean area, using monthly instead of daily SIC data, etc.). Data and methods are well described (Section 2) and data and source code are mostly openly available (or will be made available soon). Also, the results sections benefit from a clear structure with summaries at the end of subsections. The potential causes of biases are thoroughly and concisely analyzed, using correlations as main (and available) tool, and discussed in Section 4. The impact on the ocean (deep) water representation, a question motivating this study, is exemplarily discussed with well chosen indicators of the water column.
The figures and tables are overall well done.
The last chapter summarizes the main findings and new insights of the study and provides an outlook on how to improve the representation of polynyas in climate model but could benefit from minor improvements. Overall, the study is of high quality and a step towards understanding how polynya (processes) are biased in climate models.
Below, I provide more specific comments and suggestions for improvement. I am highlighting the comments I find most important as bold:
- Figure 2: are the models ordered in any particular way (e.g., based on resolution or the sea ice model used)?
- Line 218 ff. how could this value (the percentage of region where polynyas have opened for more than 25 years) be impacted by changing SIC/marginal ice zone conditions across models/years, e.g. in the mentioned East Siberian Sea?
- Line 235 ff. Trend: could it be related to retreating sea ice/“false“ detection of polynyas in the marginal ice zone? On page 9 you discuss the relation of SIA/SIE to polynya activity, I suggest you add a sentence about the relation of SIA/SIE trends and polynya trends if possible.
- Line 250 ff.: Clustering by sea ice model: I find this hard to tell from the Figure. I agree that a clustering of star symbols is visible for region 5 but I do not see it for region 6 as stated in line 252, for example. I suggest to either try to illustrate it more clearly in the figure (or an extra figure), maybe with semi-transparent symbols or a reduction to the two most common ice models, or by writing it in the text more explicitely (à la “clustering in the upper right quadrant…“). I find this especially important as part of the conclusions are drawn from this (see Abstract: „with most activity in the models whose thermodynamics scheme enhances ice growth/melt and least activity in models whose dynamics scheme dampens the influence of wind on ice“). Maybe you could indicate which sea ice model is used per ensemble in Figure 2.
- line 257: how short is short-lived?
- Figure 5: please remove the plotting artefact
- Figure 5, j): what is going on around 70°E? The white stripe looks suspicious.
- Section 3.2.: Using monthly SIC data comes with many drawbacks and the section nicely dissects them. However I find its size compared to the other sections disproportional for the (somewhat) expected results and recommendations (at least it distracted me a bit from the main story) and therefore I suggest to consider moving one of the two figures or even part of the text to the appendix.
- Section 4: Please define autumn and summer somewhere here or earlier (as e.g. summer sea ice thickness is often referred to in this section).
- Line 319: „by definition where there is sea ice, polynyas can open“ This could be true also for other regions (?), consider adding a sentence why this is seen only for Svalbard.
- Line 322 ff. and also line 394-396: Have you considered applying methods like time-lagged correlation analysis to back up your statement?
- Line 348: Point 3, correlation to the equilibrium climate sensitivity: This hit me by surprise as the sentence starts with „To summarise“ but climate sensitivity has not been mentioned anywhere before. Please elaborate.
- Line 359 I suggest you add something like „ In sea ice models, it is the rheological formulation and roughness“ (or „ drag“)
- Line 391: As you consider December-March I would rephrase (it sounds as if warm air thins, that is, melts, the ice, I guess what is meant is that growth is slowed down). Another question I have here is whether the warmer winter air temperatures could be a consequence of larger polynya areas. Was this feedback considered?
- Section 4.2. line. 404 ff. : I find the conclusions drawn here plausible, but I am missing a statement about the link of summer thickness and autumn thickness (how are they related/correlated?) or even something like a time-lagged correlation analysis to confirm that the thin summer ice is a consequence of large polynya activity while the thin autumn ice pre-conditiones for polynya opening.
- Section 5.1 line. 521. I would mention that this includes the “outlier“ model“
- Figure 7 and Figure 8: I suggest to use different shadings of blue and red to distinguish between the models
- Section 5.2. Line 464-465: “At the end of Section 4, we suspected that the models with the highest activity formed polynyas by melting the ice rather than by dynamic breaking due to strong winds.“ Where can I find this statement in Section 4? I understood there, that the thin autumn ice (which may or may not be a consequence of more melt, it could also be caused by less growth) preconditions the ice for dynamic breaking. Please explain. Is this based on the clustering by models? I think the study would benefit from a clearer distinction between air temperatures slowing growth and actual melt.
- Line 475: „mosty likely a result of how models open their polynya“. I am curious: could it be also a result of how models close their polynya? Is there a way for a model to dynamically close the polynya (without/little ice growth and thus, no dense-water generation)?
- Line 495: „no consistent correlation at the individual model level“: this seems to contradict what you wrote in lines 366 ff or line 404 ff. Please explain.
- Line 502: „Our results do suggest however that polynya misrepresentation in CMIP6, in particular their too low frequency, may be caused by a reduced effect of wind on ice in climate models“. Please elaborate on this a bit more as this was not my take-away message from section 4. 2. For example, what makes you believe the effect is reduced?
- Line 514: „further suggesting (…) sea ice melt …“: Here, my comment refers more to the previous comment on lines 464 ff.: where was it stated that sea ice indeed melts in the models for polynyas to form? From my understanding of the section 4, the ice thickness is important but this is not necessarily a consequence of more melt. However I agree that the fresh water in the ocean models suggest it.
- Figure A3 caption: I am not sure I understood the red arrows („point in the direction of its polynya area“), does it mean the values are outside of the x-axis limits?
technical:
- Consistent writing of Section vs. section
- Table 1: more complete if sivol is added in „comment“ column
- line 146 missing „and“
- line 154: refer to figure number
- line 426 typo „because of“