the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 19 Dec 2025
Climatology and Interannual Variations in Arctic Winter Sea Ice Leads in the ICESat-2 Era
Abstract. Sea ice leads play a key role in polar air-sea heat, moisture, and gas exchanges, ocean heat and salinity variations, and ecosystem processes. However, their small-scale nature challenges efforts to assess spatiotemporal variability on a pan-Arctic basis. Here, we use six years of high spatial resolution surface type (ATL07) and freeboard (ATL10) products from Ice, Cloud, and Land Elevation Satellite-2 (ICESat-2) to characterize Arctic winter sea ice leads. Both products reveal consistent climatological spatial patterns, with lead fractions generally higher near the ice edge and coastal regions, and lower over the central abyssal plains. Lead sizes follow a power-law distribution, with the exponent increasing with size. We identify four distinct features in the temporal evolution of lead fraction over the ICESat-2 era, including a maximum in winter 2020–2021; increases in lead fraction are primarily driven by changes in the number of larger (>100 m) leads. Our findings show that ICESat-2 measurements provide robust regional-scale characterization of spatiotemporal variability in winter ice leads, which will in turn better inform their underlying response to, and influence on, Arctic climate.
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Status: open (until 30 Jan 2026)
- RC1: 'Comment on egusphere-2025-6155', Anonymous Referee #1, 07 Jan 2026 reply
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RC2: 'Comment on egusphere-2025-6155', Anonymous Referee #2, 19 Jan 2026
reply
General Comments
In this manuscript, the authors map sea ice leads from ICESat-2 across the Arctic in winter from 2018 to 2024, using both ATL07 and ATL10. Interannual and regional variations in lead fraction, and differences between ATL07 and ATL10, are presented and discussed. The paper is well written and of interest to the community. I have some general comments to improve the depth of the analysis and discussion but recommend publication following these revisions.
There are discrepancies between ATL07 and ATL10 regarding the magnitude of lead fraction; in Section 5 Line 246, the magnitude of ATL10 lead fraction is noted as ~5 times greater than in ATL07. The choice of freeboard threshold is interrogated in Section 5.2. which is useful, but I think further interrogation of ATL07 compared to ATL10 is important. The strict classification of specular leads in ATL07 reduces the number of leads detected, but how does this compare to where ATL10 detects leads? Are they detecting the same leads? Perhaps comparison to some imagery would be useful to understand where the differences are coming from.
In Section 4.1.1., and Figure 2, there are notable differences between ATL07 and ATL10 specifically over the Canadian Arctic Archipelago (CAA) and south of the New Siberian Islands. This is an interesting result which should be developed further; the difference is briefly mentioned in Section 5 Lines 245-249, but there is not discussion about why these differences are present specifically in these regions, and what this means for interpreting the ATL07/ATL10 lead climatology and resultant sea surface height and freeboard in these areas.
Specific Comments
Line 7 – I think a full stop would be better here, i.e. ‘…including a maximum in winter 2020-2021. Increases in lead fraction are primarily driven by changes in the number of larger (>100 m) leads.’
Line 23 – Could you include a reference for ‘small leads comprise the majority of the lead area’?
Line 28 – I think it’s important to clarify the different challenges for the different types of sensor, e.g., SAR vs thermal infrared imagery.
Table 1 – I find the layout of this table quite hard to read. Please resize the table to improve the formatting.
Table 1 – Could you comment on how the different sea ice concentration filters in ATL07 and ATL10 may influence their lead detection? ATL10 will exclude areas with a lower ice concentration and presumably therefore exclude areas with a higher lead fraction.
Figure 1 – Please adjust the figure size so that less of the text in (a) is overlapping with the plot.
Figure 1 – Why does the number of segments increase towards the pole hole and then decrease again (but remain above zero)? This may just be my misunderstanding, but I would’ve expected the number of segments to increase with the increase in track density towards the pole, but then be zero above 88˚N.
Section 3.2. – Does the lead fraction calculation require a minimum number of valid points? For example, cloud coverage could obscure the majority of a track in a grid cell, which would distort the resultant lead fraction.
Figure 4, caption – I’m not sure I understand what is meant by ‘only grid cells with the same signs in the two products are shown’.
Figures 5 and 6 – I don't think that Archipelago should be the plural ‘Archipelagos’ here, assuming it is only referring to the Canadian Arctic Archipelago? Perhaps it would be useful to use the CAA acronym on the Figures.
Figure 7. Please increase the size of the points in the legend in (d), it is very difficult to make out the colors.
Line 277 – I think this paragraph could be further developed. What do MODIS, Envisat, and CryoSat-2 show in these studies? Are the spatial patterns/magnitudes of lead fraction consistent? There is also a more recent MODIS climatology, from:
Willmes et al. (2023). Patterns of Arctic sea-ice leads and their relation to winds and ocean currents. The Cryosphere, 17, 3291-3308, DOI:10.5194/tc-17-3291-2023.
Grammatical Comments
Line 26 – CryoSat-2
Line 89 – selected to be a sea surface
Line 122 – problems
Line 152 – the smallest ratio is present near the ice edge… opposite to the lead fraction
Line 153 – potential reasons
Line 160 – south of the New Siberian Islands
Line 162 – the total number of leads
Line 163 – lead numbers in both products
Line 164 – similar conclusions can be drawn
Line 235 – more large leads
Line 276 – consistent
Please check for consistency in capitalizing regions, e.g., Section 4.2.1. and Figure 6 ‘Central Arctic’ versus ‘central Arctic’.
Citation: https://doi.org/10.5194/egusphere-2025-6155-RC2
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- 1
General comments
This manuscript presents a valuable pan-Arctic analysis of winter lead fraction and lead-size statistics derived from ICESat-2, using two complementary products (ATL07 and ATL10). The topic is timely and relevant, and the results (spatial patterns, regional interannual variability, and lead-size scaling behaviour) will be of broad interest. I have several comments that would improve clarity and strengthen the ATL07–ATL10 comparison and the interpretation of the power-law results.
Specific comments
1. Sect. 2 (ATL07 vs ATL10 comparability; ice concentration filtering):
The manuscript applies different sea-ice concentration filters for ATL07 and ATL10 (and the products have different lead definitions). Perhaps discuss how the differing concentration masks could influence the ATL07–ATL10 comparison (e.g., particularly near the ice edge and in marginal seas)/
2. Sects. 2.1–2.2 (methods clarity):
The processing description would benefit from a flow chart alongside the text; this would help readers follow the differences between the ATL07 and ATL10 workflows and provide a nice visual summary.
3. Definition of “climatological wintertime lead fraction” (p7):
Formally define what is meant by the “climatological wintertime lead fraction” (e.g., is the climatology computed by pooling all winter observations across years in each grid cell).
4. Sect. 4.1.2 (power-law / scaling context):
The power-law analysis is interesting and could be better contextualised within the broader literature on geometric scaling in sea ice (while being clear about differences between lead-size and floe-size statistics). The discussion could reference, for example:
o Stern, H. L. (2018). On reconciling disparate studies of the sea-ice floe size distribution. Elementa: Science of the Anthropocene, 6, 49. https://doi.org/10.1525/elementa.304
o Stern, H. L., et al. (2018). Seasonal evolution of the sea-ice floe size distribution in the Beaufort and Chukchi seas. Elementa: Science of the Anthropocene, 6, 48. https://doi.org/10.1525/elementa.305
(These are floe-size focused, but may help frame interpretation of scaling regimes and physical processes.)
5. Figure 3a (readability):
Figure 3a is difficult to read at its current size. Consider splitting into two panels/figures (one for lead-size distribution, one for segment-number distribution) or enlarging/adjusting layout so axes and legends are legible.
6. Figure 6 (visual consistency):
The mix of lines (ATL10) and bars (ATL07) is visually confusing. I recommend using a consistent representation (e.g., lines for both products, or separate panels). Also please adjust titles/labels to avoid overlapping plotted data.
7. Sect. 5 (interpretation of apparent scaling changes):
The apparent change in power-law scaling is an important result. It would benefit from a deeper discussion of plausible mechanisms and why they might differ by region and year (e.g., fracture/strain regimes, wind forcing, consolidation/thermodynamic growth, lead refreezing).