| 19 Nov 2025
Sea-ice ridges are a major component of Arctic sea-ice export through the Fram Strait
Abstract. This study presents seven years (2012–2019) of Arctic sea-ice draft observations from upward-looking sonars combined with coincident observations of ice drift velocity from four moorings located across the Arctic outflow in the Fram Strait at 78.83° N. The data set covers in total about 150 000 km of drifting Arctic sea ice, at a 1 m spatial resolution, providing one of the most extensive spatially referenced sea-ice draft records in the Arctic available today. Level ice makes up about 40–50 % of the ice cover, with modal ice thickness varying between 1 m and 2.5 m, and thicker level ice westward towards the east Greenland shelf. Using local level-ice thickness and a variable-threshold ridge detection algorithm, we identify and quantify the sizes of sea-ice ridges, including shallow ridges with keel drafts less than 5 m deep, often overlooked by traditional methods using a fixed threshold. The study highlights ridges as a significant component of the sea-ice cover, with keels covering some 20–30 % of the ice bottom and contributing 28–55 % of the total sea-ice volume. The typical spatial density varies from 6 to 9 individual ridges per kilometer of sea-ice, with approximately 3000 to 5500 ridges per month at each site. A westward increase in ridge frequency and coverage was associated with the differences in the origin of sea ice arriving at the mooring locations. Further, we show that shallow ridges comprise up to 80 % of all ridges and 35–45 % of the ridged ice volume, and thus play an important role in the sea-ice volume budget. Thus, shallow ridges deserve greater attention, especially given the ongoing changes in the Arctic sea-ice cover.
Competing interests: Sebastian Gerland is an editor in The Cryosphere
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This paper presents seven years (2012 to 2019) of spatially referenced sea‑ice draft observations from upward‑looking sonars, combined with ADCP‑derived ice‑drift measurements at four moorings in Fram Strait. Using these data, the authors reconstruct meter‑scale spatial profiles of sea‑ice draft and quantify ridge frequency, keel‑depth distributions, and the areal and volumetric contribution of ridged ice to Arctic sea‑ice export.
Overall, the long‑term mooring record combined with spatial reconstruction is particularly valuable, and the focus on shallow ridges is timely given ongoing Arctic sea‑ice thinning. The main results are convincing and relevant for sea‑ice mass‑balance studies, ice-ocean drag parameterizations, and satellite-retrieval insights.
Main Comments
The title and main stated objective doesn't quite feel right to me. The real strength of the study to me lies in quantifying the contribution of sea‑ice ridges to regional sea‑ice mass balance in the region, which is articulated well at the beginning of the conclusion. Given this, I would encourage the authors to make this contribution/advance more clear in the title and framing.
In addition, this dataset represents an important modern complement to historical Fram Strait observations (largely from submarine sonar). Clearer and more quantitative comparisons with these prior estimates would significantly strengthen the manuscript, the introduction of proper campaigns and estimates is introduced quite sporadically and vaguely.
The manuscript often refers to “ice thickness,” yet the analysis is based entirely on ice draft I think! This is confusing and should be corrected throughout. If thickness is inferred or discussed conceptually, the assumptions and limitations should be clearly stated.
While the authors appropriately acknowledge the limited record length and data gaps, discussion of declining ridge numbers and shallower keel depths should be framed more explicitly as suggestive, rather than a robust trend analysis of any kind, given the significant interannual variability.
Adaptive approaches for estimating level‑ice surfaces are not really new, despite the framing of this paper. Relevant examples include methods based on low elevation change over a given along‑track distance (e.g., Wadhams and Horne, 1980) and modal elevation detection (e.g., Williams et al., 2015; sail analysis in Petty et al., 2016). I think it's better to make that much clearer.
The distinction between ice rubble and ridges (L143) is introduced rather briefly, despite being a potentially important conceptual and methodological point. A clearer explanation is needed I think!
The discussion in Section 3.4 is overly qualitative and vague. This section would benefit from clearer, more quantitative comparisons with existing studies of ridge fraction and ridge volume, ideally incorporated directly into figures rather than solely discussed in the text, to better contextualize these data.
The manuscript reports both ridge count and ridge density. The rationale for including both metrics and how they differ in interpretation should be explained explicitly or dropped, I didn't quite understand why ridge count was included.
The macroporosity discussion is confusing to me. Macroporosity affects ridge density rather than geometric volume I think. While excluding air pockets may be justified in terms of actual ice volume, I don't really understand if this was needed here..
Minor Comments
Figure 2: Since longitude is fixed for each mooring, a table might be more efficient. “2019-2019” should read “2012-2019.”
Figure 3: Clarify in the caption that fractions are calculated relative to ice‑covered periods only (open water excluded). Clarify the “mixed” category in the legend.
Figure 4 is difficult to interpret. Violin plots may provide a clearer representation of the thickness/draft PDFs. The meaning of the black line is unclear, and the phrase “maxima in modal” is confusing.
Figure 5: A similar analysis for daily ridge density within each year could be useful. While there are hints of seasonality, it is difficult to assess this from the current presentation. The added value of showing daily data as plotted should be clarified. Note that daily ridge densities are calculated over short drift distances (typically <20 km), contributing to high variability.
Figure 6: The explanation of symbols and intervals is unclear. The caption should clearly define that circles and horizontal lines show median ridge densities and the 67% prediction intervals?
Figure 10 caption: Clearly state which macroporosity assumption is used for the reported mean values and trends.
L11-12: This statement is speculative (though reasonable) and likely too strong for inclusion in the abstract.
L20: The claim that ridges are the “least studied component” of Arctic sea ice is too strong given extensive submarine, airborne, and growing satellite-based papers. Consider softening or qualifying.
L24: This sentence appears unnecessary given the breadth and maturity of the existing literature.
L29: Please verify this statement.
L33: When stating that the dataset is “one of the most extensive,” briefly clarify the basis for comparison (e.g., time span, spatial coverage).
L42: Please reword for clarity. More explicitly explain that ice velocity is required to convert time‑series draft measurements into spatial profiles.
L57: Too vague, please clarify whether this refers to ridging studies in general or specifically to keel measurements.
L74: Clarify whether the quoted ULS footprint diameter refers to the nominal instrument depth or varies with actual mooring depth.
L87: If ASL is a company, this detail may be unnecessary. Focus instead on how the processing is done and how the data are provided.
L89: Consider reporting ridge numbers per day or per hour, which may be more informative.
L91: A citation is needed; the stated value seems quite low to me.
L95-100: The discussion of draft uncertainty conflates instrument precision and interpretive uncertainty. Clarify that ~10 cm represents an estimated total uncertainty, including both random and systematic components.
L123-129: Could the same effect be achieved by simply lowering the threshold?
L200-205: When comparing level‑ice fractions to historical submarine studies, briefly note differences in season or sampling that limit direct comparison.
L272: “This analysis” is not an effective sentence opening, please rephrase.
References:
Petty, A. A., M. C. Tsamados, N. T. Kurtz, S. L. Farrell, T. Newman, J. P. Harbeck, D. L. Feltham, and J. A. Richter-Menge (2016), Characterizing Arctic sea ice topography using high-resolution IceBridge data, The Cryosphere, 10 (3), 1161–1179, doi:10.5194/tc-10-1161-2016.
Wadhams, P. and Horne, R. J.: An analysis of ice profiles obtained by submarine sonar in the Beaufort Sea, J. Glaciol., 25, 401–424, 1980.
Williams, G., Maksym, T., Wilkinson, J., Kunz, C., Murphy, C., Kimball, P., and Singh, H.: Thick and deformed Antarctic sea ice mapped with autonomous underwater vehicles, Nature Geosci., 8, 61–67, doi:10.1038/ngeo2299, 2015