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Classifying sea ice leads and floes in Swot data

Image of the Month - August 2026

Top, sea ice classification from Swot KaRIn measurements west of Banks island : leads in dark blue, floes in white, unsure in intermediate blues (righy), true color image from the MODIS instrument acquired on 4th April 2024 (left) Bottom, SSHA (left) and sigma 0 (right) from for all the SWOT ascending passes around Banks island from March 28th to April 10th 2024 (period used for the top right classification) (Credit Legos)
Top, sea ice classification from Swot KaRIn measurements west of Banks island : leads in dark blue, floes in white, unsure in intermediate blues (righy), true color image from the MODIS instrument acquired on 4th April 2024 (left)
Bottom, SSHA (left) and sigma 0 (right) from for all the SWOT ascending passes around Banks island from March 28th to April 10th 2024 (period used for the top right classification) (Credit Legos)

The Swot satellite observes sea ice along its two 60-km-wide swaths with a resolution of 250 m, while nadir altimeters observe it only along a single line directly beneath their trajectory. This new two-dimensional view allows us to observe leads (open water channels within sea ice areas) and floes (floating ice in the same areas) up to 78° latitude, covering the entire Southern Ocean and a large part of the Arctic Ocean.

For both studies of the ice and those focusing on the ocean, we need to know the sea level, which requires distinguishing between leads and floes, as the water height is observed in the leads. This enables us to reconstruct the Sea Leval Anomalies (SLA) as far as the polar oceans and to calculate the ice freeboard, and subsequently its thickness.

A study conducted by Legos establishes a classification of sea ice based on Swot data, distinguishing between leads, floes and "uncertain" intermediate states for each 250-metre pixel. This classification is now calculated as part of routine operations in Cnes's L3 250 m ocean products, and provided in the disseminated products (in the quality_flag variable). This will allow to improve our understanding of polar ocean circulation, variations in ice volume, sea ice dynamics and sea ice forecasts, whether for polar navigation or for climate studies.

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References:

  • Jestin, G., S. Fleury, M. Raynal, M. Alves, F. Piras, A. Treboutte, F. Boy2, I. Benabdillah, L. Yu, and G. Dibarboure, New high resolution sea ice leads and floes classification in the CNES SWOT product, submitted to The Cryosphere, https://doi.org/10.5194/egusphere-2026-2569
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