Impacts of air fraction increase on Arctic sea-ice thickness retrieval during melt season

• Main Authors: Salganik, Evgenii, Crabeck, Odile, Fuchs, Niels, Hutter, Nils, Anhaus, Philipp, Landy, Jack Christopher
• Format: Text
• Language: English
• Published: 2024
• Subjects: Arctic; Sea ice
• Online Access: https://doi.org/10.5194/egusphere-2024-2398; https://egusphere.copernicus.org/preprints/2024/egusphere-2024-2398/

Uncertainties in sea-ice density lead to high uncertainties in ice thickness retrieval from its freeboard. During the MOSAiC expedition, we observed the first-year ice (FYI) freeboard increase by 0.02 m while its thickness decreased by 0.5 m during the Arctic melt season in June–July 2020. Over the same period, the FYI density decreased from 910 kg m -3 to 880 kg m -3 , and the sea-ice air fraction increased from 1 % to 6 %, due to air voids expansion controlled by internal melt. This increase in air volume substantially affected FYI density and freeboard. Due to differences in sea-ice thermodynamic state (such as salinity and temperature), the air volume expansion is less pronounced in second-year ice (SYI) and has a smaller impact on the density evolution of SYI and ridges. We validated our discrete measurements of FYI density from coring using co-located ice topography from underwater sonar and an airborne laser scanner. Despite decreasing ice thickness, a similar counter-intuitive increasing ice freeboard was observed for the entire 0.9 km 2 MOSAiC ice floe, with a stronger freeboard increase for FYI than for less saline SYI. The surrounding 50 km 2 area experienced a slightly lower 0.01 m ice freeboard increase in July 2020, despite comparable half-meter melt rates obtained from ice mass balance buoys. The decreasing draft to thickness ratio from 0.92 to 0.87 observed FYI complicates the retrieval of ice thickness from satellite altimeters during the summer melt season and underlines the importance of considering density changes in retrieval algorithms.