Annual Cycle of Antarctic Sea Ice Deformation from ICESat-2

Since 2015, Antarctic sea ice extent has declined by ~53%, and reached consecutive record lows in the austral summers of 2022 and 2023. These events have raised major concern as to whether a regime shift towards more extreme and frequent low sea ice extents has begun. Potential impacts on the climate system are far-reaching and continent-wide studies are urgently needed. Due to Antarctica's remote and harsh environment, in situ observations are however sparse. NASA's ICESat-2, a laser altimeter launched in 2018, provides precise sea ice surface elevation data at high-resolution, with along-track sampling every ~0.7 m. This sampling allows us to resolve meter-scale features in the sea ice pack, such as pressure ridges, which modify the shape of the ice thickness distribution and play an important role in ocean-atmosphere momentum flux through form drag. We apply a bespoke processing technique, called the University of Maryland-Ridge Detection Algorithm (UMD-RDA), to derive the surface topography of sea ice in the Southern Ocean over a 5-year period, spanning 2018-2023. Using the UMD-RDA we can capture seasonal and interannual variability in surface roughness, pressure ridge sail height, and ridge sail spacing. We find that during the 2018-2023 period, on average, across the full Southern Ocean ice pack, sea ice surface roughness reaches a maximum in January/February followed by a minimum in April. Sail height is at its maximum in January and minimum in June, while sail spacing is at its minimum in January and maximum in June. Interannual variability shows that the 2023 season is an outlier with respect to the 5-year 2018-2023 average. In 2023 there was a decrease of ~20% in surface roughness during the April minimum, a decrease of ~8% in sail height during the June minimum, and an increase of ~31% in sail spacing during the June maximum. This suggests that, in 2023, the sea ice pack was less deformed overall than in preceding years. We also assess seasonal and interannual variability in surface roughness and ridge morphology in five distinct regions including the Amundsen-Bellingshausen (A-B), Ross, Pacific, Indian, and Weddell Seas. The A-B, Ross, and Pacific sectors showed the greatest change in 2023, with respect to the 2018-2023 average, with a decrease in surface roughness of ~34%, ~26%, and ~14%, respectively. Sail spacing within the A-B, Ross, and Pacific sectors, with respect to the 2018-2023 average, increased by ~83%, ~61%, and ~35%, respectively. Furthermore, the ICESat-2 ATL10 sea ice freeboard dataset shows a ~50% decrease in freeboard within the A-B sector in 2023. These results provide evidence that a substantial amount of thicker, older, and rougher ice was likely exported out of the A-B region. Our results can provide insight into the mechanisms responsible for the recent record low sea ice extents and could uncover new relationships between deformation, roughness, and ice extent.