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Abstract

Southern Ocean sea ice has undergone dramatic losses in extent in recent years, this year reaching a record (40-year) low of 1.91 million square kilometres in early February, weeks before its minimum annual extent. These drastic losses have garnered much attention from the scientific community and the media, but for the moment our understanding of these events remains two-dimensional because observations of sea ice thickness and volume are limited. Were changes in extent caused by increased convergence of the sea ice pack with a little overall change in volume? or did thermodynamic forcing during low-extent years promote thinner ice more prone to summer melt?Here we utilise data from the ESA CryoSat-2 and Copernicus Sentinel-3 radar altimetry missions, combining radar freeboard with a newly developed snow-on-Antarctic-sea-ice model to derive circumpolar sea ice thickness and volume from 2011 to 2023. We assess the variability in sea ice thickness in the run-up to the February minimum, focussing on the difference between below-average and above-average extent years to ascertain if low summer extent was facilitated by a thinner winter-spring ice pack.Using sea ice concentration and motion data, we then decompose sea ice thickness changes into dynamic and thermodynamic components to understand the principal driving mechanisms behind sea ice thickness variability between 2011-2023, and the dramatic extent losses of 2017, 2022 and 2023.This work forms part of the NERC DEFIANT project, which combines new in-situ and satellite data sets with advanced modelling techniques to better understand the drivers and effects of Antarctic sea ice variability.