The Antarctic sea ice cover from ICESat-2 and CryoSat-2: freeboard, snow depth and ice thickness

We offer a view of the Antarctic sea ice cover from lidar (ICESat-2) and radar (CryoSat-2) 5 altimetry, with retrievals of freeboards, snow depth, and ice volume that span an 8-month winter between April, 2019 and November 16, 2019. Snow depths are from freeboard differences. The remarkable response of the ice cover to mechanical convergence in the coastal Amundsen Sea was captured in the correlated increase in local freeboards and thickness. The multiyear ice in the West Weddell sector also stands out with mean sector thickness of > 2 m. Thinnest ice is found near polynyas (Ross Sea and Ronne) where new ice areas are exported seaward and entrained in the surrounding ice cover. For all months, the results suggest that ~60-70% of the total freeboard is comprised of snow. While the spatial patterns in the freeboard, snow depth, and thickness composite are as expected, the observed seasonality in these variables is surprisingly weak, likely attributable to the competing processes (snowfall, snow redistribution, snow-ice formation, ice deformation, basal growth/melt) that contribute to uncorrelated changes in the total and radar freeboards. Evidence points to biases in CryoSat-2 freeboards from high salinity (>10 psu) in the basal snow-layer resulting in lower/higher snow depth/ice thickness retrievals. Adjusting CS-freeboards by 6 cm gives a circumpolar ice volume of 12,500 km3 in October, for an average thickness of ~0.93 m. Validation of Antarctic sea ice parameters remain a challenge, there are no seasonally and regionally diverse data set that could be used to assess the large-scale satellite retrievals.