Antarctic sea ice formation and melt rates estimated from under-ice ocean observations

Southern Ocean sea ice thickness is an important climate variable due to its impact on freshwater fluxes, ocean-atmosphere heat exchange, and momentum transfer. Yet monitoring sea ice thickness from satellites has proved difficult, in part owing to a sparsity of in situ measurements for validation purposes. Here we estimate sea ice formation and melt rates around Antarctica by constructing local upper-ocean salinity budgets based on Argo profiling float and instrumented seal observations. Vertical Ekman flux and geostrophic advection terms are computed using wind stress from ERA5 atmospheric reanalysis, satellite retrievals of sea ice drift and concentration, and dynamic topography from CryoSat-2. Freshwater fluxes associated with snow are investigated by reconstructing snow accumulation and advection and applying a simple model of snow loss processes, including snow-ice formation. Sources of uncertainty and bias are evaluated and discussed. We present a circumpolar climatology of the upper-ocean salinity tendency due to winter sea ice growth and melt. From this, Antarctic sea ice thicknesses are derived and compared with an empirical thin sea ice thickness retrieval from SMOS.