Highlights on key polar processes driving the Antarctic surface mass balance

Abstract: Ice sheet mass balance results of the small imbalance between ice flow discharging in the ocean and net snow accumulation on the ice sheet surface, i.e. the surface mass balance (SMB). Past change in ice sheet mass can be assessed using satellite altimetry, gravimetry, or the input-output method, which all request SMB estimates.The input-output method, which consists in separately modeling ice dynamics and SMB, is also the only way to project future trends. Here we use the polar-oriented regional climate model MAR to estimate the Antarctic SMB for the period 1979-2018 using forcing from 7 reanalyses: ERA-5, ERA-Interim, JRA-55, MERRA2, NCEP1, NCEP2, and 20CRv2. We show that spatial patterns of SMB modeled by the reanalyses diverge significantly between each-other, whereas the spread is insignificant after downscaling with MAR. SMB modeled with MAR is markedly closer to observations than the original reanalyses outputs. Divergences in SMB between reanalyses and MAR are driven by divergences in snowfall amounts, which are explained by processes related to clouds and precipitation in the atmosphere (notably cloud-precipitation conversion, precipitation advection and precipitation sublimation) rather than by model resolution. The temporal variability of downscaled SMB is however strongly related to the forcing reanalysis variability. This study emphasis the importance of polar-specific processes and of their implementation in climate models, particularly in view of atmosphere-ice sheet coupling