Contemporary Characteristics of the Antarctic Firn Layer (1979-2020)

Firn covers ~99% of the Antarctic ice sheet, in which nearly all (>99%) of the surface meltwater refreezes or is retained in liquid form. The fate of meltwater is highly dependent on the firn density, firn height and temperature of the underlying firn. While this refreezing and retainment prevents meltwater to be released as runoff and contribute to sea level rise, it has an impact on the firn density, temperature and liquid water content. Such firn conditions are extremely important for the Antarctic ice sheet: saturation of firn layers can potentially lead to disintegration of ice shelves. Multilayer firn models can be used to simulate firn processes and identify firn conditions. In addition, simulated firn air content can be used to convert altimetry-derived surface height change to ice sheet mass change. In this study, we present the contemporary characteristic of the Antarctic firn layer simulated with the IMAU Firn Densification model (IMAU-FDM) for the period 1979-2020. Improvements have been made regarding (i) fresh snow density and (ii) firn compaction rate by using an extensive dataset of in-situ density measurements covering 148 locations. RMSE of the simulated depth of = 830 kg m-3 and of the surface snow density have been reduced by 33 and 24%, respectively, with overall reduced surface snow densities and increased firn air content, and reduced firn air content in very high and very low accumulation areas. We present the spatial, seasonal and decadal variability of firn density and firn air content (Figure). We find substantial liquid water content storage and firn aquifers in the Antarctica Peninsula, and substantial liquid water storage in West Antarctic and Shackleton ice shelves. The contribution of firn compaction to firn air content and surface height elevation changes is significant. In addition, the large spatial (up to 40 m) and temporal (up to 5 m) variations in firn air content demonstrates the importance to consider firn air content variations for altimetry studies.