On the importance of atmosphere-snow humidity exchange for the climate signal stored in the snow isotopic composition

Research over the last five years dedicated to identifying and quantifying the processes responsible for driving the climate signal in the isotopic composition of the snow have documented the role of the humidity exchange between the snow and atmosphere in changing the initial precipitation isotopic composition. Laboratory and field experiments combined with direct vapor isotope flux measurements have shown that not only does the depositional flux changes the surface snow isotopic composition, but sublimation from the snow surface induces isotopic fractionation leading to changes in the snow isotopic composition. Thus, it was shown that for the EastGRIP ice core location, including fractionation during sublimation, atmosphere-snow exchange processes explain between 35 and 50 % of the day-to-day variations in the snow surface signal when no precipitation occurs.Until recently, it was unknown on which time scales these surface exchange processes are important for the isotope signal.Here we combine direct accumulation and eddy-covariance humidity flux measurements with high resolution regional climate model simulations. Focusing on the EastGRIP ice core site, we find that during the summer season up to 40% of the accumulation is sublimated and about 10% is re-deposited. Such relative high fluxes compared to the amount of precipitated snow would naturally lead to an influence of the seasonal isotopic composition of the snow.By combining outputs from an isotope-enabled general circulation model (ECHAMwiso) and a regional polar climate model (MAR) with the SNOWISO exchange and snowpack model, we find that the influence on the snowpack isotopic composition is not only isolated to the summer isotope signal but influences the full seasonal cycle. In fact, we find that that the atmosphere-snow exchange influence on the annual mean isotopic composition results in a significant bias in the source region condition deduced from the isotopic composition of the ice core.