A Case Study on Drivers of the Isotopic Composition of Water Vapour at the Coast of East Antarctica

Stable water isotopes (SWIs) contain valuable information on the past climate and phase changes in the hydrologic cycle. Recently, vapour measurements in the polar regions have provided new insights into the effects of snow-related and atmospheric processes on SWIs. The purpose of this study is to elucidate the drivers of the particularly depleted vapour isotopic composition measured on a ship close to the East Antarctic coast during the Antarctic Circumnavigation Expedition in 2017. Reanalysis data and backward trajectories are used to model the isotopic composition of air parcels arriving in the atmospheric boundary layer (ABL) above the ship. A novel approach is developed to account for moisture exchanges with the snow surface. The model generally reproduces the observed trend with strongly depleted vapour δ18O values in the middle of the 6-day study period. This depletion is caused by direct air mass advection from the ice sheet where the vapour is more depleted in heavy SWIs due to distillation during cloud formation. The time spent by the air masses in the marine ABL shortly before arrival at the ship is crucial as ocean evaporation typically leads to an abrupt change in the isotopic signature. Snow sublimation is another important driver because the air masses and the sublimation flux will differ substantially in their isotopic composition if the air masses cross the ocean-snow boundary or descend from higher atmospheric levels. Although our model makes strong simplifications, it is a useful and computationally efficient method for understanding SWI dynamics at polar sites.