A Case Study on Drivers of the Isotopic Composition of Water Vapor 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, vapor 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 vapor 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 simple model is developed to account for moisture exchanges with the snow surface. The model generally reproduces the observed trend with strongly depleted vapor d(18)O 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 vapor 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 when the isotopic composition of the sublimation flux differs substantially from that of the advected air mass, for example, marine air arriving at the coast or free-tropospheric air descending from high altitudes. Despite strong simplifications, our model is a useful and computationally efficient method for understanding SWI dynamics at polar sites.Plain Language Summary Stable water isotopes are useful to reconstruct historical temperature conditions from ice cores. This method is possible because phase changes of water alter the isotopic composition. For example, if an air mass cools down, forms clouds, and produces rain or snowfall, the water vapor preferentially loses heavy water molecules. This study aims to explain a remarkable vapor isotopic signal measured on a ship close to the East Antarctic coast during 6 days in 2017. We model the isotopic composition of air parcels along their pathways to the ship and develop a novel approach to represent moisture exchange with the snow surface. The modeled vapor isotopic composition at the ship reaches a distinct minimum, similar to the measurements, when the air parcels move directly from the ice sheet to the ship. As expected, the vapor isotopic composition is lower over the ice sheet than over the ocean, largely due to cloud formation. However, moisture uptake from the snow surface and from the ocean shortly before arrival at the ship can strongly and abruptly influence the isotopic signature of the air masses. Although our model is not perfect, it helps to improve the interpretation of isotope measurements at polar sites.