Antarctic Water Stable Isotopes in the Global Atmospheric Model LMDZ6: From Climatology to Boundary Layer Processes

Abstract Observations of water stable isotopes in Antarctic surface snow, precipitation and water vapor are key for improving our understanding of the atmospheric water cycle and past climate reconstructions from ice cores. In this study, we use isotopic observations in Antarctica to assess the skill of the isotope‐enabled atmospheric general circulation model LMDZ6, nudged to ERA5 above the boundary layer (1980–2023 period). The model has no significant bias for time‐mean temperature and snow accumulation over the ice sheet. Sensitivity test on parameterized supersaturation strength highlights its opposite effect on precipitation O and d‐excess. Selecting an intermediate supersaturation strength resulted in a minimal bias for surface snow O across the continent, with a reduced but systematic positive bias in surface snow d‐excess (5‰). We then assessed seasonal and diurnal isotope variability with daily precipitation and continuous vapor isotopes at Dumont d’Urville (DDU, coastal station) and Concordia (inland station). On a seasonal scale, LMDZ6iso accurately reproduces the seasonal cycle of precipitation O and d‐excess at both stations. Moving from statistical evaluation to physical analysis, we use the individual process contributions to boundary‐layer water vapor isotopes to identify the main drivers controlling the clear‐sky isotopic daily cycles. At Concordia, daily isotope variations are mainly driven by surface sublimation, whereas at DDU they are driven by surface sublimation and advection by the katabatic flow. Our results suggest that to further improve water isotopes in LMDZ6iso, fractionation during surface sublimation should be included and fractionation at condensation for low temperature should be better constrained.