Assessing synoptic-scale variability in isotope-enabled climate models: challenges for water vapor isotopes over the Southern Ocean

Stable water isotope ratios (WIRs) are sensitive to phase changes and are crucial for understanding water cycle processes. However, limited observational data and model uncertainties in the southern hemisphere pose challenges for accurately reproducing these ratios. This study compared isotope-enabled climate models (IsoGSM and MIROC5-iso) with ship-based observations over the Indian Ocean sector of the Southern Ocean in January 2006. The models accurately simulated temperature, pressure, and humidity but faced challenges with vapor δ18O and δD. By categorizing atmospheric conditions into southerly flows from Antarctica, northerly flows with atmospheric rivers (ARs), and the other cases, we found varying model performance. In the other cases, models showed agreement with observations. In contrast, in southerly cases, the models failed to capture substantial decreases in δD, likely because of inadequate representation of Antarctic land-surface processes. In northerly cases, both models showed model-dependent discrepancies in δD. A newly developed simple WIR model indicated these discrepancies stem from inadequate precipitation processes. These findings reveal that synoptic-scale processes influence WIR variations in ways not fully captured by current climate models. As ARs deliver precipitation to Antarctica that forms ice cores, these limitations impact paleoclimate interpretations, highlighting the need to improve synoptic-scale processes.