Moisture sources and dynamics over the Southeast Tibetan Plateau reflected in dual water vapor isotopes

Abstract

The Southeast Tibetan Plateau (SETP) has experienced a significant drying trend in recent decades, which is likely linked to shifts in moisture sources. To investigate the roles of ocean surface evaporation, continental air mass intrusion, and precipitation-vapor interactions, we present a three-year daily time series of near-surface vapor δ18O and d-excess data from the SETP station. Our analysis reveals that apparent negative correlations between d-excess and relative humidity over the Indian Ocean are primarily due to anticorrelated seasonal cycles, which become insignificant or marginal when examined seasonally. This result underscores the need for caution in interpreting d-excess as a conservative tracer of ocean surface evaporation. Instead, we identify local and upstream specific humidity as the primary determinants of non-monsoon season d-excess variability, which is influenced by the intrusion of cold and dry air from upper levels. During the summer monsoon season, both d-excess and δ18O reflect the effect of raindrop evaporation during transport, which decreases δ18O but increases d-excess. These findings offer new insights into the use of water isotopes to track moisture sources and circulation changes across the SETP, especially under varying seasonal circulation systems. In particular, the findings for d-excess will contribute to our understanding of shifts in moisture sources and provide a framework for interpreting d-excess in various hydroclimatic applications, including ice core studies.