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

Abstract. The Tibetan Plateau (TP) serves as a water tower for major rivers in Asia, and mountain valleys in southeastern TP are key channels for moisture entering the TP. Water resources on the TP are experiencing spatially opposite changes due to climate change, and understanding the sources and dynamics of atmospheric moisture is vital. To investigate the role of ocean surface evaporation, continental air mass intrusion, and rain-vapor interaction, we present a three-year daily time series of near-surface water vapor isotope compositions (δ¹⁸O and d-excess) from the South-East TP station. We find that apparent negative correlations between d-excess and relative humidity over the Indian Ocean mainly reflect their similar seasonality. When analyzed for different seasons, the correlation is insignificant or only explains a marginal fraction of variance. Therefore, caution is required when interpreting the d-excess as a conservative tracer of ocean surface evaporation. Instead, local and upstream specific humidity is the main factor determining non-monsoon season d-excess variability due to the intrusion of cold and dry air from upper levels. During the summer monsoon season, d-excess and δ¹⁸O mainly reflect the effect of raindrop evaporation on humidity during transport which decreases lower vapor δ¹⁸O but increases d-excess values. These findings provide new insights into the significance of using water isotopes to track moisture sources and dynamics over the TP with seasonally alternating circulation systems. Particularly, the findings for d-excess will improve the understanding of different moisture sources and guide the interpretation of d-excess derived from other water bodies and ice cores.