Control of seasonal water vapor isotope variations at Lhasa, southern Tibetan Plateau

摘要

Water isotopes in precipitation provide new insight on the controlling factors for moisture source variations on the Tibetan Plateau. However, understanding the infra-annual water isotope cycle is difficult for a region lack of winter precipitation. Here, we reported water vapor delta O-18 and delta H-2 time series over two and half years at Lhasa on the southern Tibetan Plateau. We compare our values with concurrent daily precipitation isotopes to assess the seasonal controls of vapor isotopes and possible correlation to precipitation isotopes. Results show an intraseasonal dependence of water vapor delta O-18 on large scale meteorological regime. During the winter months, vapor water delta O-18 shows positive dependence on regional-scale temperature, confirming a Rayleigh-distillation controlling moisture transport. During the summer months, lower vapor delta O-18 is weekly in association with the heavy precipitation over tropical Indian Ocean and on the Tibetan Plateau, and this relation is further enhanced by the evaporation of precipitation at the sampling site. We also found a weak seasonal variation of vapor dexcess, likely resulted from climate seasonality over the large moisture sources region. Moreover, the isotope dataset contains a high-frequency of fluctuations for both delta O-18 and d-excess lasting several days during the summer monsoon season. These high frequency fluctuations are in phase with precipitation events, confirming significant local evaporation effects on the short-term water cycle, which further complicated the control of near surface water vapor isotopes. These findings improve our regional scale understanding of hydrological cycle on the southern Tibetan Plateau, and will potentially improve our understanding of isotope variations in proxy archives in the Tibetan Plateau.