Vehicle-based in situ observations of the water vapor isotopic composition across China: spatial and seasonal distributions and controls

Stable water isotopes are natural tracers in the hydrological cycle and have been applied in hydrology, atmospheric science, ecology, and paleoclimatology. However, the factors controlling the isotopic distribution, both at spatial and temporal scales, are debated in low and middle latitude regions, due to the significant influence of large-scale atmospheric circulation and complex sources of water vapor. For the first time, we made in situ observations of near-surface vapor isotopes over a large region (over 10 000 km) across China in both pre-monsoon and monsoon seasons, using a newly designed vehicle-based vapor isotope monitoring system. Combined with daily and multiyear monthly mean outputs from the isotope-incorporated global spectral model (Iso-GSM) and infrared atmospheric sounding interferometer (IASI) satellite to calculate the relative contribution, we found that the observed spatial variations in both periods represent mainly seasonal mean spatial variations, but are influenced by more significant synoptic-scale variations during the monsoon period. The spatial variations of vapor delta O-18 are mainly controlled by Rayleigh distillation along air mass trajectories during the pre-monsoon period, but are significantly influenced by different moisture sources, continental recycling processes, and convection during moisture transport in the monsoon period. Thus, the North-South gradient observed during the pre-monsoon period is counteracted during the monsoon period. The seasonal variation of vapor delta O-18 reflects the influence of the summer monsoon convective precipitation in southern China and a dependence on temperature in the North. The spatial and seasonal variations in d-excess reflect the different moisture sources and the influence of continental recycling. Iso-GSM successfully captures the spatial distribution of vapor delta O-18 during the pre-monsoon period, but the performance is weaker during the monsoon period, maybe due to the underestimation of local or short-term high-frequency synoptic variations. These results provide an overview of the spatial distribution and seasonal variability of water isotopic composition in East Asia and their controlling factors, and they emphasize the need to interpret proxy records in the context of the regional system.

Automated summary

Stable water isotopes are important tracers in the hydrological cycle, and their spatial and temporal distributions are influenced by various factors. In this study, the authors conducted in situ observations of near-surface vapor isotopes over a large region in China, and found that the spatial variations in isotopes are mainly influenced by seasonal mean variations during pre-monsoon and monsoon seasons, but also by synoptic-scale variations during the monsoon period. Factors such as moisture sources, continental recycling processes, and convection play significant roles in controlling the isotopic composition of water vapor in different periods. The results highlight the need to consider regional factors when interpreting proxy records.