Isotopic variation of precipitation over eastern Mongolia and its implication for the atmospheric water cycle

SummaryTo investigate the atmospheric water cycle over eastern Mongolia and its surrounding regions, a monitoring network for stable hydrogen and oxygen isotopes in precipitation has been established. We obtained 66 monthly samples at six stations for a year from October 2002 to September 2003 and 204 daily samples at seven stations during the warm period from April to September of 2003. The observed isotopic composition showed considerable coherence in space and was characterized by a strong correlation with air temperatures (and a weak correlation with precipitation). A Rayleigh-type model involving the temperature and amount of precipitation successfully reproduced day-to-day variations in the observations with root mean square errors of 30‰ for δD and 2.7‰ for δ18O. This good reproducibility demonstrates that isotopic variation of precipitation can be explained by rainout history during transportation from a single vapor-source reservoir. The conditions of the reservoirs generally corresponded with those for a subtropical marine atmosphere, indicating that continental recycling generally has a minor effect at least on the temporal variability of isotopes in the precipitation. In July, however, observed δ values were very much lower than predicted. Considering the atmospheric circulation pattern and its relationship to isotopic data, it is inferred that the additive water vapor with low δ might be supplied from regions in the south (e.g., rice paddy fields in southeast China). Transient eddies can play an important role in transporting such water vapor to eastern Mongolia.