Rainwater isotopes in central Vietnam controlled by two oceanic moisture sources and rainout effects

Oxygen isotopes are commonly used proxies in paleoclimate research, however, a correct interpretation requires a detailed understanding of processes controlling isotope variability for a specific site.  A common interpretation for oxygen isotopes in precipitation across the Asian monsoon region, links the seasonal and interannual variability to changes in the summer monsoon strength.

However, some locations within tropical Asia do not receive rainfall during the summer monsoon season. In central Vietnam most of the annual rainfall falls during autumn instead of summer, making central Vietnam ideal to investigate processes controlling rainwater isotope variability, independent from the summer monsoon. By using rainwater isotopes, collected over five years, and moisture uptake simulations for these time periods, we investigate the seasonal cycle and interannual variability of hydrological processes in central Vietnam.

Our results show that the seasonal variability is dominated by a shift in moisture source from the Indian Ocean in summer to the South China Sea (western Pacific) from autumn to spring. The different source locations are reflected by an increase in δ¹⁸O values from around − 8 to − 10‰ during summer to values between 0 and − 3‰ during winter/spring. Further, we show that the amount effect and the occurrence of tropical cyclones, which are typical for the region, have no effect on a seasonal to interannual scale. Instead, we find that the timing of the seasonal ITCZ migration is a driving component of variability on these time scales.