Atmospheric processes control the stable isotopic variability of precipitation in the middle–lower reaches of the Yangtze River Basin, East Asian monsoon region

Paleoclimatic records in the East Asian monsoon region (EAM) depend on precipitation isotopes as proxies for past hydroclimatic variability. However, the potential mechanisms controlling precipitation isotopic variability in this region remain poorly understood. This study collected daily precipitation isotopes in the middle-lower reaches of the Yangtze River Basin from 2017 to 2020 to explore how convective behaviors and moisture sources determining the variability of precipitation stable isotopes. Daily precipitation δ18O presented a weak inverse relationship with local precipitation amount and air temperature. Results of correlation analysis showed that the maximum correlation zones of precipitation δ18O between different cloud levels and outgoing long wave radiation were identified in the upstream regions such as the Indian Peninsula, BoB, and Indo-China Peninsula during the rainy season. Back-trajectory analysis showed that substantial water vapor from maritine sources in the Indian Ocean and the Bay of Bengal (BoB) largely contributed to the lower δ18O values during the rainy season (−7.5 ‰) because of intense rainout processes in the upstream sourced regions, demonstrated by significant regional correlation between precipitation δ18O and regional precipitation amount. The regional correlations were stronger in the upstream regions than the local influences, indicating that precipitation isotopes in this region are more influenced by regional convective processes in the upstream region during the rainy season. Using the modified Stewart model, the mean subcloud evaporation fraction estimated was greater in rainy season (24.1 % vs 36.1 % for HK and HZ, respectively) than that in dry season (15.1 % vs 19.9 % for HK and HZ, respectively). These findings highlight the importance of upstream processes in controlling precipitation isotopes in the EAM region and provide reliable evidence on the interpretation of monsoonal climate variability.