Monsoon vs. non-monsoon precipitation isotopes in Southwest China: Contrasting roles of moisture sources, Tibetan hydroclimate, and local amount effects

Natural stable isotopes in precipitation provide important information on water vapor sources and atmospheric convection processes, which play key roles in climate change, hydrological cycle, and paleoclimate reconstruction. However, the controlling mechanisms of precipitation isotopes in subtropical regions remain insufficiently understood. This study aimed to clarify the dominant controls on precipitation isotope variability and their seasonal differences in a typical subtropical region of Southwest China. Monthly precipitation samples were continuously collected at the Yanting experimental station in Sichuan, Southwest China from 2012 to 2020. The oxygen isotope ratio (δ¹⁸O) and deuterium excess (d-excess) in precipitation were analyzed, and their relationships with precipitation amount, large-scale atmospheric circulation, and moisture source regions were examined using statistical analysis and a structural equation model. The results showed a significant negative relationship between anomaly precipitation amount and monthly δ¹⁸O values, particularly during the monsoon season, indicating a pronounced precipitation amount effect. Structural equation modeling revealed that this effect was mainly driven by hydrometeorological conditions over the seas and Tibetan Plateau. Moisture from the Bay of Bengal was identified as a more influential source than the South China Sea, largely associated with the El Niño-Southern Oscillation, and its contribution has increased in recent years. In contrast, during the non-monsoon season, precipitation δ¹⁸O variations showed no significant relationship with any local factor, suggesting a combined influence of teleconnections and local processes. These findings demonstrate that precipitation isotopes in Southwest China are governed by distinct seasonal mechanisms, underscoring the critical roles of large-scale atmospheric circulation, Tibetan Plateau hydroclimate, and moisture source dynamics in modulating isotope variability across subtropical monsoon regions.