Interannual variability of H₂¹⁸O in precipitation over the Asian monsoon region

The stable isotopic composition of water has been used as a paleoproxy to reconstruct past climates over the Asian monsoon region, but the main controls on the variability of isotopes of water in precipitation have not been characterized quantitatively in this region. Therefore, we used an atmospheric general circulation model incorporating stable water isotope physics to quantitatively estimate the relative contributions to isotope variability in precipitation falling in the Asian monsoon region. As in previous research, we identified two primary factors controlling the interannual variability of δ 18 O precip (defined as (Rsample/R VSMOW − 1) × 1000, where R VSMOW is the 18 O ratio in Vienna Standard Mean Ocean Water) and its correlation with El Niño–Southern Oscillation (ENSO) events: the amount of precipitation at the observation site, and distillation during transport from source regions. Two sensitivity experiments revealed that distillation during transport from source regions was the dominant controlling factor; at Bangkok, Bombay, and Hong Kong, the amount of local precipitation contributed 27%, 33%, and 25% while distillation processes contributed 70%, 60%, and 70%, respectively. Similarly, distillation processes accounted for 80%, 82%, and 83% of observed differences in δ 18 O precip between El Niño and La Niña years at these three cities, respectively. Therefore, interannual variability of δ 18 O precip at the three stations primarily reflects distillation during transport from source regions, and it is also governed by the large‐scale tropical variability (ENSO).