ENSO modulated upstream convection as the primary control on interannual δ¹⁸O variability in East Asia

The mechanisms driving oxygen isotope variations in East Asian summer monsoon (EASM) precipitation (δ¹⁸O p ) and their proxy archives remain controversial. Here, we use IsoGSM3—an isotope-enabled climate model with moisture-tagging capability and rigorously validated against observations, to show that the El Niño Southern Oscillation (ENSO) is the primary mode of interannual δ¹⁸O p variability across East Asia. ENSO’s imprint arises primarily through its modulation of upstream deep convection and rainout, altering the isotopic signature of moisture transported downstream into East Asia. It further strengthens the imprint of low-level monsoon circulation variability on δ¹⁸O p , producing larger-amplitude and more spatially extensive anomalies over the EASM domain. An additional pathway involves ENSO’s influence on the subtropical westerly jet, with El Niño events accentuating its southward displacement during the late and post-monsoon seasons, leading to positive δ¹⁸O p anomalies over the EASM domain. Despite these coherent dynamical influences, the ENSO-driven leading mode of interannual δ¹⁸O p variability captures only ~21% of the total variance, a signal that, though clearly discernible in the model and observations, can be potentially obscured in archives that integrate δ¹⁸O p over multiple years, making seasonal or annually resolved records essential for capturing its variability.