Prolonged QBO-ENSO phase alignment caused unbalanced water resources across East Asia

Since the 1960s, the southern floods-northern droughts dipole pattern of East Asian summer precipitation became abnormally active in the early 1990s but weakened markedly after the late 2000s. During its active periods, the dipole produced widespread flooding south of the Yangtze River basin and severe droughts to the north. However, the mechanisms underlying this precipitation dipole remain inconclusive, leaving uncertain whether it may re-intensify and threaten East Asia's hydrological balance in the future. Here we show that a prolonged phase alignment between the stratospheric quasi-biennial oscillation (QBO) and the tropospheric El Niño-Southern Oscillation (ENSO) exerted a dominant influence by modulating the East Asian subtropical westerly jet, thereby shaping the observed precipitation dipole. This mechanism is confirmed by climate simulations, in which prescribing historically observed QBO and ENSO forcings in the BCC-CSM2-MR model successfully reproduces the southern floods-northern droughts dipole pattern that became pronounced after the early 1990s. Future projections indicate that such QBO-ENSO alignments may recur under any emission scenario, suggesting that the currently weakened dipole could re-emerge in a future climate. Our findings underscore the critical role of stratosphere-troposphere coupling in regulating East Asian hydroclimate variability and offer an early-warning perspective for future shifts in regional water resources.