MJO unlocks the stratospheric polar vortex influence on winter extreme precipitation over South China

Abstract. Winter extreme precipitation over South China (SC) exerts profound impacts, yet its stratospheric drivers remain limited understood, with a long-standing puzzle of the statistically weak linkage between the stratospheric polar vortex (SPV) and SC rainfall extremes. Here, using observational analyses and CMIP6 historical simulations, we demonstrate that a Eurasia-shifted SPV (ESSPV) can increase SC winter extreme precipitation, but this modulation is strictly phase-locked to the Madden–Julian Oscillation (MJO), occurring predominantly during MJO phases 2–5. Two synergistic pathways explain this phenomenon. First, the ESSPV excites a Rossby wave across Eurasia, inducing a low-level northeasterly anomaly north of SC that strengthens local moisture convergence. Second, the ESSPV couples with the westerly quasi-biennial oscillation (WQBO), which amplifies the MJO-associated anomalous anticyclone over the western North Pacific and enhances low-latitude moisture transport toward SC. CMIP6 multi-model analyses further confirm that the fidelity of MJO phase 2–5-associated western North Pacific anticyclone determines a model's ability to reproduce the observed linkage, firmly establishing the MJO as the indispensable gateway linking the SPV to SC extreme precipitation.