<p>Sea surface temperature (SST) anomalies profoundly shape regional precipitation variability, yet their pathways are often obscured by seasonal/monthly mean-state analyses overlooking daily atmospheric dynamics. Focusing on monsoon-affected South China, we show that the rainy-season rainfall interannual variability is primarily driven by shifts in daily circulation regimes rather than rainfall intensity. Objective classification identifies four dominant regimes: two low-pressure system–related types, a pre-onset monsoon regime, and a South China Sea monsoon–dominated regime. Moisture tracking and precipitation decomposition confirm that regime frequency dominates year-to-year rainfall variations. SST anomalies in the Indian, Pacific, and Atlantic Oceans modulate these frequencies by triggering large-scale atmospheric responses, particularly anomalous cyclones/anticyclones over the western North Pacific. These atmospheric changes alter convection and instability, driving low-pressure activity and monsoon onset. Our findings reveal a regime-mediated mechanism linking remote ocean variability to regional hydroclimate extremes, offering new insights for improving climate prediction and projection under global warming.</p>