Formation Mechanism for Persistent Heavy Rainfall in Southern China during the Onset of South China Sea Summer Monsoon

Abstract The onset of the South China Sea (SCS) summer monsoon (SCSSM) has considerable impacts on the weather and climate in East Asia and beyond. Southern China often experiences persistent heavy rainfall during the SCSSM onset, and this rainfall can be attributed to the 10–30-day intraseasonal oscillation (ISO) originating from the equatorial western Pacific and propagating northwestward. Before the monsoon onset, the SCS is controlled by a strong negative phase of ISO featured by an anomalous low-level anticyclone while active convection dominates the Bay of Bengal (BOB). In this condition, anomalous southwesterlies at the northwestern flank of the SCS anticyclone carry abundant water vapor to southern China. Meanwhile, the convective forcing over the BOB moves to the southern Tibetan Plateau and triggers an anomalous upper-tropospheric anticyclone. This anticyclone extends downstream with the background westerlies and brings northerly anomalies to southern China, inducing anomalous ascending motions through altering potential vorticity advection. Thus, the negative phase of ISO induces heavy rainfall in southern China through enhancing both moisture supply and ascending motions. When the ISO evolves into a positive phase, convective activity and low-level westerlies intensify over the SCS, indicating the onset of the SCSSM. Afterward, the above dynamic and thermodynamic conditions over southern China are reversed and then the heavy rainfall recedes. We highlight that the ISO not only plays a significant role in triggering the onset of the SCSSM but also causes a persistent heavy rainfall process in southern China during the monsoon onset. Significance Statement Persistent heavy rainfall in southern China can have significant societal impacts, including severe floods, agricultural damage, and human health. It is found that during the onset of SCSSM, southern China often experiences persistent heavy rainfall lasting nearly 1 week, characterized by a significant increase before monsoon onset but a quick reduction afterward. Why is this heavy rainfall process generated during the SCSSM onset and how does it quickly decay afterward? We emphasize the key role of the 10–30-day intraseasonal oscillation (ISO) originating from the equatorial western Pacific in triggering this persistent heavy rainfall process. This study helps to improve the understanding of the SCSSM onset and its relationship with the weather and climate in southern China.