This study reveals a significant in-phase relationship between the South China Sea summer monsoon (SCSSM) withdrawal date and tropical cyclone (TC) genesis over the western North Pacific (WNP). The number of TCs generated over the WNP from mid-September to mid-October is positively correlated with the SCSSM withdrawal date during the period of 1979–2016. The decreased (increased) number of TCs generated during early (late) SCSSM withdrawal years is attributed to both internal atmospheric dynamics and external sea surface temperature (SST) forcing. Through dynamic (Rossby wave response) and thermodynamic (increased moisture) processes, the warm SST anomalies during late withdrawal years over the tropical WNP contribute to maintaining the monsoon trough (MT) in the boreal autumn and moisturizing the mid-level atmosphere, providing a favorable environment for TC genesis. The remaining MT can facilitate the conversion of mean kinetic energy into eddy kinetic energy (EKE) and enhance synoptic-scale waves. In addition, upper-level baroclinic energy conversion also contributes to EKE development. Both barotropic and baroclinic processes favor TC genesis over the WNP. In contrast, colder SSTs during early withdrawal years induce the early withdrawal of the MT, leading to depressed enhancement of the EKE and weakening the northwestward propagation of synoptic-scale waves. Hence, fewer (more) TCs tend to be generated during early (late) withdrawal years.

这项研究揭示了南海夏季风（SCSSM）撤除日期与北太平洋西部（WNP）上的热带气旋（TC）成因之间存在显着的同相关系。9月中旬至10月中旬，WNP上产生的TC数量与1979-2016年SCSSM退出日期呈正相关。在SCSSM撤出初期（晚期）产生的TC数量减少（增加）的原因是内部大气动力学和外部海表温度（SST）强迫。通过动态（罗斯比波响应）和热力学（湿度增加）过程，热带WNP退水后期的SST异常温暖，有助于在北方秋季维持季风槽（MT）并滋润中层大气，为TC的产生提供了有利的环境。其余的MT可以促进将平均动能转换为涡动能（EKE）并增强天气尺度波。此外，上斜压能量转换也有助于EKE的发展。正压和斜压过程都比ＷＮＰ更有利于ＴＣ的发生。相反，在退役初期，较冷的海温会诱发MT的早退，从而导致EKE的增强减弱，并减弱天气尺度波向西北传播。因此，在提早（后期）撤出期间倾向于产生更少（更多）的TC。高层斜压能量转换也有助于EKE的发展。正压和斜压过程都比ＷＮＰ更有利于ＴＣ的发生。相反，在退役初期，较冷的海温会诱发MT的早退，从而导致EKE的增强减弱，并减弱天气尺度波向西北传播。因此，在提早（后期）撤出期间倾向于产生更少（更多）的TC。高层斜压能量转换也有助于EKE的发展。正压和斜压过程都比ＷＮＰ更有利于ＴＣ的发生。相比之下，退役初期的SST较冷会诱发MT的早退，从而导致EKE的增强受到抑制，并减弱天气尺度波的西北传播。因此，在提早（后期）撤出期间倾向于产生更少（更多）的TC。