Critical role of intraseasonal oscillations in shaping extreme rainfall from tropical cyclones over the South China Sea

Extreme rainfall induced by tropical cyclones (TCs) poses a major hazard across South China, with its intensity and distribution strongly modulated by monsoon activity over the South China Sea (SCS). This study quantifies the role of intraseasonal oscillations (ISOs), specifically the 10–20-day quasi-biweekly oscillation (QBWO) and 30–60-day Madden–Julian oscillation (MJO), in shaping extreme accumulated rainfall (EAR) from TCs over the SCS. Removing ISO components results in 63%–78% of EAR events falling below the extreme rainfall threshold, demonstrating ISO’s critical contribution to extreme rainfall occurrence. Based on the dominant ISO signals, EAR events are classified into three types: Type A (significant in both QBWO and MJO bands; 57.6%), Type B (QBWO-only dominant; 36.4%), and Type C (MJO-only dominant; 3.7%). ISO-related rainfall contributions decrease with increasing TC intensity, indicating that weaker storms rely more heavily on favorable ISO phases. In cases with pronounced ISO influence, a clear phase-locking relationship emerges: peak rainfall in Type A and B events aligns with the QBWO crest, whereas in Type C events it coincides with the MJO’s rising-to-peak phase. ISO-scale cyclonic circulations strengthen low-level westerlies south of the rainfall maximum, enhancing moisture convergence into the TC. While QBWO-related circulation structures are similar in Type A and B events, MJO-associated patterns in Type C are characterized by a more persistent and zonally extended moisture belt. These findings highlight the regulatory role of ISO phase and amplitude in TC-induced extreme rainfall over the SCS and provide a physical basis for improving extended-range prediction of high-impact hydrometeorological events.