Modulations of Marine Boundary Layer Jets and Associated Heavy Rainfall by Quasi‐Biweekly Oscillations Over the South China Sea During June

Abstract Convection over the South China Sea (SCS) exhibits distinct quasi‐biweekly oscillations (QBWO). The QBWO_SCS modulates marine boundary layer jets (MBLJs) embedded in the southwesterly monsoon flow during June, the late Meiyu season. During the first half cycle from Phase 1 (P1) to Phase 2 (P2), convection is most suppressed over the SCS in P2, while enhanced convective activity is observed within the Meiyu trough over southeastern China. An anomalous low‐level anticyclonic circulation over the SCS leads to increased (decreased) MBLJ frequency over the northern (southern) SCS, and these anomalies reverse in Phase 4 (P4). In P2, QBWO_SCS‐intensified MBLJs near the southeastern coast of China create favorable conditions for heavy inland rainfall. First, strengthened southwesterly MBLJs enhance moisture transport from the SCS to inland southeast China and increase moisture convergence within the Meiyu trough. Second, warm, moist air advected inland meets continental cold air, enhancing baroclinicity and frontogenesis, which strongly correlates with inland rainfall maxima. Third, the topography of southeast China amplifies rainfall through orographic lifting as moisture‐laden MBLJs impinge on elevated terrain. In contrast, during P4, weakened MBLJs reduce moisture transport, convergence, and frontogenesis, leading to diminished inland rainfall. Meanwhile, QBWO_SCS‐induced convection‐coupled circulation in P4 supports monsoon trough development and enhances heavy precipitation over coastal south China, the northern SCS, and southern Taiwan. About 25% of QBWOs over SCS are positively correlated with low‐level cyclonic flow originating from QBWOs over the tropical western Pacific, 10% are weakly modulated by the boreal summer intraseasonal oscillation, and 65% are caused by other processes.