South China Sea summer monsoon withdrawal and the synoptic‐scale wave train over the western North Pacific

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AbstractBased on NCEP/NCAR reanalysis data (1951–2016), this study focuses on the in‐phase relationship between the South China Sea summer monsoon (SCSSM) withdrawal date and the strength of the synoptic‐scale wave train. The synoptic‐scale wave train during early autumn, which features northwest–southeast‐oriented cyclonic and anticyclonic anomalies, is the dominant synoptic mode over the tropical western North Pacific (WNP). The synoptic‐scale wave train is observed to be significantly strengthened (depressed) during the late (early) SCSSM withdrawal years. Concurrent with the late SCSSM withdrawal, the monsoon trough remains stronger than that in the early withdrawal years. The anomalous low‐level cyclone over the South China Sea favours the barotropic energy conversion from the mean flow to synoptic‐scale eddies. Moreover, the anomalous easterly wind shear over the SCS and the increased moisture over the tropical WNP provide a favourable environment for the development of the synoptic‐scale wave train. This study interprets the inner dynamics of tropic atmospheric activity based on a wave perspective. These results may suggest that synoptic‐scale wave train‐related tropical cyclone activity and precipitation are enhanced in the late SCSSM withdrawal years.