Close Linkage of the South China Sea Summer Monsoon Onset and Extreme Rainfall in May over Southeast Asia: Role of the Synoptic-Scale Systems

Abstract

Abstract Based on multiple datasets and methods, this study reveals a close relation between the South China Sea summer monsoon (SCSSM) onset and the occurrence frequency of extreme rainfall over Southeast Asia (including the Indochina Peninsula, Hainan Island, and the Philippines) in May. Specifically, an early SCSSM onset tends to be accompanied by a higher chance of extreme rainfall over Southeast Asia, while a late SCSSM onset is likely to coincide with less heavy rainfall in May. It is suggested that the amplitude of tropical synoptic-scale systems may play an important role in the linkage between the SCSSM onset and the extreme rainfall over Southeast Asia. Accompanying an early SCSSM onset, anomalous cyclonic circulation appears over the SCS and Philippine Sea, which tends to increase the quantity of midtroposphere moisture via moisture advection and Ekman pumping. Besides, the barotropic energy conversion (from mean kinetic to eddy kinetic energy) associated with this anomalous cyclone will facilitate the development of the synoptic-scale systems. The vertical easterly wind shear accompanying an early SCSSM onset will confine the synoptic-scale wave train to the lower troposphere, which is also favorable for its growth. Once these synoptic-scale systems develop, the baroclinic process (from eddy available potential energy to eddy kinetic energy) will further promote their development. As such, the intensity of synoptic-scale systems is much stronger in early than late SCSSM onset years. The increased moisture and vigorous synoptic-scale systems further lead to an increase in the occurrence of extreme rainfall over Southeast Asia.