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

Abstract

Based 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.

Influences of central Pacific warming on synoptic-scale wave intensity over the northwest Pacific

“…Synoptic-scale waves (SSWs) play an important role in the tropical summertime climate over the tropical northwest Pacific (NWP), which influences tropical rainfall (Hurley and Boos 2015;Feng et al 2020a, b), tropical cyclone (TC) activity (Chen and Huang 2008;Chen and Chou 2014;Huangfu et al 2017a;Zhou et al 2018a) and the South China Sea (SCS) summer monsoon (Huangfu et al 2018;Hu et al 2020). SSWs can be studied as synoptic-scale convectively coupled equatorial waves (CCEWs), which include mainly mixed Rossby gravity waves, tropical depression (TD)-type waves and Kelvin waves (Matsuno 1966;Takayabu 1994;Wheeler and Kiladis 1999;Kiladis et al 2009).…”