Distinctive Features of Tropical Waves Before and After the South China Sea Summer Monsoon Onset

Abstract This study reveals the remarkable differences in the synoptic‐scale tropical waves before and after the South China Sea (SCS) summer monsoon (SCSSM) onset. Before the monsoon onset, the dominant synoptic‐scale systems around the SCS are the eastward‐propagating equatorial Kelvin waves. After the SCSSM onset, while the Kelvin waves remain largely unaltered, the transitions from mixed Rossby‐gravity waves to tropical depression (TD)‐type waves become more pronounced. Such distinctive features of the tropical waves before and after SCSSM onset are attributable to the adjustments in atmospheric mean flow. An anomalous low‐level cyclone develops around the SCS after the monsoon onset. The convergence and shear of this anomalous low‐level cyclone contribute to the barotropic energy conversion from the mean flow to the synoptic‐scale systems, which is conducive to the development of TD‐type waves and genesis of tropical cyclones.