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

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.