The role of synoptic‐scale waves in the onset of the South China Sea summer monsoon

The role of premonsoon synoptic-scale waves (2- to 10-day oscillation) in the onset of the South China Sea summer monsoon (SCSSM) is investigated in this study. The results indicated that the onset date of the SCSSM from 1979-2016 was related to the intensity of premonsoon synoptic-scale waves. The more active the premonsoon synoptic-scale waves, the earlier the SCSSM tended to be established. This study focused on the accumulative effects of premonsoon synoptic-scale waves on the SCSSM onset because active years often included more than one process. According to the regression analysis results, three premonsoon synoptic-scale waves influenced the onset of the SCSSM. First, waves over the southern Bay of Bengal (BOB) were related to Kelvin waves, which accelerated the intrusion of southwesterly winds over the BOB into the South China Sea (SCS). Second, waves over the eastern SCS were related to tropical depression (TD)-type waves. Westerly anomalies south of this type of synoptic-scale wave were favorable for the onset of the SCSSM. Third, waves originating from the north were hypothesized to be related to the mid-latitude fronts. The mid-latitude vortex moved northeast of the SCS and induced the intrusion of southwesterlies over the SCS. All three waves continuously enabled the westerly anomalies and affected the SCSSM. In addition, the combined effects of these synoptic-scale waves were investigated. The results showed that active waves led to significant anomalous westerlies along the boreal tropical region and cyclonic circulation over the Philippine Sea. Anomalous convection was enhanced over the BOB and SCS, and the western Pacific subtropical high was suppressed. The combined intensities of these premonsoon synoptic-scale waves indicated their potential for use as a new predictor of the onset of the SCSSM.