The Intensity of the South China Sea Summer Monsoon Onset: Characteristics, Dynamics, and Impact

Abstract The onset of the South China Sea (SCS) summer monsoon (SCSSM) is regarded as the prelude of the East Asian summer monsoon. Compared with the extensively studied monsoon onset date, little attention has been paid to the onset intensity. In this study, an index for the intensity of the SCSSM onset is defined, which shows obvious interannual variability during 1980–2022. Composite results reveal that in strong (weak) SCSSM onset years, the SCS exhibits more (less) pronounced variations in low‐level circulation, monsoon convection, and middle‐to‐upper thermal structure during the monsoon onset. It is found that the 10–30‐day intraseasonal oscillation (ISO) originating from the tropical western Pacific exerts the dominant influence on the SCSSM onset intensity. More vigorous ISO activities can induce more pronounced atmospheric changes over the SCS before and after monsoon onset, thereby enhancing the monsoon onset intensity. Furthermore, the diagnoses of eddy kinetic energy (EKE) reveal the dynamic processes responsible for the stronger ISO activities in strong onset years, in which the dominant roles of the generation of eddy available potential energy and its subsequent conversion to EKE are emphasized. More importantly, the intensity of the SCSSM onset shows obvious impact on regional precipitation in southern China. Specifically, a rapid transition from wet to dry before and after the SCSSM onset is prone to occur in southern China when the monsoon onset intensity is strong. Therefore, monitoring and predicting the intensity of the SCSSM onset are of great scientific and practical significance.