The complexity of South China Sea summer monsoon onset

The complexity of the South China Sea (SCS) summer monsoon (SCSSM) onset is mainly reflected in the interaction of multiscale processes that include the seasonal cycle, 10-25-day ISO (HISO), 30-60-day ISO (LISO). In this study, the characteristics and mechanism of the HISO and LISO and their interaction with the background field are investigated when they trigger the SCSSM onset base on newly released reanalysis and remote sensing data for the period of 1979–2020.

The SCSSM onsets always are triggered by the second westward HISO or first northward LISO when the control of subtropical high pressure weakens on the SCS. The first HISO can be seen as a signal that the control is weak enough, and the SCSSM is about to onset. The SCSSM can also be established without the effects of the HISO or LISO, but the date would be put off. Based on the budget analysis of column-integrated moist static energy (MSE), the interaction between the easterly trade winds and the zonal gradient of MSE anomalies is considered the dominant reason for the HISO that can successively propagate westward from the western North Pacific. The SSTa-induced turbulent heat flux and the interaction between the mean southerly and the meridional gradient of the MSE anomaly are both important for the northward LISO from the equatorial Indian Ocean when it triggers the SCSSM onset. For the simulation and forecasting of the SCSSM onset, we put more emphasis on the role of the HISO because it is a more active process.