Impacts of the South China Sea Summer Monsoon Onset on the Mesoscale Convective Systems over Southern China

This study investigates the response of mesoscale convective systems (MCSs) over southern China to the South China Sea summer monsoon (SCSSM) onset during 2001-2020, using the high-resolution satellite data from the Global Precipitation Measurement (GPM) mission and an iterative tracking algorithm for MCS identification. Composite analysis reveals that the MCS frequency over southern China decreases by more than half during about 10 days after the SCSSM onset. The decline in MCS frequency substantially contributes to the reduction in total precipitation over southern China after the SCSSM onset. The MCS-related precipitation explains 60% of the total rainfall reduction, and the meso-α-scale convective systems (MαCSs) contribute a larger share of this reduction than the meso-β-scale convective systems (MβCSs). Furthermore, the decrease in MαCSs explains a larger fraction of the reduction in precipitation as precipitation intensity increases. The MCS activity suppression over southern China can be explained by the significant weakening of both the upward motion and low-level vertical zonal wind shear. The weakened upward motion is due to the southerly weakening in the lower troposphere, and the weakened vertical shear is due to a diminished meridional temperature gradient in the lower troposphere.