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

This study investigates the response of mesoscale convective systems (MCSs) over southern China to the onset of the South China Sea summer monsoon (SCSSM) during 2001–20, using 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 SCSSM onset. The decline in MCS frequency contributes substantially to the reduction in total precipitation over southern China after SCSSM onset. MCS-related precipitation accounts for 60% of total rainfall reduction, and meso-α-scale convective systems (MαCSs) contribute a larger share of this reduction than 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 suppression of MCS activity 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 weakening of the southerly flow in the lower troposphere, and the weakened vertical shear is due to a diminished meridional temperature gradient in the lower troposphere.