After the onset of the South China Sea (SCS) summer monsoon, monsoon southwesterlies undergo several multiday active periods, named southwesterly monsoon surge, when persistent extreme precipitation is usually observed in southern China. Coupling with the regional forcings over the SCS and southern China, the surge of southwesterly may exhibit large or small diurnal cycles and result in different rainfall patterns, which is worthy of further research. To address this issue, the present study analyzes the diurnal cycles of southwesterly monsoon surge to clarify the coupling physical mechanisms and joint rainfall impacts of large-scale monsoonal disturbances and regional diurnal forcings.
        There are 51 episodes of southwesterly monsoon surge identified in 2014–2023, most of which are observed from mid-May to mid-July. They tend to accompany with the 10–20-day quasi-biweekly oscillation in the strengthening phase of the 30–60-day intraseasonal oscillation. Besides the intraseasonal oscillations, these monsoon surge episodes occur with strong pressure gradients over the SCS induced by the synoptic disturbances of a northwestward-moving anomalous low and the westward-expanding WPSH. The strong pressure gradients enhance the geostrophic winds that greatly contribute the southwesterly monsoon surge, but the wind diurnal amplitudes are mainly regulated by the inertial oscillations of ageostrophic wind. It is found that the diurnal amplitudes of southwesterly monsoon surge are associated with atmospheric tide and land-sea thermal contrast. The decay of atmospheric tide and the cooling of the boundary layer in southern China and the SCS can greatly facilitate the large-scale sea breezes in nighttime, which are characterized by the nocturnal northward surge of active southwesterly.
        The rainfall patterns in southern China vary with the wind diurnal cycles during the episodes of southwesterly monsoon surge. Focusing on Guangdong Province, the most populous province of China, the surge episodes with large diurnal cycles can yield heavy rainfall extensively from the southern coastal area to the northern mountainous area, while the rainfall during the episodes with small diurnal cycles is mostly concentrated in the southern coastal area. Such differences in rainfall spatial patterns are closely related to the different rainfall activities in the morning. During the surge episodes with large diurnal cycles, the morning rainfall tends to be remarkable in the northern mountainous area, which is enhanced by the nocturnal acceleration of monsoon southwesterly. Active monsoon southwesterlies in nighttime can transport abundant warm moist energy to southern China. They also lead the rainfall systems in southwestern China eastward with positive vorticity and dynamic lifting in the windward of Nanling mountain. As a comparison, the dynamic and thermodynamic conditions for precipitation in the surge episodes with small diurnal cycles are confined to the southern coastal area where the rainfall is concentrated. These results highlight that the large-scale monsoonal disturbances and regional diurnal forcings have a close relationship with each other and jointly influence rainfall in southern China.

Asian Summer Monsoon,Diurnal variation;,Precipitation