The Influence of +SR/−EAP Combined Pattern on Persistent Rainfall During the Pre-Rainy Season in South China and Its Associated Energetic Conversion

Precipitation data provided by the China Meteorological Administration and ERA5 reanalysis data from 1990 to 2020 are used to analyse the influence of +Silk Road/−East Asia-Pacific (+SR/−EAP) combined pattern on persistent rainfall during the pre-rainy season in South China (PRPSC) and its associated energetic conversion. The results indicate that the typical +SR/−EAP combined pattern is significantly correlated with the PRPSC. The development of the combined pattern results in the southward shift of the western Pacific subtropical high (WPSH), facilitating the transport of water vapour from the western Pacific to South China. Meanwhile, the South Asian high (SAH) intensifies and extends eastward, with its eastern boundary overlapping the western boundary of the WPSH along the East Asian coast, favouring the precipitation in South China. During the persistence of the combined pattern, integrated moisture flux convergence occurs in South China, with positive vorticity in the lower levels and negative vorticity in the upper levels, resulting in increased precipitation. The +SR/−EAP combined pattern develops by extracting energy from the basic flow via both barotropic and baroclinic energy conversions. However, since the barotropic energy conversion is relatively small and inefficient, the maintenance of the combined pattern mainly relies on baroclinic energy conversion to obtain available potential energy from the basic flow. Barotropic and baroclinic energy conversions primarily occur in regions north of 30°N, effectively supporting the anomalous centres of the +SR/−EAP pattern in mid-to-high latitudes. Baroclinic energy conversion is positive in both the upper and lower troposphere. However, the energy conversion in the upper troposphere exceeds that in the lower, which shows that the baroclinic energy conversion contributes more significantly to the maintenance of the +SR pattern.