Mechanisms of early and late summer precipitation in Southwest China: dynamic and thermodynamic processes

Abstract This study investigates dynamic and thermodynamic components of moisture flux convergence in Southwest China (SW-MFC) and their underlying physical mechanisms during early and late summer. Using precipitation observation and CRA-40 reanalysis datasets from 1979 to 2023, the results show that both dynamic and thermodynamic processes modulate the SW-MFC in early summer (May-June), with dynamics playing a pivotal role. In contrast, the precipitation anomaly in late summer (July-August) is predominantly driven by the dynamic factors. Meanwhile, the large-scale circulation over the northern Indian Peninsula significantly modulates the SW-MFC. In early summer, anomalous convection around the Maritime Continent with the tripole sea surface temperature (SST) mode in the tropical Indo-Pacific can trigger the formation of “double ring” vertical zonal circulation cells. A large-scale westerly anomaly at the lower troposphere over the northern Arabian Sea foster cyclone strengthening over the northern Indian Peninsula, enhancing southerly moisture transport and increasing precipitation over Southwest China. During the late summer, large-scale dipole SST pattern between the subtropical central-eastern Pacific and the Indo-Pacific warm pool generates significant easterly anomalies towards the Maritime Continent. The SST gradient stimulates an extensive anticyclonic shear zone over the western equatorial Pacific, with an intensified low-pressure zone to its north. This atmospheric pattern over Southwest China and Indian Peninsula can form a vertical circulation circle that largely intensifies widespread precipitation. Numerical model experiments can reproduce the mechanisms of tropical Indo-Pacific joint effects on the Southwest precipitation in both early and late summer, providing a theoretical basis for understanding and forecasting summer precipitation over Southwest China.