Regional Persistent Extreme Precipitation Events over Southwest China under Different Low-Latitude Intraseasonal Oscillations during the Rainy Season

Abstract

Abstract This study investigates the mechanisms of low-latitude intraseasonal oscillations affecting regional persistent extreme precipitation events (RPEPEs) over Southwest China (SWC) during rainy seasons. Most of the RPEPEs over SWC are dominated by 7–20-day variability. The RPEPEs over SWC are preconditioned by two different types of 7–20-day Rossby waves with almost opposite phases over the western North Pacific (WNP). The two types of 7–20-day Rossby waves have direct and indirect effects on type-1 and -2 RPEPEs, respectively. For type 1, a coupled 7–20-day low-level anticyclone and suppressed convection originating from the tropical WNP propagate northwestward and cover the region from the South China Sea (SCS) to the Bay of Bengal before the RPEPEs. The anticyclone triggers ascending motion over SWC and transports more moisture to SWC, favoring the SWC RPEPEs. Before the type-2 RPEPEs, a coupled 7–20-day low-level cyclone and enhanced convection propagates from the tropical WNP to the SCS. The enhanced convection over the SCS leads to the westward extension of the western Pacific subtropical high (WPSH) and the eastward shift of the South Asian high (SAH). The variations in the WPSH and the SAH directly cause SWC RPEPEs by inducing ascending motion and transporting moisture. The mechanisms for type-2 RPEPEs tend to work under the background with a strong WPSH. Using a Lagrangian model, we found that both the 7–20-day oscillations and their background atmospheric circulations result in significant differences in moisture sources for the two types of RPEPEs. These findings benefit a better understanding of SWC extreme precipitation events.