Remote Forcing and Local Response: Distinct Mechanisms Govern the Interannual Variability of Extreme and Common Precipitation Frequency in Southwest China during Rainy Season

This study investigates the interannual variations of extreme precipitation frequency (EPF, above 90th percentile threshold) and common precipitation frequency (CPF, between 40th~60th percentile threshold) in Southwest China during rainy season from 1981 to 2024, and explores their underlying mechanisms using observations, reanalysis dataset, and the Linear Baroclinic Model (LBM). We found that EPF exhibits dominant periods of 2–3 and 4–8 years, while CPF shows quasi-2-year and quasi-4-year oscillations. During the positive phase of EPF and CPF, although both of them are driven by enhanced low-level moisture convergence and cyclonic anomalies in Southwest China, their underlying mechanisms and causes are distinct. The convergence for EPF is primarily linked to the remote forcing of Eurasian-like Rossby wave train, which is excited by a dipole structure of sea surface temperature anomalies over the North Atlantic. For CPF, the low-level convergence is associated with the local response of positive phase of the Pacific-Japan (PJ) atmospheric pattern. This PJ pattern is further shown to be stimulated by developing phase of La Niña-like pattern in the Pacific during rainy season, which suppress convection near the Philippines and then trigger the PJ pattern via the wave propagation. Numerical experiments with the LBM corroborate these observed dynamical linkages.