Propagation Diversity of 20–40-Day Oscillation of Summer Precipitation in the Middle and Lower Reaches of the Yangtze River

The summer precipitation in the middle and lower reaches of the Yangtze River (MLYR) is characterized by obvious intra-seasonal oscillations. This study investigates the propagation diversity of the 20–40-day summer precipitation oscillation in MLYR and its underlying mechanism. The 20–40-day oscillation events manifest as three types: northward propagation, southward propagation, and local oscillation. For northward and southward events, the propagation of precipitation is accompanied by the movement of 20–40-day low-frequency cyclonic vortices in the lower troposphere. Further investigation into the mechanism of anomalous vorticity propagation accompanying low-frequency vortices reveals that positive relative vorticity advection in the north of the vortex facilitates its northward propagation. Moreover, the positive advection is primarily reliant on the background southerly winds to transport the low-frequency vorticity. Diabatic heating dominates the southward propagation of low-frequency precipitation. The center of the latent heating is located in the south of the low-frequency vortex, which steers its southward migration. Furthermore, the uneven spatial distribution of latent heating across the vortex may be attributed to the non-uniform distribution of mean humidity. Based on the above results, sensitivity experiments are conducted using a regional climate model (RegCM4.6). The results demonstrate that when the southerly winds are increased (decreased) or the specific humidity gradient is decreased (increased), the southward (northward) events weaken and disappear, or transitioned into local oscillations or northward (southward) events. This further validates the physical processes through which the basic flow and humidity inhomogeneities affect the meridional propagation of the 20–40-day oscillations of precipitation.