Subregional division of the rainy season onset over Southwest China and its distinct mechanisms

The rainy season onset (RSO) over Southwest China (SWC) critically affects regional droughts and floods. Most previous studies treated the regional RSO over SWC as a single object, thereby overlooking its pronounced spatial disparities. Using the cumulative precipitation departure and rotated empirical orthogonal function, this study identifies two dominant subregional modes of RSO variability over SWC: a western mode and an eastern mode. Climatologically, the RSO mainly occurs in May over western SWC but in April over eastern SWC. More importantly, the interannual variations of the two subregional modes are governed by different physical mechanisms. For the western mode, earlier onset is associated with strengthened lower-tropospheric southwesterlies and enhanced moisture transport over the North Indian Ocean. These anomalies are closely related to a deepened India–Burma trough and an upper-level warm anticyclonic anomaly over northwestern India. The latter is embedded in a quasi-stationary Rossby wave train that may originate from the North Pacific and propagate eastward across North America, the North Atlantic, and western Europe toward northwestern India, likely associated with Pacific La Niña-like SST anomalies. For the eastern mode, earlier onset is linked to a localized anomalous cyclone over the Indochina Peninsula, which favors ascent over eastern SWC. This circulation anomaly may be connected to a subtropical Rossby wave train propagating along the subtropical westerly jet from North Africa to South China. These results highlight the necessity of considering subregional differences when investigating and predicting RSO variability over SWC.