Rainy season onset date in Southwest China and the related atmospheric circulations.

The rainy season onset date (RSOD) in the Southwest China (SWC) has important guiding significance for agricultural planting, and the abnormal RSOD is easy to cause drought or flood disasters in late spring and early summer. However, the RSOD in SWC and its related atmospheric circulation on daily scale and large regional scale has been less explored in the past literature. Therefore, this study aims to explore the circulation characteristics of RSOD on a daily scale in the whole SWC using the long-term observation and reanalysis datasets from 1961 to 2021. Results revealed that the RSOD in SWC advances from the east toward the west, with an average RSOD of May 27 and a standard deviation of 9.19 days. It is found that from around 10 days before the RSOD, the intensity of the India-Burma Trough and the Somali cross-equatorial jet stream were enhanced; the East Asian subtropical westerly jet begin to southward shift, which bring the upward movement, water vapor accumulation and the intersection of cold and warm air in southwest China, causing an earlier onset of rainy season in SWC. Moreover, May 5–25 are crucial to the early or late onset of the rainy season in SWC. When an anomalous zonal dipole of geopotential height in the upper level and an anomalous meridional dipole of geopotential height in the lower level occurred in May 5–25, the rainy season onset earlier. The atmospheric circulation changes approximately 10 days before the RSOD, and the anomalous circulation modes revealed in this study are of great importance for the prediction of RSOD in SWC. • Atmospheric circulation obviously converts or intensifies approximately 10 days before the onset of the rainy season in Southwest China. • May 5–25 are crucial to the early or late onset of the rainy season in SWC. • The dipole modes of geopotential height anomaly in the upper and lower levels affect the early and late onset of the rainy season in SWC. [ABSTRACT FROM AUTHOR]