Interdecadal Variation of Autumn Precipitation in Yunnan since the 1980s and Its Causes

Utilizing observational data of surface station precipitation in autumn in Yunnan from 1981 to 2023, along with concurrent reanalysis data, this study employs cluster analysis to examine the regional variation characteristics of autumn precipitation in Yunnan. The findings are as follows: (1) From the 1990s to the 2000s, autumn precipitation in Yunnan underwent a process of reduction starting from the southeast and gradually extending northwestward. In the 2010s, autumn precipitation began to increase in central and southeastern Yunnan, while it continued to decrease in western and northwestern Yunnan. (2) Autumn precipitation in Yunnan is closely associated with southerly moisture transport in the lower atmosphere over low- to mid-latitude regions. The westward extension of the Western Pacific Subtropical High (WPSH) significantly influences this southerly moisture transport. However, the impact of its westward extension exhibits a complex, nonlinear relationship, as the position of the WPSH differently affects low-level southerly moisture transport in various regions, thereby exerting varying influences on autumn precipitation across different parts of Yunnan. (3) When the average autumn western ridge point of the WPSH extends westward from around 140°E to approximately 110°E, it hinders southerly moisture transport to Yunnan, resulting in reduced autumn precipitation across the region. However, when the average autumn western ridge point further extends westward from around 110°E to approximately 90°E, this westward extension anomaly continues to impede southerly flow in western Yunnan but favors southerly moisture transport from the South China Sea to Yunnan. Consequently, autumn precipitation in central and eastern Yunnan turns to above-normal levels, while precipitation in western to northwestern Yunnan remains below-normal.