Abstract:Based on the precipitation observations from 723 basic meteorological stations in China and the NCEP reanalysis data, the possible causes of the 2017 autumn rainfall anomalies in West China were investigated by detailed investigations about the impacts of sub-seasonal to seasonal anomaly of atmospheric circulation, interannual anomaly of external forcing from ocean together with long-term variations of precipitation. The results show that quasi-period variations with multiple timescales, including 3-4 a, 6-9 a, 12-18 a and 36 a since 1960s, along with an increasing long-term trend since 1990 found in the autumn rainfall in Western China. The abnormal heavy rainfall in West China during autumn of 2017 resulted from the superposition influences from multi-time scale variations. In the autumn of 2017, the position of polar vortex in the Northern Hemisphere lay towards East Asia, allowing the cold air from the low trough near Lake Baikal to splitting southward. Meanwhile, the West Pacific subtropical high was excessively stronger and westward. Therefore, a stronger water vapor convergence area was formed and eventually led to the abnormal heavy rainfall in West China. On sub-seasonal time scales, the East Asian summer monsoon retreated more slowly than normal, and the northern boundary position of the summer monsoon was continuously maintained northward than normal. The main rain band was correspondingly maintained around the region of West China, Jianghuai and Hanjiang in September and October. The sub-seasonal variations of the northward water vapor transportation from the West Pacific Ocean through the South China Sea, together with the sub-seasonal activities of northeastern cold vortex, may be sub-seasonal circulation factors which influence the autumn rainfall anomaly in West China. In addition, the interannual anomaly of autumn rainfall in West China and central-eastern equatorial Pacific SST shows a remarkable negative correlation. The La Ni〖AKn~D〗a condition which developed from the late summer and early autumn in 2017 acts as the important interannual forcing of autumn rainfall anomalies. Also, the abnormal heavy rainfall in West China is related to positive phases of decadal to multi-decadal variability and long-term increasing trend in recent years.