Abstract:Based on the observed daily precipitation and sunshine hour data in Sichuan Province, the NCEP/NCAR reanalysis daily and monthly data, and the NOAA monthly sea surface temperature data, the variation characteristics of the autumn rainy and sunless weather and its mechanism of the Sichuan in 2017 have been analyzed by the modern statistics methods in this paper. The results showed that the stage characteristics of autumn rain in Sichuan from September to October of 2017 were obvious, showing the characteristics of more rainy days, less sunshine and longer duration of rainy and sunless weather. The area between Baykal Lake and Balkhash Lake was under a wide and inclined low trough in the middle layer of troposphere, which is beneficial to the movement of cold airflow to the southeast, influencing Sichuan. At the same time, the Western Pacific subtropical high was stronger, stretching more westward than normal, and finally the cold and warm airflows met in Sichuan and formed continuous precipitation in September. In October, the Western Pacific subtropical high was stronger, stretching more westward and northward than normal, while the Indian-Burma trough was deeper at 500 hPa, which is beneficial to the water vapor transport to Sichuan from Bay of Bengal and Western Pacific. This is favorable for the continuous precipitation in Sichuan. Further analysis showed that the characteristics of Rossby wave propagation is beneficial to the continuation and maintenance of the Eurasian circulation in the middle and high latitudes, and the Rossby wave energy dispersion is an important cause of maintaining the low trough on the area between Baykal Lake and Balkhash Lake in September. In addition, the sea surface temperature in the equatorial central and eastern Pacific obviously continues to be lower, making the Western Pacific subtropical high become stronger, stretching more westward and northward and Indian-Burma trough was deeper than normal from September to October. Therefore, the influence of the external forcing signal of sea surface temperature and the circulation anomaly caused by the Rossby wave energy dispersion led to the persis-tence of rainy and sunless weather in Sichuan from September to October of 2017.