Anomalous Characteristics of Autumn Rain in Western China in 2017 and Its Possible Mechanism

The autumn rain in western China is more typical in 2017. The rainfall is 49% more than that of the normal year and comes the first since 1984 (Zhi Rong, et al., 2018). Continuous rainfall caused serious flooding disasters, local landslides, debris flows and other disasters. And the continuous lack of sunshine is unfavorable to the autumn harvest and autumn sowing of crops (Feng Aiqing, et al., 2018). In this paper, based on the new monitoring indicators of autumn rain in western China published by the China Meteorological Administration in 2015 and NCEP reanalysis data, the anomalous characteristics of autumn rain in western China in 2017 and its related large-scale circulation are analyzed. The conclusions are as follows: (1) The characteristics of autumn rain in the southern and northern regions of the monitoring area are basically the same. The onset date of autumn rain in the north (south) of monitoring area in 2017 is 14 (16) days earlier than that of the normal year. And the withdrawal date is 5 days later (earlier). the autumn rain period is 54 (63) days, which is 19 (11) days longer than that of the normal year. The rainfall of autumn rain in the north (south) of monitoring area is 234.1 (322.2) mm, which is 72.6% (74.4%) more than that of the normal year. Especially, the amount of autumn precipitation in the south of monitoring area in 2017 comes the second since 1981. (2) The advanced transition of east Asian atmospheric circulation system from summer pattern to winter pattern resulted in a significant early onset of autumn rain in western China in 2017. In the upper troposphere, the westerly jet appeared about 10 days earlier than that of the normal year. In the middle and low troposphere, the meridional circulation of west high and east low pattern was observed in the middle and high latitudes of Eurasia. The cold air from the north and the warm and wet air from the southwest side of the Western Pacific Subtropical High (WPSH) converged in the southwest of China, resulting in the early onset of autumn rain in western China. (3) Influenced by the persistent warming of Indian Ocean Basin-Wide mode (IOBW) from spring to summer, the intensity of the Western Pacific Subtropical High (WPSH) extremely strong (the second strongest since 1981) and its west point extended extremely westward (also the second place since 1981) during the autumn rain period. The intensity of moisture flux from the southwest side of the Western Pacific Subtropical High (WPSH) is stronger. Additionally, the strength of the low trough of Baikal Lake is stronger. The cold air from the north and the abundant warm and wet water vapor from the south converged in Western China, resulting in more precipitation during autumn rain period.