Variation Characteristics And Influencing Factors Of Autumn Rain In West China From 1979 To 2018

West China is a typical autumn rain area in China,and its autumn precipitation is a major climatic feature in the southwest region.It is usually called the West China autumn rain.The forecasting work for it has always attracted the attention of Chinese meteorologists.This article is based on the monthly reanalysis data of ERA-Interim provided by the European Center and the daily precipitation data of the national meteorological stations from 1979 to 2018 provided by the China Meteorological Data Network,as well as the Indian Ocean Dipole Index and West Pacific Subtropical High Ridge Line Index.In this paper,the methods of EOF analysis,wavelet analysis,correlation analysis,and synthetic analysis are used to explore the reasons for the change in the amount of autumn rain in West China and the impact of the Indian Ocean dipole and the position of the ridge line of the Western Pacific Subtropical High Pressure on autumn rain from the perspective of a large circulation background.The purpose is to provide a reference for improving the level of autumn rain forecast in West China.The main conclusions of this article are as follows:(1)Inter-decadal change of autumn rain in West China.More precipitation in autumn before the 1980 s,there was a decrease from the mid-late 20 th century to the end of the 2000 s,and it gradually increased after entering 21 th.At the same time,there is periodic oscillation of precipitation.There was a quasi-ten-year periodic oscillation around 1985-1995.From 2010 to the present,the autumn rain in West China has a very obvious 2--5 year cycle.The number of autumn precipitation days in West China is divided into two stages with 1988 as the boundary.There are many rainy days in the early period,and the number of precipitation days in the later period has decreased.Three different types of autumn rain in West China reflect the regional differences of autumn rain in West China.(2)Large-scale circulation background analysis shows: The dry and cold air currents from the north and the warm and humid air currents in the south intersect frequently in the West China area,which intensifies the frontal activity,and the suction effect of the high-altitude jets enhances the ascending movement,providing good power conditions for the autumn rains in West China.At the same time,the airflows of the Arabian Sea and the Indian Ocean turning north through the Bay of Bengal and the eastward airflow on the south side of the Western Pacific Subtropical High converging with tropical water vapor provided the water vapor conditions for the west China region.(3)West China autumn rain and summer IOD index showed a significant positive correlation,with a correlation coefficient of 0.37,which passed the 95%significance test.The synthetic analysis shows: When the early IOD is positive,the probability of an increase in autumn rains in West China was high,and conversely,the probability of autumn rains decreased.(4)The autumn rain in West China was significantly negatively correlated with the position of the ridge line of the Western Pacific Subtropical High in summer,with a correlation coefficient of-0.26,which passed the 90% significance test.the result shows: When the ridge line of the Western Pacific Subtropical High in the early stage was south,the probability of an increase in autumn rains in West China was high,and conversely,the probability of autumn rains decreased.