Research On The Influence And Mechanism Of The Mid-high Latitude System On The Autumn Precipitation Change In West China

The autumn precipitation in West China(WCAR)is a special rainy weather phenomenon in autumn,the percentage of precipitation in the whole year is only less than that in summer,with the second largest amount of precipitation.It is also the last rainy season in China associated with the East Asian summer monsoon,which has the characteristics of continuous rainy days,but the rainfall is not so large and the intensity is weak.It is prone to floods and secondary disasters.This special climate phenomenon has an important impact on China's agriculture,economy and people's life.Therefore,it is of great significance to understand its variability and associated physical mechanism.Based on the CN05.1 gridded observed rainfall data from 1961 to 2016 and NCEP/NCAR reanalysis data,this paper analyzes the interdecadal variation characteristics of WCAR and the mechanism of the mid high latitude circulation system in the northern and southern hemispheres on it by means of synthesis,regression and correlation,and further studies the influence of the boreal summer sea ice concentration(SIC)of the South Indian and the South Pacific Ocean Basin on the variability of WCAR.The main conclusions are as follows:(1)The results show that there are significant interdecadal variations of autumn precipitation in West China from 1961 to 2016.WCAR shifted from the positive phase to the negative phase in the mid-1980 s.After entering the 2010 s,the WCAR tends to increase again,with the increasing amplitude even larger than that before the mid-1980 s.(2)The physical mechanism responsible for the interdecadal change in WCAR is further explored from the perspective of large-scale atmospheric circulations by means of synthesis and regression,particularly focusing on the similarity and difference of the atmospheric circulations for the increase of WCAR after the 2010 s and before the mid-1980 s.It is found that the main circulation systems affecting the WCAR include: the negative Asian sea level pressure anomaly,the deepening of the Lake Balkhash trough,the prevailing southerly wind in East Asia and the northward displacement of the westerly jet stream over East Asia.During these two periods,compared with the negative phase of the WCAR,both the Lake Balkhash trough and the Asian sea level pressure are strengthened,favoring the southward outbreak of cold airs from high latitudes into West China.Meanwhile,the southerly anomalies prevail in the lower troposphere of East Asia,benefiting for the transport of water vapor from low latitudes toward West China.In addition,the East Asian jet stream shifts northward,providing a favorable dynamical condition for the occurrence of WCAR.Under such a configuration,the WCAR is thus increased.With respective to the counterparts before the mid-1980 s,the blocking over Europe and the trough over Lake Balkhash are both much stronger,which may induce more cold airs from high latitudes to invade West China.Concurrently,more warm-moist airflows from the Indian Ocean in the Southern Hemisphere are transported toward West China.More cold airs and more warm-moist airflows which encounter in the targeted region contribute to more rainfall after the 2010 s than before the mid-1980 s.(3)Furthermore,the study of Antarctic sea ice reveals that variability of WCAR is positively correlated to the boreal summer SIC in the Southern Indian and Pacific Ocean basins.Such a relationship is well supported by the SIC-related changes in atmospheric circulations and water vapor transport.Corresponding to a high summer SIC,both the Lake Balkhash trough and the Asian low pressure are deepened,the East Asian jet stream shifts northward,and the water vapor transport to West China is enhanced during the boreal autumn,providing a favorable background for the increase of WCAR,and vice versa.The potential mechanism of how the boreal summer SIC affects the autumn atmospheric situation is further investigated.Results show that the atmospheric circulation in the high latitudes of the Southern Hemisphere possesses a seasonal persistence from the boreal summer to autumn,which may result from the persistence of sea ice.Thus,the boreal summer SIC-related anomalies in the upper-tropospheric winds over the Southern Indian and Pacific Ocean basins can persist to the following autumn and subsequently exert substantial influences on the Eurasian atmospheric circulations via two meridional teleconnections.The counterpart in the lower troposphere contributes to the water vapor transport toward West China.Both of the northern and southern hemispheres' circulation system in the middle and high latitudes plays an important role in the influence of WCAR.The two hemispheres can be connected by the wind field at high level and low level and the water vapor transport in the two ocean channels of the Pacific Ocean and the Indian Ocean.The combination of various meteorological elements has an important impact on WCAR.