Study On Characteristics Of Wet And Dry Season In Southwestern China

Southwest China is located in subtropical monsoon region,which has distinct the wet and dry season.By utilizing daily precipitation dataset from 1961 to 2014 in China,the characteristics of wet and dry season in southwest China were analyzed.The results indicated that the climatological onset features in the region started from the eastern and northwestern part,and then advanced to the western Yunnan Province and Sichuan Basin.Meanwhile,the dry season started from the Sichuan Basin,southern Guizhou,and then advanced to northern Sichuan,Chongqing and southwestern Yunnan.The climatological length of wet season was longer in the sub-regions as the central Sichuan,Chongqing,Guizhou and southern Yunnan than in northwestern Yunnan and Sichuan Basin,and vice versa in dry season.The onset date of wet season exhibited abrupt shift in the mid-1970s and the recent five years,while the onset date of dry season became earlier in central and eastern Southwest China.In addition,it is very significant that the length of wet(dry)season became shorter(longer).It was also found there were large differences between the spatial and temporal features of the precipitation in the wet and dry season.There was more precipitation in wet season in southeastern part than in northwestern area,and the decreasing trend was observed in eastern part while increasing trend in the western part.For the dry season,there was more precipitation in the eastern Southwest China than in the western part,the precipitation in Chongqing and Guizhou showed an increasing trend,while a decreasing trend was found in the Sichuan.In the anomalous wet(dry)years of wet season,the onset date was earlier(later),the end date tended to be later(earlier),the length of wet season was longer(shorter).In the anomalous earlier(later)onset years of dry season,the length of dry season was longer(shorter),and more(less)precipitation was observed in the season.We further analyzed the possible cause for the interannual variation of wet and dry season.In the anomalous wet(dry)years of wet season,the La Nina(El Nino)type SST anomalies was observed,with covering of negative(positive)SST over the eastern tropical Pacific and the Indian ocean.Under the circumstance,the intensity of South Asian high and Northwestern Pacific subtropical high was weak(strong)than normal,the Northwestern Pacific subtropical high also shifted eastward(westward),and hence the southwest monsoon was strong(weak).In the anomalous early(later)onset of dry season,the positive(negative)SST anomalies covered the Indian Ocean and the Pacific Ocean,hence the Northwestern Pacific subtropical high was strong(weak)with westward(eastward)shifting of its position,the southwest monsoon was weak(strong).