The analysis of atmospheric circulation during the years with spring drought in Shaanxi

Using the observed precipitation data from March to May during the past 47 years in 51 stations in Shaanxi,as well as the NCEP/NCAR reanalysis data,this paper determines the criterion of years with spring drought and rainy years in Shaanxi by the Synthetical Drought Index,and analyses compositely different atmospheric circulations in years with spring and early summer droughts and rainy years in Shaanxi.The results display that the Shaanxi spring droughts are long-lasting and more droughts happen in northern part than in south,showing inconsistent distribution.There are some key areas with more spring droughts happen,which are the soil erosion areas along the Great Wall over the northern part of Shaanxi,Guanzhong Plain and Qinba mountainous areas.During the years with spring drought,'-+-' height anomaly distribution in mid-high latitudinal of Eurasia at 500 hPa field reveals that mid-Asian ridge and east-Asian trough are much intense,which results in the significant winter monsoon circulation with southern area of warm and cold air;On the contrary in the years with rainy spring,the atmospheric circulation is latitudinal,and the east-Asian trough is weaker and lies in more east area.At the same time,there are more lows in Caspian Sea and Aral Sea,with intense trough in the Bay of Bengal.During late spring and early summer,the height anomaly is '-+-+',which means that more latitudinal circulation with stronger ridge in mid-Asia.As a result,the cold air can move toward southeast by the northern way and the west Pacific high is less strong and lies in its south-east;Oppositely,during the rainy years in late spring of Shaanxi,the Ural ridge is more intense,and more troughs active in the Lake Baikal area.At the same time,the west Pacific high is much stronger,and stronger south wind at the lower troposphere is more prevail than in normal times.At last,the negative correlation between spring droughts and the preceeding-winter height distribution displays that if the Ural ridge is more intense(weaker),Shaanxi would have drought(much rain).