Atmospheric circulation during the spring drought and rainy years in Shaanxi Province.

Using the observed precipitation data in March and April from 1961 to 2007 in 86 stations in Shaanxi Province and the NCEP/NCAR reanalysis data,this paper determines the criterion of spring drought and rainy years in Shaanxi Province with the Standard Precipitation Index,and analyses the different atmospheric circulation in spring drought and rainy years in Shaanxi compositely.The results displays that the precipitation in southern part is much than that in northern part of Shaanxi,with significant yearly variations in Shaanxi.During the spring drought years,spring height anomaly distribution in -+- pattern in mid-high latitudinal of Eurasia at 500 hPa field reveals the atmospheric circulation with intense long-wave ridge in Middle-Asia and stronger trough at East Asia;On the contrary in the spring rainy years height anomaly at 500 hPa field displays opposite distribution,with the negative height anomaly in Okhotsk,Caspian Sea and Aral Sea,while positive height anomaly in Japanese Sea and Ural Mountains.This height anomaly distribution reflects that long-wave troughs move easterly by the Tibetan Plateau with the blocked high sets up and collapses,and the latitude-circulation develops in East Asia,which can easily make cold air accumulated in the northern region,at the same time the active warmer wet air can go northerly,resulting in multiple spring precipitations in Shaanxi.At the 850 hPa stream fields,the rainy(drought) spring years usually mean the stronger(weaker) southerly wind in lower troposphere in the eastern coast of Asian continent.Moreover,there are close relationships between the last winter circulation of weak(intense) high ridge and East Asia trough with the rainy(drought) spring in Shaanxi Province.