Frequency distribution of drought intensity in Northeast China and relevant circulation background

Considering the possible effect of the climate warming on the severe drought,a drought strength index was defined by using the monthly precipitation and air temperature data from 17 stations in Northeast China during the period 1951-2007.Based on the drought strength index the periodic change of drought strength and frequency distribution were analyzed,and contemporary relationship between the variation of summer drought strength and water vapor transportation anomalies were also analyzed.The result shows that the variation of drought strength has a remarkable 2~3 year period and underwent 4 climate phases during the last 57 years.Occurrence of drought is most frequent and 4.1 months in each year during 1996-2007 may occur with various drought grades,the rate is 34%.The drought rate in summer is the highest,reacges 34.5% and in spring is second,reaches 32.25%,winter's drought rate is the least,and autumn's drought rate is 27.5%.When the summer,Mongolian high pressure is higher,the Western Pacific subtropical high is weaker and southterly,the westerly moisture transfers and the Bay of Bengal southwestern water vapor transportation weakens,and these are primary circulation background to cause drought in summer in Northeast China