Characteristics Of Drought In East China Based On SPEI And Its Relationships With East Asian Summer Monsoon And SST

Based on the CRU precipitation and temperature data from 1961 to 2011, the wind field and the vertical velocity data supplied by NCEP/NCAR, a new drought index——the standardized precipitation evapotranspiration index (SPEI) are used in this study as the criterion to classify the drought level, and the statistical test methods, such as the empirical orthogonal function (EOF), the synthetic analysis are also used. The characteristics of drought variance on different temporal scales over east China and its relationship with East Asian summer monsoon and SST are analyzed on this study. The results show that:(1) The frequency of drought is highest over the east China in the drought variance on the time scale of 3,6, and 12 months. Analysis of EOP applied to SPEI on different time scales show that, drought of the past 51 years over east China exhibits obvious interdecadal variation, and summer is the most significant season that influences drought variance on the scale of year.(2) Over the past 51 years, the highest occurrence frequency appears in the east China during the summer, and the biggest variability appears in north China, and the severity of drought increases in the past 20 years. Among the three sectors of east China, drought over north China is affected mostly by temperature.(3) Drought over north China, the Yangtze River downstream areas and Southern China undergoes significant interdecadal changes during the past 51 years. Drought-prone areas has transferred from the Yangtze River downstream areas before 1979 to Southern China after 1979 and then to north China in the middle of 1990s.(4) East Asian summer monsoon index presrnts the significant interdecadal variations during the last 51 years. Before the mid-70s of the 20th century, there is an obvious difference in droughts between Yangtze River and the southern China, with the stronger East Summer monsoon. Accompanied by the East Asian summer monsoon weakening after the mid-1970s, obvious changes in droughts are only caused in North China.(5) For the drought and flood conditions of east China during summer, the area that influenced mostly by East Asian summer monsoon is located in the Jianghan Plain. When the summer monsoon is strong, the local drought condition is aggravated, otherwise, when the monsoon is weak, the drought condition eased.(6) Tropical eastern Pacific Ocean is the key to affect the drought over the North China, the simulation results of numerical experiments verify that the change of the SST over the key ocean has an important impact of the drought decadal variations in the North China.