Characteristics Of Atmospheric Circulation And Causes For Summer And Autumn Droughts In The Eastern Region Of Southwest China

Based on NCEP/NCAR reanalysis data and rainfall data of753weather stations in China from1961-2010, the causes for autumn and summer droughts in the eastern region of the southwest part of China(ESWC) are discussed in this paper. The main results of the research are as follows:(1) There is an obvious decline trend in the autumn precipitation in the ESWC. The main characteristics of the interdecadal change of autumn precipitation is that a sudden shift from wetness to dryness appears around the late1980s.(2) The interannual variations of precipitation is highly related to the SSTA over the regions from the eastern Indian Ocean(around Sumatra) and western Pacific(EIWP) to eastern Pacific(EP). By inducing abnormal atmospheric heating, the positive and negative abnormal pattern in SST over the EIWP and EP induces cyclonic circulations around South China Sea and Bay of Bengal,together with enhanced Hardly Cell. In this case, the area of interest is then controlled by both northerly wind and enhanced downward vertical motion while the water vapor from then Bay of Bengal is reduced. All of the above reasons lead to autumn droughts in the studied region.(3) The above observational results are further confirmed by a series of numerical model simulations using NCAR CAM3.0(Community Atmosphere Model3.0).The simulations also show that the SST of the western Indian Ocean has no impact on the autumn rainfall in the area of interest.(4) The interdecal change and linear trend of autumn precipitation in ESWC is closely related to the enhancement of Hardly cell, which is directly caused by the increase of SST around the tropic, and the cooling in the upper level in China is also an important reason for the subsidence north of the Hardly cell.(5) The summer precipitation in the area of interest is highly related to the heating source in the eastern part of the Tibetan Plateau(ETP).When the heating around ETP is stronger, the rainfall in ESWC would be more, vice versa.(6) The abnormality of the heating source in ETP may affect the summer precipitation in ESWC by causing abnormal movement of south Asian high and western Pacific subtropical high.