Characteristic analysis of atmospheric circulation for extreme heavy drought in Yunnan Province in autumn of 2009

Based on monthly precipitation data of the 30 stations in Province Yunnan and the NCEP/NCAR reanalysis data from 1961 to 2009,the rainfall anomaly and the atmospheric circulation features in autumn 2009 were analyzed.The atmospheric circulation features were analyzed with water vapor flux features,Westerly wind anomalies and subtropical high.The results show that the rainfall in Yunnan Province is reduced in autumn 2009.At the same period,warm moisture flux from the Bay of Bengal region and the South China Sea decreased.The interaction between cold moisture flux from Inner Mongolia and warm moisture flux from the Bay of Bengal and North Indochina was weakened.Autumn moisture convergence reduced in Yunnan Province in 2009.Zonal westerly circulation enhanced in September and October in 2009,and in November,meridional westerly circulation become stronger,but the 700-500hPa meridional wind vector over the western Inner Mongolia,Ningxia,Sichuan weakened.The west Pacific subtropical high became stronger and moved west in September and October 2009,Yunnan was under the control of the west Pacific subtropical high,this made water vapor convergence reduced;in November 2009,the west Pacific subtropical high stretched toward the Bay of Bengal region,southerly wind only exists over the north Bay of Bengal region to the north of 17.5°N,this made Southwest China moisture flux from the Bay of Bengal weakened.Based on the above analysis,authors think water vapor flux anomaly,westerly circulation anomaly and the west Pacific subtropical high being stronger and moving west are important factors affecting the autumn rainfall in Yunnan.