Climatic causes of continuous drought over Yunnan Province from 2009 to 2012

By using the observed monthly precipitation data( 1961—2012) of 124 stations of Yunnan Province,the monthly mean atmospheric circulation data and global SST data from NCEP / NCAR reanalysis and the outgoing longwave radiation( OLR) data,the climatic features of continuous drought over Yunnan from 2009 to2012 and the causes of summer drought over Yunnan from 2009 to 2012 with the atmospheric circulation anomaly,SST anomaly and local convection have been studied. The results indicate that firstly,the continuous drought over Yunnan from 2009 to 2012 was a serious drought across whole Province,which lasted for a longer time and took place from spring,summer,autumn to winter. The most serious areas were mainly located in the middle and the southeast of Yunnan. Secondly,the continuous drought over Yunnan from 2009 to 2012 took place against the big climatic background of precipitation reduction. Thirdly,the summer 500 h Pa height field distributions of high,medium and low latitudes over Eurasia were not very similar from these in 2009 to 2012,but the summer circulation configurations in the four years indicated that the cold air over East Asia shifted more east than the normal and the impacts of cold air in southwest China were weaker,so it was unfavourable to cold air southward affecting on Yunnan. In addition,subtropical high extended to the west of its normal position and its strength was stronger in 2009 and 2010 so that it was unfavourable to rainfall over Yunnan Province under the control of subtropical high. In 2011 and 2012,subtropical high extended to the north and east of its normal position so that it was unfavourable to transporting its peripheral vapor to Yunnan Province. The height of most of Yunnan Province and Bay of Bengal was above normal continuously from 2009 to 2012. The India-Burma trough near the bay of Bengal was not active or weak so that it is unfavourable to transporting the south vapor to Yunnan. In addition,during the continuous 4 years summer drought over Yunnan,the SST of tropical Pacific Ocean shows different abnormal stages respectively. Developing stages of El Nino events,decaying stages of La Nina event and the warmer Indian Ocean SST were favorable to the occurrence and development of summer drought over Yunnan. Tropical SST anomaly was one of the important external force factors of summer drought over Yunnan from 2009 to 2012. Finally,the divergence at lower level companied with the convergence at the higher level over most of Yunnan and its south from 2009 to 2012 was unfavourable to the occurrence and development of upward movement. The bay of Bengal and south China sea area was mainly controlled by northwest wind or easterly wind and the northward vapor transport was below normal,which was one of direct causes of continuous 4-year summer drought over Yunnan from2009 to 2012. The local summer convection over Yunnan was weaker than normal,which was one of important factors of summer rainfall being less than normal over Yunnan from 2009 to 2012.