Diagnosis of the Severe Drought in Autumn/Winter 2009-2010 in Yunnan Province

Severe drought disaster occurred in a large area of Yunnan during autumn and winter 2009-2010,that is the heaviest one in the province since meteorological record was established. The formation of the drought is analyzed in this paper. It is found that the drought event was directly related with the abnormal atmospheric circulation systems. The main causes that lead to the event were that the polar vortex at 500hPa slanted towards the west hemisphere and its intensity wass weaker than average. The western Pacific subtropical high at 500hPa located at a more western position and was stronger than normal. The water vapor brought to Yunnan was inhibited as the Bay of Bengal was controlled by northeast flow at 700hPa flow field. The dry flow was dominant over Yunnan and no cold air was brought to the province during autumn and winter 2009-2010. OLR was higher in the Bay of Bengal and Yunnan. Convective activities were inhibited and very few rainfall cloud systems could be formed.