ASYMMETRIC VARIATION OF THE CIRCULATION OVER THE SOUTH CHINA SEA AND THE BAY OF BENGAL AND ITS IMPACTS ON THE SERIOUS DROUGHT IN SOUTHWEST CHINA IN AUTUMN OF 2009

Based on reanalysis data of NCEP and the monthly precipitation data of 160 stations in China during 1971～2009,the characteristics of circulation and water vapor transport at the middle and low latitudes of East Asia in the autumn of 2009,as well as the impact of circulation and water vapor transport features on a drought of the southwest China,were analyzed.Furthermore,the differences of the water vapor transport in autumn over East Asia in different ENSO state were discussed,and the situation in 2009 was compared.The result shows that the circulation situation between the Bay of Bengal and the South China Sea showed asymmetric variation in the autumn of 2009,causing pressure gradients to decrease between the two regions,and anomalously anti-cyclonic circulation and cyclonic circulation appeared over the Bay of Bengal and the South China Sea separately.The southwest China and the Indo-China Peninsula were lying between two centers of anomalous circulation,causing the southwest air current to move into China to weaken anomalouslly and changing water vapor transport accordingly.The water vapor circulation cell of a monsoon depression in the south of the South China Sea was anomalously strong,and the water vapor from the Bay of Bengal and the South China Sea returned to the South China Sea through the Indo-China Peninsula,without re-entry into China,decreasing the precipitation in the southwest China and resulting in the drought eventually.During this period,descending motion was abnormal,convective activity was inhibited,and the degree of drought was increasing over the southwest China.The water vapor flux increased more significantly in the El Nino year than in the La Nina year in autumn over the southwest region and the southern part of the middle and lower reaches of Yangtze River basin,while decreasing over the areas of north and northwest China.The distribution of precipitation in China and the water vapor transport feature of South China Sea in the autumn of 2009 were different from that of normal El Ni o years and even opposite to it.After simple simulation reduction and analysis of the characteristics of El Ni o effect in East Asia in the autumn of 2009,it is considered that the difference mentioned above may be related to the relatively northward location of El Ni o anti-cyclonic circulation.