Relationship Analysis between South Branch Trough and Severe Drought of Southwest China during Autumm and Winter 2009/2010

By use of the monthly precipitation dataset of 160 stations in China from 1951-2010 and NCEP/NCAR reanalysis data, the evolution characteristics of severe drought in southwest China from October of 2009 to March of 2010 are studied.At the same time, the relationships between south branch trough and severe drought in southwest China during the winter are analyzed by using wavelet analysis, Singular Value Decomposition and composite analysis.The results show that the large area drought take place in Yunnan in October as a sigh of the beginning of severe drought in southwest China, from November to February it spreads all over the southeast China, and in March it relieves.The beginning, evolution and weaken of severe drought in southwest China from Oct.to Mar.is closely related with the geopotential height at 500 hPa and the vertically integrated water vapor flux of the same time.The precipitation in southwest China from Nov.to Mar.and south branch trough index in Nov.both have the significant time-scale change of the 10 to 12-year, and the phases of them are significant opposite.When the geopotential height of south branch trough area in Nov.is negative anomaly or south branch trough index is weaker than usual, the rainfall in southwest China is more and vice versa.SVD analysis shows that in the left homogeneous correlation field of the first mode the Bay of Bengal region (18°~20 °N, 84°~92 °E) is significant key region, and it reflects the information of south branch trough changes.The distribution of the first mode indicates that when 500 hPa geopotential height is negative anomalies in significant key region from Nov.to Mar., the precipitation in southwest China from Nov.to Mar.will be positive anomalies, while in the left homogeneous correlation field of the second mode the 5°~22 °N, 70°~110 °E region is significant key region, and it reflects the information of westerly changes on the north side of the Tibetan Plateau.The distribution of the second mode indicates that when 500 hPa geopotential height is negative anomalies in significant key region from Nov.to Mar., the precipitation in southwest China from Nov.to Mar.will be negative anomalies.The previous 500 hPa geopotential height is a forecast signal for the anomalies of precipitation of the same time.