2013年长江中下游夏季高温干旱演变过程及环流异常成因简析

利用改进的CIn指数，结合NCEP/NCAR再分析资料，逐候分析2013年夏季长江中下游地区高温干旱演变过程和高温异常成因。结果表明：改进后的CIn指数能够准确识别此次干旱过程，整体持续时间近一个月。旱情从7月第4候湖南南部开始，8月第3候干旱程度最强，特旱中心位于湖南省中东部，浙江省为高温中心，8月第4候旱情得到缓解。在干旱成因上，极涡位置异常偏西，影响长江中下游地区冷空气偏弱，南亚高压东伸与西太平洋副热带高压西伸明显；鄂霍次克海至菲律宾一带呈较强的EAP遥相关型，其中长江中下游地区位势高度的正异常加强了西太平洋副热带高压的中心强度并使其位置偏西；乌拉尔山地区的阻塞高压与西太平洋副热带高压相对峙，有利于西太平洋副热带高压长期稳定地控制在长江中下游地区；同时欧亚大陆与西太平洋海陆温差增大，东亚夏季风偏强，西太平洋副热带高压异常偏北，长江中下游地区下沉气流强盛，在西太平洋副热带高压和南亚高压共同作用下，造成持续的高温干旱过程。

Evolution and abnormal causes for high temperature and drought in the middle and lower reaches of Yangtze River in summer of 2013

Based on the improved drought index of CIn and the NCEP/NCAR reanalysis data, the evolution and the causes of summer droughts over the middle and lower reaches of Yangtze River in 2013 were analyzed. Results show that the improved drought index of CIn is good for monitoring the duration of the drought, which lasted for a month. The event began from the fourth pentad in July in the southern Hunan province, and the third pentad in August had the heaviest drought whose center was located in the middle and eastern Hunan province. Zhejiang province was the high temperature center and the drought alleviated at the fourth pentad in August. It was shown that the extremely westerly extension of polar vortex led to the weak cold air in the middle and lower reaches of Yangtze River. As a result, the South Asia high pressure moved abnormally eastward while the Western Pacific Subtropical High (WPSH) moved obviously westward. The area covered from Okhosk Sea to the Philippines showed strong EAP teleconnection patterns, in which, the geopotential height of the positive phase in the middle and lower reaches of Yangtze River strengthened the center of WPSH and made its location westerly. When the blocking high over Ural Mountains and WPSH confront each other, which was favorable to control stably the middle and lower reaches of Yangtze River for a long term. Meanwhile, the temperature difference between the Eurasian continent and the West Pacific ocean increased, the East Asian summer monsoon became strong, the northward extension of WPSH became anomalous, and the sinking air current in the middle and lower reaches of Yangtze River was strong, all of which caused the continuous high temperature and drought.