2011年长江中下游地区旱涝急转成因分析

Cause Analysis for the Sharp Turn from Drought to Flood in the Middle and Lower Reaches of the Yangtze River During 2011

利用NCEP和国家气候中心提供的再分析资料,对2011年发生在长江中下游地区的旱涝急转事件的成因进行了初步分析。结果表明:此次旱涝急转发生在特定的大尺度环流背景下,5月极涡偏强,而南亚高压和西太平洋副热带高压均偏弱、偏南;6月极涡强度变弱、范围偏小,而南亚高压增强、西移,西太平洋副热带高压加强、西伸。季风活动异常是引发此次旱涝急转的重要因素,前期南海夏季风强度弱,且中间一度中断,6月初突然增强并北推至长江中下游地区。前期我国主要的3条水汽输送带均明显偏弱,6月初又明显加强,导致大量暖湿气流汇集于长江中下游地区并与南下的冷空气交汇,为旱涝急转的发生提供了良好的水汽条件。2010年7月开始至2011年4月结束的拉尼娜事件是此次旱涝急转事件可能的重要外强迫条件之一。

Some kinds of climate monitoring and diagnosis data provided by National Climate Center（NCC） were used to preliminarily analyze the reasons for the sharp turn from drought to flood （STDF）in the middle and lower Yangtze River during 2011. The results showed that firstly this STDF occurred under a special large-scale circulation background, the polar vortex was stronger in May 2011, while both the south Asia high and subtropical high were weaker. In June the polar vortex became weaker with smaller area, meanwhile the south Asia high strengthened and moved northwestward, the subtropical high strengthened and extended westward. Secondly, abnormal monsoon activity was an important factor to causing this STDF, the southwest monsoon was weaker in early May 2011 and interrupted for a time, then suddenly intensified in early June and extended northward to the middle and lower of Yangtze River. Thirdly, three main moisture transportation channels to China were noticeably weaker at the early stage and strengthened in early June, which caused abundant warm and humid air to gather and converge with cold air in the middle and lower of Yangtze River, and provided moisture conditions for the occurring of STDF. Fourthly, the La Ni？a event beginning in July 2010 and ending in April 2011 was probably an important external forcing condition for this STDF.