我国西南地区夏季长周期旱涝急转及其大气环流异常

利用1961~2015年中国567站逐日降水资料，定义了一个西南地区夏季长周期旱涝急转指数，结果表明:1961~1970年夏季旱转涝多于涝转旱，1971~1980年夏季涝转旱年较多，1981~2000年旱转涝与涝转旱年相当，21世纪初以来，指数又呈现出负值的趋势，涝转旱年偏多。选取西南地区夏季旱涝急转典型年，对旱涝急转年的大气环流和水汽输送异常进一步分析发现，旱转涝年旱期西太平洋副热带高压偏西偏强，中高纬西风带偏强，冷空气不易南下，垂直场上表现为下沉运动，来自孟加拉湾和南海的水汽输送异常偏弱，西南地区亦处于水汽辐散区，因此降水偏少。而涝期中高纬环流的经向运动增强，乌拉尔山以东的槽加深，东亚沿岸脊加强，中高纬西风带偏弱，在垂直场上表现为上升运动，孟加拉湾和南海为西南地区提供了充足的水汽，有利于该地区降水增多；涝转旱年则相反。

Characteristics of Long-Cycle Abrupt Drought-Flood Alternations in Southwest China and Anomalies of Atmospheric Circulation in Summer

Based on daily precipitation data collected at 567 stations in China from 1961 to 2015, an index of abrupt drought-flood alternation in Southwest China in summer was defined. The results show that the alternation events from drought to flood are more than those from flood to drought in the summers during 1961-1970, the alternations from flood to drought are more during 1971-1980, and the alternations from drought to flood are equal to those from flood to drought during 1981-2000. Since the early 21st century, abrupt alternations from flood to drought tend to increase. Typical abrupt drought-flood alternation events in summer were selected and the characteristics of the anomalous atmospheric circulation and water vapor transportation were analyzed. It was found that the western Pacific subtropical high extended westward and became stronger during the drought period before the conversion from drought to flood. The westerlies at the middle and high latitudes also became stronger, preventing cold air from moving southward. In the vertical, downdraft prevailed and the moisture transport from the Bay of Bengal and South China Sea was weaker than before. Southwest China was located at the moisture divergence area. In addition, there was less rainfall during drought periods. During the flood period after the conversion from drought to flood, meridional circulation at the middle and high latitudes tended to be stronger. The trough to the east of the Ural Mountains deepened and the eastern Asia ridge strengthened. The westerlies at the middle and high latitudes became weaker and updrafts prevailed in the vertical. Also, large amounts of moisture were transported from the Bay of Bengal and South China Sea to Southwest China, which explains why rainfall became abundant there. The opposite was true in the years of conversion from flood to drought.