Drought-Flood Abrupt Alternation and Its Atmospheric Circulation Characteristics during Flood Season in Southern China

Based on the daily precipitation data of the 753 Chinese stations provided by the National Meteorological Information Center, the drought-flood abrupt alternation phenomenon during the 1960-2015 flood seasons (April to September) was investigated, an evaluation index of drought-flood abrupt alternation was defined, and the atmospheric circulation characteristics of the drought-flood abrupt alternation events were studied. The results show that the drought-flood abrupt alternation events happened 17 times in southern China during the flood seasons of 1960 to 2015, and the frequency of their occurrences declined from three times in the 1960s to once in the 1970s and the 1980s, and then increased rapidly to four times in the 1990s and eight times in the early 21st century. According to the abrupt alternation time, the events are divided into two groups: Drought-to-flood events in late spring and early summer and flood-to-drought events in midsummer. During the drought period of drought-to-flood events in late spring and early summer, the location of the western Pacific subtropical high was more southerly; the eastern part of China was mainly affected by a dry and cold ridge, and the negative vorticity development in the lower-level southern China was accompanied by the strengthening of divergence and descending movement and the weakening of the water vapor transport, which caused less rainfall in southern China. During the flood period of drought-to-flood events in late spring and early summer, the western Pacific subtropical high stretched toward the north; the southern branch trough of the Bay of Bengal was abnormally stronger; southern China was located in the west side of the subtropical high and southeast side of the South Asia high; and the positive vorticity development in the lower-level southern China was accompanied by the strengthening of convergence, ascending movement, and water vapor transport, which caused more rainfall in southern China. During the flood period of drought-to-flood events in midsummer, the location of the western Pacific subtropical high was more westerly; southern China was located in the northwest side of the subtropical high and the southeast side of the South Asia high; and the positive vorticity developed in the lower-level southern China with the strengthening of convergence, ascending movement, and water vapor transport, which caused the precipitation in southern China to increase. During the drought period of drought-to-flood events in midsummer, the western Pacific subtropical high jumped significantly to the north; the South Asia high extended eastward and northward to the middle and lower reaches of the Yangtze River; southern China was controlled by the subtropical high and the upper east wind jets; negative vorticity developed in the lower-level southern China with the strengthening of divergence and descending movement and the weakening of the water vapor transport, which caused the precipitation in southern China to decrease.