Analysis on anomalous characteristics of the summer drought-flood transitions in Hunan

Based on the summer monthly precipitation data from 89 meteorological stations in Hunan, NCEP/NCAR reanalysis data and sea temperature data during 1961-2015, the drought-flood abrupt index (IDFAI) in the past 55 years are calculated, and the atmospheric circulation of the same period and previous winter's SST field characteristics of the drought-flood abrupt (the drought to flood situation and the flood to drought situation) years are analyzed. The results are as follows. (1) In drought period of the drought to flood years, Okhotsk High is found, Western Pacific Subtropical high (WPSH) is abnormally westward and southward, and Hunan is controlled by west airflow in mid-latitude. At the same time South Asia high is found to be more northerly and strengthened. With strong sinking movement and water vapor divergence over Hunan, it leads to little rainfall and results in drought. In flood period, WPSH is more southerly than the same period, and Hunan is influenced by mid-latitude trough and WPSH. Meanwhile, South Asia high moves northward. The eastward-stretch ridge point is located in the junction of Sichuan and Chongqing and, its center presents Tibetan mode. With strong upward movement and water vapor convergence over the Hunan, it results in vast precipitation and floods. (2) In flood period of the flood to drought years, WPSH is abnormally eastward, and cold and warm air masses converge at Hunan. South Asia high is found to be more southerly and weak. With strong upward movement and water vapor convergence over the Hunan, it results in vast precipitation and floods. In drought period, WPSH is more westerly than in normal years, and Hunan is controlled by WPSH. At the same time, South Asia high is found to be more easterly and strengthened. The eastward-stretch ridge point is located in Hubei, and its center presents Iranian mode. With strong sinking movement and water vapor divergence over the Hunan, it leads to less rainfall and drought. (3) IDFAI index has significant connection with previous year's Pacific Ocean SST, which provides the reference signals for the prediction of summer drought-flood abrupt mode in Hunan.