湖南夏季旱涝转折异常特征分析

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

利用1961—2015年夏季(5—8月)湖南89个台站的逐月降水资料和NCEP/NCAR再分析资料、海温资料,计算了湖南近55 a的旱涝急转指数(LDFAI),挑选出湖南夏季旱涝急转(旱转涝和涝转旱两种类型)异常年,分析了异常年的同期大尺度环流和前期海温的基本特征,结果表明:(1)旱转涝年,旱期对流层中层鄂霍次克海有阻塞高压,副高偏西偏南,湖南受中纬度偏西气流控制,南亚高压较常年整体偏北偏强,湖南上空伴随着下沉运动加强,水汽辐散,致使湖南少雨干旱;涝期副高较同期偏南,湖南受中纬度低槽和副高共同影响,南亚高压北移,东伸脊点位于川渝交界附近,且高压中心呈青藏高压模态,湖南上空伴随着强烈的上升运动和水汽汇合,导致湖南降水增多。(2)涝转旱年,涝期副高较常年偏东,冷暖空气交汇在湖南地区,南亚高压整体较常年偏南偏弱,湖南上空伴随着上升运动和水汽汇合,湖南偏涝;旱期副高较常年偏西,湖南受副高控制,此时南亚高压主体偏强偏东,东伸脊点位于湖北一带,高压中心呈伊朗高压模态,加上湖南上空下沉运动和水汽输送辐散异常偏强,干旱少雨。(3)LDFAI指数与前期(前一年夏、秋、冬季和当年春季)太平洋相关海区海温存在显著相关性,这为湖南夏季旱涝急转类型的预测提供了参考信号。

Based on the summer monthly precipitation data from89meteorological stations in Hunan,NCEP/NCAR reanalysis data and sea temperature data during1961-2015,the drought-flood abrupt index(IDFAI)in the past55years 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.