Abstract: Based on the monthly precipitation data from 57 weather stations in the Loess Plateau from 1961 to 2015, the standardized precipitation index (SPI) was calculated as the index of drought and flood to study the circulation characteristics in abnormal drought and flood years in April in the Loess Plateau and their earlier forcing signals. The results are as follows: (1) The ridge from Ural Mountains to Xinjiang in China was stronger, and East Asian trough was deeper on 500 hPa, and the south wind was weaker on 700 hPa over the Loess Plateau and its east, and the temperature was  lower on 700 hPa from east China to Japan, and the westerly jet from east China to the ocean on the east of Japan moved southerly  and  became stronger. For these reasons, it became drought in April in the Loess Plateau, vice versa. (2) The frequency of the blocking in April in the range of 180°E-150°W in mid-high latitude of the Northern Hemisphere was more in the abnormal flood years than in the abnormal drought years. (3) The south Indian ocean dipole (SIOD) or the southwest Indian ocean SST could be used as a previous forcing signal for drought and flood in April. There was a significant negative correlation between SPI in April and SIOD of December in the previous years. (4) The responses of the circulation field to the positive and negative phase anomalies of SIOD were mainly reflected in the differences of the Ural high pressure ridge, the East Asian trough on 500 hPa and the circulations over the area from Lake Baikal to northeast China and Ural Mountains on 700 hPa.

Key words:  drought/flood in the Loess Plateau, standardized precipitation index (SPI), circulation characteristics, the south Indian 

摘要： 基于1961—2015年黄土高原57个基准站逐月降水资料，计算标准化降水指数（SPI），将其作为旱涝指标研究4月黄土高原旱涝异常年环流特征及其前期外强迫信号。结果表明：（1）当500 hPa乌拉尔山至中国新疆高压脊偏强、东亚大槽偏深，700 hPa黄土高原及其以东南风偏弱，700 hPa中国东部至日本温度偏低，中国东部到日本以东洋面西风急流南压加强时，黄土高原易旱，反之易涝。（2）4月北半球中高纬度150°W—180°E区域异常涝年阻塞出现次数多于异常旱年。（3）南印度洋偶极子（SIOD）或西南印度洋海温可以作为4月旱涝的前期强迫信号，4月SPI与前一年12月SIOD存在显著负相关关系。（4）黄土高原旱涝对SIOD正负位相异常的响应主要反映在500 hPa乌拉尔山高压脊、东亚大槽以及700 hPa贝加尔湖至中国东北以及乌拉尔山的环流差异上。

关键词: 黄土高原旱涝, 标准化降水指数, 环流特征, 副热带南印度洋偶极子