关键词: 梅雨, 异常降水, 旱涝急转, 季风

Abstract:

 Based on the NCEP/NCAR reanalysis data with the 2.5°×2.5°resolution, the OLR (Outgoing Longwave Radiation) data from the NOAA, the observed precipitation data and the recorded Meiyu index, the evolution characteristics of the South Asia high, the subtropical high, southwest monsoon and convective system in 2011 were analyzed in order to reveal the cause of abnormal precipitation during the Meiyu period. The results show that the abnormal characteristics during the Meiyu period include the advanced beginning and ending dates, the sudden turning from drought to flood, intensive and abnormal abundant precipitation in 2011. The dates of northward jump of the South Asia high and the subtropical high, adjustment of the northern westerly belt circulation at 500 hPa and pushing northward to the Yangtze River valley of southwest monsoon are all earlier than those under the normal conditions, which result in the earlier beginning of Meiyu. The main reason for the earlier ending of Meiyu is that the subtropical high moves northwards and eastwards which is caused by ITCZ (Intertropical Convergence Zone) strengthening and surging northwards with the northward STS (Severe Tropical Storm) “Meari”. The position and intensity of the South Asia high and subtropical high adjust rapidly, so is the circulation of middle-high latitudes. At the same time, the Southwest monsoon and moisture flux transformation are also strengthened. All these factors result in the change from winter monsoon circulation to convergent zone of dry-cold air and warm-wet air in the region of middle and lower reaches of the Yangtze River. The atmospheric stratification is not stable and precipitation intensity is strong during the Meiyu period. Thus, the above-mentioned factors lead to the abrupt change from drought to flood. During the Meiyu period, the subtropical high and the upper westerly jet are stable and strong. The stronger monsoon surge, dry cold air from the middle-high latitudes and propagating eastward convection disturbance over the Tibetan Plateau are favorable to the abnormally abundant precipitation during the Meiyu period in 2011.

Key words: Meiyu, Abnormal precipitation, Sharp turn from drought to flood, Monsoon