华北夏季降水转型环流特征分析

Analysis on the Circulation Characteristicsof Summer Precipitation Conversion in North China

摘要: 本文利用日降水量资料和NCEP/NCAR再分析大气环流等资料，对华北夏季降水雨型年代际转换及环流特征作综合分析，结果表明：1) 我国东部夏季降水型的转换可划为5个时段，1961~1965年华北、东北都明显多雨；1966~1980年华北多雨，东北正常偏少；1981~2000年东北多雨，华北正常偏少；2001~2010年华北、东北少雨，淮河流域明显偏多；2011~2013年华北、东北多雨，江淮流域偏少。2) 近几年华北、东北夏季雨季与1961~1965年相似，降水量明显偏多，但环流形势与1961~1965年有明显不同。在夏季，海平面气压场上，1961~1965年，蒙古低压中心位于蒙古中南部，低压显著加深；而2011~2013年，蒙古低压中心位于蒙古东部至华北、东北地区，低压明显偏弱。在500 hPa高度场上，1961~1965年，西伯利亚槽、华北槽都有所加深，贝加尔湖脊减弱，“阶梯槽”的形势造成华北、东北降水偏多；而2011~2013年，贝加尔湖脊正常，鄂霍次克海位势高度升高，副高偏北，华北槽受东部阻挡的作用加强，结果造成华北、东北夏季降水偏多。在850 hPa风场上，1961~1965年，东亚有明显偏南风异常，蒙古地区有强大气旋性环流异常，与东亚偏南气流在华北、东北西侧产生风向辐合，造成华北、东北降水异常偏多；而2011~2013年，东亚地区无南风异常，但有明显的东南风异常，风速明显小于1961~1965年偏南风，再加上蒙古地区无气旋性环流辐合带来的动力上升条件，造成华北、东北夏季降水虽然比常年偏多，但少于1961~1965年。所以，虽然近年华北和东北夏季降水同时明显增多，与1961~1965年类似，但环流特征明显不同，突出的是1961~1965年为偏南风异常，即东亚夏季风偏强，而2011~2013年为东南风异常，而东亚夏季风并无明显加强。

Abstract: In this paper, a comprehensive analysis on decadal shift of precipitation rainfall pattern and cir-culation characteristics during summer in North China is made based on such data as daily preci-pitation, general atmospheric circulation reanalyzed by NCEP/NCAR. The results of the analysis indicate that: 1) The shift of precipitation rainfall pattern during summer in Eastern China can be divided into 5 periods, i.e., 1961-1965 with significantly more rainfall both in North China and Northeast China, 1966-1980 with more rainfall in North China but normally less rainfall in North-east China, 1981-2000 with more rainfall in Northeast China but normally less rainfall in North China, 2001-2010 with less rainfall in North China and Northeast China but significantly more rainfall in Huaihe River Basin, and 2011-2013 with more rainfall in North China and Northeast China but less rainfall in Yangtze-Huaihe River Basin. 2) In recent years, the precipitation in rainy season (summer) in North China and Northeast China is similar to that during 1961-1965, which is significantly high. However, the circulation pattern is significantly different from that during 1961-1965. For the sea-level pressure field in summer, the low-pressure center in Mongolia was located in South Central Mongolia during 1961-1965, with low pressure trough significantly dee-pened; while it was located in Eastern Mongolia and toward North China and Northeast China during 2011-2013, with low pressure trough significantly weakened. At 500 hPa height field, Siberia trough and North China trough were deepened but Baikal Lake Ridge was weakened during 1961-1965. Besides, the “step trough” brought more rainfall to North China and Northeast China. During 2011-2013, however, the Baikal Lake Ridge was normal, the geopotential height of Okhotsk Sea was increased, the subtropical high was located northward, and the blocking effect by the eastern part on the North China trough was strengthened, resulting in more rainfall during summer in North China and Northeast China. At 850 hPa wind field, southerly wind was obviously abnormal in East Asia and strong cyclonic circulation anomaly also occurred in Mongolian region during 1961-1965. Wind direction convergence was caused in the west of North China and Northeast China with the southerly airflow in East Asia, resulting in abnormally more rainfall in North China and Northeast China. During 2011-2013, however, south wind was normal in East Asia but southeast wind was obviously abnormal, with wind speed significantly lower than that of southerly wind during 1961-1965. Moreover, there was no dynamic condition created by cyclonic circulation convergence in Mongolian region. As a result, the rainfall in summer in North China and Northeast China is more than that under normal condition but less than that during 1961- 1965. Therefore, similar to the period of 1961-1965, the precipitation in North China and Northeast China has increased significantly in recent years, but the circulation characteristics are markedly different. A significant difference was that the southerly wind was abnormal during 1961- 1965 (namely the summer monsoon in East Asia was strong); while the southeast wind was abnormal during 2011-2013 (namely the summer monsoon in East Asia was not markedly strong).