近40年我国东部降水持续时间和雨带移动的年代际变化

Decadal Variability of Rainfall Persistence Time and Rainbelt Shift over Eastern China in Recent 40 Years

利用我国地面观测站降水资料以及欧洲中期数值预报中心(ECMWF)的月平均再分析资料,研究了在全球平均表面气温偏冷和偏暖阶段,我国东部降水开始和结束时间以及雨带南北移动的变化,并分析了与东部降水变化相关联的大气环流特征。结果表明:近40年,20世纪60—70年代全球平均表面气温处于一个相对偏冷时期,而80—90年代处于偏暖时期;在这样的变暖背景下,我国东部地区年总降水量呈现出“南涝北旱”异常特征,与冷位相比较,在暖位相阶段长江流域年总降水量明显增加,而华北地区降水量减少,其中长江流域降水的增加主要是由夏季降水增加引起的,3月长江中下游降水增加也很重要,北方的降水减少主要是由从盛夏到初秋的降水减少引起的。平均而言,暖位相阶段我国南方强降水开始时间较早、结束较晚,持续时间较长,而北方强降水开始较晚,持续时间较短。从春末到夏季,冷位相时我国东部强降水带表现出从华南、经过长江流域向华北移动的特征,而在暖位相时强降水主要集中在长江流域,从华南向华北移动的特征不明显。雨带的这种异常变化与东亚大气环流有关,在暖位相时夏季东亚大陆低压比冷位相时弱,而鄂霍次克海高压偏强,西太平洋副热带高压位置偏南,使夏季东亚副热带地区的西南风减弱,梅雨锋加强,导致雨带滞留在长江流域,使长江流域降水增加、北方降水减少。

Using observations of precipitation from rain gauge stations in China for the period of 1960-1999 and the monthly European Center for Medium-Range Weather Forecast reanalysis for the period from 1958 to 2001, the variations of the beginning and ending time of the rainfall and rainbeh shift over eastern China between the cold and warm periods of globally averaged surface air temperatures are examined. The associated atmospheric circulation over East Asia is also analyzed. The results show that in recent 40 years, the relative cold period of the surface air temperature appears in the 1960s and 1970s, while the warm period appears in the 1980s and 1990s. Compared to the cold period, the annual total rainfall in the warm period is characterized by the feature of southern floods/northern droughts, namely that the annual total rainfall increases along the valley of the Yangtze River in the warm period and decreases over North China. The increase in the valley is mainly due to the increase of rainfall in summer （June to August） and March, while the decrease is attributed to the decrease from July to September. On the average, the persistent heavy rainfall over southern China （25°-30°N, 115°-120°E） begins earlier, ends later, and maintains longer in the warm period than in the cold period. Over northern China （34°-40°N, 115°-120°E）, the persistent heavy rainfall begins later and maintains shorter in the warm period. Moreover, there is a significant difference in the rainbeh shift over eastern China from late spring to summer between the cold and warm periods. In the cold period, the rainbeh shift shows a pronounced feature from South China via the valley of the Yangtze River to North China. However, in the warm period, the rainbeh mainly appears along the valley, not showing a striking shift from South China to North China. The southern floods/ northern droughts over eastern China are associated with anomalies of the East Asian atmospheric circulation under global warming. In the warm period, the low over the Asian continent and the high over the Sea of Okhotsk are strong during summer, accompanied by the anomalous northeasterlies from the south of the Sea of Okhotsk to mid-latitudes of East Asia, which may strengthen the activities of cold air at these latitudes. Meanwhile, the subtropical high over the western North Pacific is more northward in the warm period. Corresponding to these anomalies, the East Asia summer subtropical monsoon weakens and the Meiyu front strengthens along the valley of the Yangtze River, which makes the rainfall stay in the valley, resulting in the increase of local rainfall. Anomalous downward motions appear over northern China, resulting in the decrease of local rain.