OLR与长江中游夏季降水的关联

ASSOCIATION BETWEEN GLOBAL OLR AND SUMMER RAINFALL OVER THE MIDDLE REACH OF YANGTZE RIVER

用SVD方法分析了1、4、7月全球OLR与夏季(6—8月)中国华中区域降水场的关系,结果表明:若1月南非东部沿岸至西印度洋、北美北部OLR(Outgoing Longwave Radiation)偏低(偏高),或北非、美国西南沿岸及近海OLR偏高(偏低),则夏季长江中游降水将偏多(偏少)。若4月澳大利亚至东印度洋、日界线以东热带太平洋OLR偏低(偏高),或西北太平洋偏高(偏低),则夏季长江中游降水将偏多(偏少)。若7月东印度洋—澳大利亚大陆、东亚OLR偏低(偏高),则夏季华中区域长江及其以北降水将偏多(偏少),湖南和江西南部降水将偏少(偏多)。夏季长江中游旱、涝年前期OLR明显的区别在于热带太平洋:涝年1月东、西太平洋为明显负、正异常,4月这种异常进一步加剧;旱年1月正好相反,东、西太平洋为微弱的正、负异常,4月转为东、西太平洋为微弱的负、正异常。太平洋暖池OLR低值区(强对流区)4、7月持续偏南,是夏季长江中游降水偏多的另一重要信号。冬、春季OLR与夏季长江中游降水大尺度关联的可能机制为:若1月热带东、西太平洋OLR为明显负、正异常,4月这种异常进一步加剧,也即冬、春季热带太平洋Walker环流持续减弱,从而使夏季暖池对流活动减弱,热带辐合带偏南,Hadley环流偏弱,使夏季西太平洋副热带高压主体位置偏南,导致中国夏季主雨带不能北推至黄河流域,而长期滞留长江中下游,最后造成长江中游降水异常。

Relationships between January, April and July monthly global outgoing longwave radiation （OLR） and summer （JJA） rainfall over Central China were studied by using the SVD. Results indicate that if January OLR is lower （higher） over the Eastern coast of Southern Africa to the Western Indian Ocean, and the north of northern America, or higher （lower） over the northern Africa, and the southwest coast of U. S. A and the offshore region, then the JJA rainfall will be excessive （deficit） over the middle reach of the Yangtze River; when April OLR is lower （higher） over Australia to the east of the Indian Ocean, and the eastern tropical Pacific Ocean, or higher （lower） over the northwest Pacific Ocean, then the JJA precipitation will be heavier （lighter） ; when July OLR is lower （higher） over the eastern Indian Ocean to the Western Pacific Ocean, Australia and the East Asia, then the JJA rainfall will be more （less） over the middle Yangtze River and the North of it, but less （more） over the South of of Hunan and Jiangxi province. There is significant difference in early stage OLR over the tropical Pacific Ocean between summer drought and flood years for the middle reach of the Yangtze River. In the summer flood years, the distinct negative/positive anomalies occupy respectively the Eastern/Western tropical Pacific in January, those anomalies become more remarkable in April; but in summer drought years, the weak positive/negative anomalies lie respectively over the eastern/western tropical Pacific in January, and they become weak negative/positive anomalies in Aril, respectively. It is another important signal for excessive summer rainfall over the middle reach of the Yangtze River that the low OLR （strong convection） region over the warm pool of the western Pacific Ocean lies south of normal persistently from to July. Mechanism for large scale association between OLR in winter and spring and summer rainfall over the middle reach of the Yangtze River is possibly that if January OLR is distinctively lower/higher over the East/West of tropical Pacific Ocean, and the anomalies continuously intensify in April, that is to say, the Walker Circulation weakens persistently from winter to spring over tropical Pacific Ocean, thus leading to the attenuation of convection over the warm pool, and the southward shift of ITCZ as well as the weakening of Hadley Circulation compared with normal. The west Pacific subtropical high lies south than normal, obstructing the northward march of the summer rain belt. Retardation of the rain belt brings about excessive precipitation over the middle reach of the Yangtze River.