夏季热带西太平洋对流与长江中下游降水关系的研究

A Study of the Relationship Between Summer Tropical Convection over the Western Pacific and the Rainfall in the Middle-Lower Reaches of the Yangtze River

利用1979～2001年NCEP/NCAR月平均再分析格点资料、OLR和中国160站的月平均降水资料,研究了夏季热带西太平洋对流活动与长江中下游降水的关系.结果表明,夏季热带西太平洋上空对流活动强(弱)时,长江中下游地区夏季降水显著偏少(多).而西太平洋副热带高压在夏季热带西太平洋对流和长江中下游地区降水之间起到了桥梁作用,通过副高的东西位置、面积、强度异常等影响东亚夏季风异常,进而造成长江中下游地区夏季降水异常.另外,热带西太平洋对流异常显著对应着在热带和中纬度地区呈经向分布的Rossby波列(即东亚遥相关波列),该波列结构的异常与长江中下游地区夏季降水异常也有密切的关系.

Using the NCEP/NCAR （National Centers for Environmental Prediction / National Center for Atmospheric Research） monthly reanalysis data, OLR （Outgoing Long-wave Radiation） data and monthly precipitation data of 160 stations from 1979 to 2001, the relationship between the summer tropical convection over the western Pacific and the precipitation in the middle- lower reaches of the Yangtze River is studied. Enlightened by the distribution of correlation coefficients between summer rainfall in the middle- lower reaches of the Yangtze River and OLR over the East Asia- the western Pacific region from 1979 to 2001, summer rainfall index （Imin） is defined as the averaged precipitation over 17 stations in the middle- lower reaches of the Yangtze River, summer tropical convective index （IoLR） is defined as the regional averaged OLR over the region （10°N. 20°N, 120°E- 150°E） over the western Pacific. The years with greater IOLR anomalies （weak convection）, like 1980, 1983, 1993, 1996 and 1998, and the years with less IOLR. anomalies （strong convection）, like 1981, 1984, 1985, 1999, 2000 and 2001 are chosen to compose geopotential height and wind fields at 500 hPa and 850 hPa levels respectively. In order to highlight the differences in the associations of convection, the relationship between IokR and the western Pacific subtropical high （WPSH） indices and the relationship between IOLR and the East Asian remote connection indices （IEARC, defined based on the correlation coefficients between IOLR and 500 hPa geopotential height over the East Asia region from 1979 to 2001） are studied. The results show that tropical convection over the northwestern Pacific has a close positive relation with summer precipitation over the middle - lower reaches of the Yangtze River. If the summer tropical convection is stronger, WPSH would expand greatly and extend westward remarkably over the subtropical western Pacific; if westerlies is weaker at the northwestern edge of the WPSH, the summer precipitation in the middle- lower reaches of the Yangtze River would be less; when the summer tropical convection is weaker, WPSH would occupy less area and contract eastward remarkably over the subtropical western Pacific, resulting in stronger westerlies at the northwestern edge of the WPSH and more summer precipitation over the middle- lower reaches of the Yangtze River. Ridge of WPSH shows a significant change in the north - south direction but its northern border does not. Therefore, the WPSH serves as an important bridge between tropical convection and summer precipitation in the middle - lower reaches of the Yangtze River. The anomalous WPSH can cause the anomalies of East Asia summer monsoon, thus leading to the anomalies of summer rainfall in the middle- lower reaches of the Yangtze River. In addition, the differences in composite geopotential height and wind fields at 500 hPa or 850 hPa show that tropical convection inspires Rossby wave propagating from the tropics to high latitudes （i. e East Asian teleconnection type）. The spacial structure of the anomaly is closely related to tropical convection over the eastern Pacific and the summer precipitation anomaly over the middle- lower reaches of the Yangtze River.