长江中下游入梅指数及早晚梅年的海气背景特征

利用1957～2001年全国160站逐月降水资料和116站入梅日期资料，定义了一个长江中下游入梅指数，以定量描述长江中下游地区平均入梅的早晚，再结合ERA－40高分辨率再分析资料和ERSST海温资料，利用相关分析和合成分析， 分别研究了早、晚梅年同期（6～7月份）和前期（前一年12月份至当年5月份）的大尺度大气环流及海温的异常特征。结果表明:早梅年同期，200 hPa南亚高压偏北，印度北部、孟加拉湾－印度尼西亚－副热带太平洋地区上空的对流偏强，西太平洋副热带高压和赤道辐合带位置偏北，东亚副热带夏季风偏强，晚梅年则相反。前期1月份北太平洋涛动及4月份西太平洋暖池附近的对流与当年入梅早晚存在显著的相关关系:早梅年，1月份北太平洋涛动偏弱，4月份西太平洋暖池附近的对流活跃；晚梅年，1月份北太平洋涛动偏强，4月份西太平洋暖池附近的对流偏弱。此外， 从前期海温场来看，早梅年，1～4月份北大西洋中高纬地区海温偏低，低纬地区海温偏高，呈南北偶极子分布状态，2月份西太平洋暖池附近海域及北半球冬、春季环澳大利亚海域海温明显偏高，晚梅年情况正好相反。以上这些前期信号为长江中下游地区入梅的短期气候预测提供了参考依据。

Regional Meiyu Onset Index over the Middle and Lower Reaches of the Yangtze River and the Associated Ocean- Atmospheric Features for the Early/Late Meiyu Onset Years

A large number of previous researches about the Meiyu are chiefly on the precipitation and its influencing factors,but that on the Meiyu onset is as important as on Meiyu rainfall in scientific value and in practice.In view of the regionality of the Meiyu and based on the 1957-2001 monthly precipitation data of 160 stations and the Meiyu onset data of 116 stations over China,a regional Meiyu onset index(RMOI) is defined to describe the early or late Meiyu onset in the middle and lower reaches of the Yangtze River,quantitatively.Then ERA-40 high resolution reanalysis data and extended reconstructed sea surface temperature(SST) data are used,together with simple correlation and composite analysis,to investigate the singularities of large scale atmospheric circulation and SST in the same term of June to July as well as in the previous months from the preceding December through May for early and late Meiyu onset years.Results exhibit that during the simultaneous term of the early Meiyu onset years,the 200-hPa South Asia high lies more northward,and convection tends to strengthen over northern India,the Bay of Bengal,Indonesia,the subtropical western Pacific,associated with the more northward western Pacific subtropical high(WPSH).Meanwhile,cross-equatorial jet flow near Somali in the lower troposphere and the Indian summer monsoon are more intense.The East Asian summer monsoon is stronger in the early Meiyu onset years,which is chiefly contributed by the stronger flow in the southern swing of the WPSH.Previously,the RMOI has anomalous relationship with the North Pacific oscillation(NPO) in January and with convective activities over the western Pacific warm pool in April,that is,weaker NPO and stronger convection over the warm pool in the early Meiyu onset years contrary to stronger NPO and weaker convection over the warm pool in the late ones.In addition,as far as previous SST is concerned for the early Meiyu onset years,lower SST emerges in the mid-and high latitudes of the North Atlantic(NA),associated with higher SST in the low latitudes of the NA from January to April,which shows a south-north dipole pattern.Anomalously higher SST appears in the western Pacific warm pool region in February and over the waters around Oceania in the previous spring.The situations are just opposite for the late Meiyu onset years.All these offer some references for the short-term prediction of the Meiyu onset.