ENSO对中国夏季降水可预测性变化的研究

ON THE WEAKENING RELATIONSHIP BETWEEN SUMMER PRECIPITAION IN CHINA AND ENSO

众多研究表明,ENSO对东亚夏季风尤其是中国夏季降水存在很大影响,已成为中国夏季降水首要的预测因子。传统的预测模型认为,当前期ENSO为暖位相状态时,夏季中国主要雨带位置偏南,长江流域降水偏多;反之,当前期ENSO为冷位相状态时,夏季中国主要雨带位置偏北,长江流域降水偏少。基于1951—2003年中国160站月平均降水资料和同时段的NOAA ERSST海表温度资料,讨论了中国夏季降水和前冬Nino3区海温关系的年代际变化。分析结果显示,近20年来二者相关性已大大衰减。作为中国夏季降水的主要预测指标,ENSO的指示意义也相应减弱。在1951—1974年,依据前冬Nino3区SSTA预测夏季降水符号准确率在67%以上的站数有43站,但在1980—2003年,同样准确率的站数只有15站。在前一个研究时段,这43站呈区域性分布于东北地区、黄河和长江流域,但后一个研究时段内的15站分布分散,不利于区域性预测。相关分析结果表明,在20世纪70年代中期之前,当前冬赤道东太平洋海温偏高时,华北和江南南部的多数测站夏季降水偏多,淮河流域降水偏少,同时梅雨开始偏晚。反之,当前冬赤道东太平洋海温偏低时,华北和江南南部夏季降水易偏少,淮河流域降水则偏多,同时梅雨开始偏早。但在20世纪80年代之后,上述对应关系较难成立。因此,在汛期预测业务中参考ENSO的作用时必须充分考虑年代际背景的差异。

As explored by previous studies, ENSO has a significant influence on the East Asian summer monsoon, especially on the summer precipitation in China. So, it has been the most important predictor for the summer rainfall. According to those researches, a warmer ENSO phase （El Nino event） in the preceding winter always leads to a more southward shifted summer rainfall belt in Eastern China and more precipitation over the Yangtze River valley than normal; however after a cooler ENSO phase （La Nina event）, the summer rainfall belt will be more northward, and the precipitation over the Yantze River valley will be less. Based on monthly rainfall data from 160 primary stations in China during 1951 - 2003 and the NOAA ERSST data in the same period, the interdecadal variation of the relationship between summer precipitation in China and SSTA Nino3 in the preceding winter has been examined in this paper. Results show that the relationship has weakened in the past two decades. As a predictor, the significance of ENSO has also decreased remarkably. For example, during 1951 - 1974 there were 43 stations in total whose summer rainfall anomalies can be predicted by the SSTA Nino3 of the preceding winter with an accuracy of anomaly sign more than 67 %, while the total number of such stations has reduced to 15 in 1980 - 2003. In the first period, the 43 stations are mainly concentrated in northwestern China, the Yellow River valley and the Yangtze River valley, but the distribution of the 15 stations in the second period is rather scattered, which is obviously unfavorable for regional prediction. Correlation analysis results and the composite maps also indicate that before the late 1970s, when the above-normal （below-normal） SSTs occurred over the central and eastern Pacific in the preceding winters, then more （less） summer rainfall was found in North China and the .south of the Yangtze River valley, and less （more） summer rainfall over the Huaihe River valley, and meanwhile the China Meiyu began later （earlier） ; and vice versa. However, all these relationships have obviously weakened after the 1980s. Therefore, the background of the interdecadal variation of the predictor must be fully considered when the effect of ENSO is taken account in the operational prediction of summer precipitation.