青藏高原异常雪盖和ENSO在1998年长江流域洪涝中的作用

The Role of the Anomalous Snow Cover over the Qinghai-Xizang Plateau and ENSO in the Great Floods of 1998 in the Changjiang River Valley

１９９７／１９９８年冬季青藏高原大部地区积雪异常偏多，出现了历史上罕见的严重雪灾。作者在回顾关于青藏高原雪盖与中国季风雨关系的基础上，分析了１９９８年夏季各月中国东部降水分布和主要雨带移动的特点，并与青藏高原多雪年夏季我国主要雨带活动的统计特征进行对比分析．探讨了１９９８年夏季长江流域洪涝的成因。结果表明，１９９８年夏季降水的一些重要特征：如６月强降雨出现在湖南、江西、浙江一线，７月二度梅发生在湖南北部、湖北南部、江西北部一带，８月长江上游、汉水流域和黄淮地区降水频繁，以及夏季我国主要雨带北移明显推迟，都与多雪年的情况非常相似。突出地反映了１９９８年夏季长江流域洪涝的发生，前期冬季青藏高原出现的积雪异常起着重要的作用。多雪年夏季西太平洋副热带高压北移也明显偏迟，致使中国主要雨带持续偏南，造成长江流域降水异常偏多。另外，分析表明，它还与 １９９７年强厄尔尼诺的共同作用有密切关系。

An abnormally deep and long-lasting snow cover over the Qinghai-Xizang Plateau (QXP) occurred in the winter of 1997 / 1998 prior to the great floods in the middle reaches of the Changjiang River in the summer of 1998. In this paper, we first review our previous findings in analyzing the relationships between the summer monsoon rainfall in China and the snow cover over the QXP. On this basis, the features of the rainfall anomaly distribution and the seasonal northward progress of main rainband in the eastern China during the great floods of 1998 are analyzed. A comparative study is made on the movement of the main rainband in summers of 1998 and the rich snow cover year. Finally, we discuss the roles of the anomalous snow cover over the QXP in the great floods in the Changjiang River Valley. The results show that some of the important features of the summer rainfall of 1998 over the eastern China such as the heavy rainfall of June in Hunan, Jiangxi and Zhejiang, the second period of the Meiyu rainfall of July in the northern Hunan, southern Hubei and northern Jiangxi, the frequent rainfall of August in the upper reaches of the Changjiang River, Hanshui River valley and Huanghe-Huaihe River region, and the delay of the northward progress of main rainband, are very similar to those in rich snow cover years. This suggests that the great floods of 1998 in the Changjiang River valley are closely related to the anomalous snow cover over the QXP. During the rich snow cover years, the seasonal northward advance of the western Pacific subtropical high may obviously be delayed, which may result in enhanced summer rainfall in the Changjiang River valley. It is also seen that its joint effects with the EI Nino event of 1997-1998 are the important reasons resulting in the great floods of 1998 in the Changjiang River valley.