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
青藏高原异常雪盖和ENSO在1998年长江流域洪涝中的作用

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.