Abstract:

Based on meteorological and hydrological observation and NCEP/NCAR (National Center for Environmental Prediction/National Center for Atmospheric Research) reanalysis data, the variation characteristics of compound dry and hot events, and their causes and impact on runoff in the main confluence area of the upper Yellow River in summer were analyzed. The results show that, from the average spatial distribution, the number of high temperature days in summer increases gradually from southwest to northeast in the main confluence area of the upper Yellow River, and the opposite is true for summer precipitation. From temporal and spatial distribution, the number of summer high temperature days in the main confluence area of the upper Yellow River has been increasing consistently since 1961. The precipitation shows fluctuating consistently changes from a long-term trend perspective, but increased consistently after 2000. The compound dry-hot events have increased significantly since this century. From the perspective of multi-time scale changes, the summer compound dry-hot events in the main confluence area of the upper Yellow River exist mainly inter-annual changes and trend changes, and the significant increase of summer compound dry-hot events since 2000 is mainly caused by trend changes. From the perspective of influencing factors, the changes of summer compound dry-hot events in the main confluence areas of the upper Yellow River are mainly influenced by multiple circulation factors, but the influencing factors differ greatly on different time scales. On the inter-annual scale, the influence of westerly circulation, East Asian summer monsoon, South Asian summer monsoon, plateau summer monsoon, and north wind circulation is relatively weak, on the inter-decadal scale, compound dry-hot evencs are mainly influenced by the Tibetan Plateau summer monsoon and they are also influenced by the westerly circulation, the East Asian summer monsoon, the South Asian summer monsoon, the Tibetan Plateau summer monsoon and the north wind circulation on the multi-decadal scale. From the background field of large-scale circulation, the West Pacific subtropical high is stronger and westward, the lack of abnormal water vapor transport in the southwest and the abnormal downward motion in the vertical field are the main reasons for the increase of summer compound dry-hot events in the main confluence area of the upper Yellow River since this century. The increase of compound dry-hot events in the main confluence area of the upper Yellow River will reduce the runoff of Lanzhou station in the basin, the main reason for the increase in runoff of the Yellow River Lanzhou section since 1998 is the increase in precipitation.

Key words: the main confluence area of the upper Yellow River, summer, compound dry-hot, runoff, effect

摘要：

基于气象、水文观测及NCEP/NCAR（National Center for Environmental Prediction/National Center for Atmospheric Research）再分析数据对黄河上游主要汇流区夏季复合干热事件变化特征及其成因以及对径流的影响进行了分析与探讨。结果表明，在平均空间分布上，黄河上游主要汇流区夏季高温日数从西南到东北逐渐增多，而夏季降水量正好相反；从时空分布看，1961年以来黄河上游主要汇流区夏季高温日数呈一致增多趋势；夏季降水从长期趋势看呈一致波动变化，但2000年以后呈一致增加趋势；复合干热事件本世纪以来显著增多。在多时间尺度变化上，黄河上游主要汇流区夏季复合干热事件主要以年际和趋势变化为主。环流影响因子方面，黄河上游主要汇流区夏季复合干热事件变化受多环流因子共同影响，但在不同时间尺度上影响因子差异较大，年际尺度上西风环流、东亚夏季风、南亚夏季风、高原夏季风及北风环流的影响程度均较弱，年代际尺度上主要受高原夏季风环流影响，多年代际尺度上同时受西风环流、东亚夏季风、南亚夏季风、高原夏季风及北风环流共同影响。大尺度环流背景场上，西太平洋副热带高压偏西偏强、缺少异常的西南水汽输送及垂直场上异常的下沉运动是本世纪以来黄河上游主要汇流区夏季复合干热事件增多的主要原因。黄河上游主要汇流区夏季复合干热事件的增加会使得流域兰州站的径流量减少，而1998年以来黄河兰州段径流量的增加主要原因是降水增加。

关键词: 黄河上游主要汇流区, 夏季, 复合干热, 径流, 影响