Upper- and Lower-tropospheric Circulation Anomalies Associated with Interannual Variation of Pakistan Rainfall during Summer

Abstract

This study investigated the large-scale circulation anomalies, in both the upper and lower troposphere, associated with the interannual variation of rainfall in Pakistan during summer, using the station observation data in this country and circulation data of the NCEP-NCAR reanalysis from 1981 to 2017. Results showed that the upper- and lower-tropospheric circulation anomalies associated with monthly rainfall variability exhibit similar features from June to August, so analyses were performed on June–August circulation and Pakistan rainfall data. The analyzed results indicated that summer rainfall in Pakistan is enhanced when there is an anticyclonic anomaly to the northwest of Pakistan in the upper troposphere and easterly anomalies along the southern foothills of the Himalayas in the lower troposphere, and vice versa. These upper- and lower-tropospheric circulation anomalies were found to be related, but show unique features. The upper-tropospheric anticyclonic anomaly is closely related to the Silk Road Pattern along the Asian westerly jet, while the lower-tropospheric easterly anomalies are related to the cyclonic anomaly to the south of Pakistan, i.e., intensified South Asian monsoon trough. The results presented here suggest that the interannual variability of summer rainfall in Pakistan is a combined result of upper- and lower-tropospheric circulation anomalies, and of extratropical and tropical circulation anomalies.

摘要

本文研究了与巴基斯坦夏季降水年际变化有关的高层、低层大尺度环流异常。研究使用了巴基斯坦的站点观测资料和NCEP–NCAR的再分析环流资料,研究时段为1981–2017年。结果表明,无论是高层还是低层,与降水年际变化有关的环流异常在6–8月都表现出相似的特征,因而我们针对6–8月平均的环流和巴基斯坦降水资料进行了分析。分析结果表明,当对流层高层巴基斯坦西北部出现反气旋式环流异常、低层沿喜马拉雅山南麓为东风异常时,巴基斯坦降水增强,反之降水减弱。高层的反气旋式环流异常和沿亚洲急流的丝绸之路遥相关关系密切,而低层的东风异常和巴基斯坦南部南亚季风槽的加强有关。这些结果一方面说明,巴基斯坦夏季降水的年际变化是高层-低层、热带-热带外环流共同作用的结果;另一方面也说明,我们在研究与巴基斯坦夏季降水有关的高层、低层环流异常时,要注意不同层次物理过程的不同。