Contrasts between the Interannual Variations of Extreme Rainfall over Western and Eastern Sichuan in Mid-summer

Abstract

Rainfall amount in mid-summer (July and August) is much greater over eastern than western Sichuan, which are characterized by basin and plateau, respectively. It is shown that the interannual variations of extreme rainfall over these two regions are roughly independent, and they correspond to distinct anomalies of both large-scale circulation and sea surface temperature (SST). The enhanced extreme rainfall over western Sichuan is associated with a southward shift of the Asian westerly jet, while the enhanced extreme rainfall over eastern Sichuan is associated with an anticyclonic anomaly in the upper troposphere over China. At low levels, on the other hand, the enhanced extreme rainfall over western Sichuan is related to two components of wind anomalies, namely southwesterly over southwestern Sichuan and northeasterly over northeastern Sichuan, which favor more rainfall under the effects of the topography. Relatively speaking, the enhanced extreme rainfall over eastern Sichuan corresponds to the low-level southerly anomalies to the east of Sichuan, which curve into northeasterly anomalies over the basin when they encounter the mountains to the north of the basin. Therefore, it can be concluded that the topography in and around Sichuan plays a crucial role in inducing extreme rainfall both over western and eastern Sichuan. Finally, the enhanced extreme rainfall in western and eastern Sichuan is related to warmer SSTs in the Maritime Continent and cooler SSTs in the equatorial central Pacific, respectively.

摘 要

四川东部盆地盛夏(7、8月)的降水量显著大于四川西部的高原地区。本研究发现,四川东西部极端降水的年际变化基本是相互独立的,分别对应不同的大尺度环流和海温异常。在对流层高层,川西地区极端降水的偏多与亚洲西风急流位置偏南有关,而川东地区极端降水的偏多则与中国上空的反气旋异常环流相联系。在对流层低层,川西地区极端降水的偏多对应四川西南上空的西南风异常和四川东北上空的东北风异常。在地形作用的影响下,这两支风场异常都有利于川西地区降水增加。相对而言,川东地区极端降水的偏多对应四川以东的低层南风异常。该南风异常受盆地北部的山脉阻挡,在盆地上空转为东北风异常,有利于川东地区的降水。因此,四川及其周边地区的地形是川西和川东极端降水异常的重要因素。此外,川西地区极端降水的偏多对应海洋性大陆周边的暖海温异常,而川东地区极端降水的偏多则对应赤道中太平洋的冷海温异常。