Atmospheric rivers in East Asia summer as the continuum of extratropical and monsoonal moisture plumes

Abstract East Asian atmospheric rivers (ARs) exhibit the most pronounced activity in summer with significant impacts on monsoon rainfall. However, their detailed characteristics from a synoptic perspective are yet to be revealed. In this study, we unravel the inherently complex nature of East Asian summer ARs by applying a multiscale index that quantifies the relative importance of high- (HF) and low-frequency (LF) moisture transports in AR development. It is found that both HF and LF processes contribute to shaping the summertime ARs in East Asia, contrasting to the wintertime ARs dominated by HF processes. Stratification of ARs with the multiscale index reveals that HF-dominant ARs are driven by baroclinically-deepening extratropical cyclones, analogous to the widely-accepted definition of canonical ARs. In contrast, LF-dominant ARs result from enhanced monsoon southwesterly between a quasi-stationary cyclone and anticyclone with the latter being the anomalous expansion of the western North Pacific subtropical high, reminiscent of the classical monsoon rainband. While HF-dominant ARs are transient, LF-dominant ARs are quasi-stationary with a higher potential for prolonged local impacts. The intermediate ARs, constituting a majority of East Asian summer ARs, exhibit synoptic conditions that combine HF- and LF-dominant ARs. Therefore, East Asian summer ARs cannot be explained by a single mechanism but should be considered as a continuum of extratropical cyclone-induced and fluctuating monsoon flow-induced moisture plumes. This finding would serve as a base for the advanced understanding of hydrological impacts, variability, and projected change of East Asian ARs.