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East Asian atmospheric rivers (ARs) exhibit the most pronounced activity in summer with significant impacts on monsoon rainfall. However, their occurrence mechanisms are yet to be revealed in detail. 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-frequency (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 an enhanced monsoon southwesterly between a quasi-stationary cyclone and an anticyclone with the latter being the anomalous expansion of the western North Pacific subtropical high. Such a pattern is 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 parent system 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. Significance Statement: Despite the accumulation of studies on summertime atmospheric rivers (ARs) in East Asia, a comprehensive explanation for their occurrence mechanisms remains elusive. This study disentangles their complicated nature through case-level multiscale analyses. In contrast to wintertime ARs, summertime ARs are shaped by both high- and low-frequency moisture transports. The high-frequency moisture transport is associated with migratory extratropical cyclones which are suppressed but still active in summer, while the low-frequency moisture transport arises from the fluctuation of a quasi-stationary monsoon southwesterly along the periphery of the western North Pacific subtropical high. The varying relative contribution of high- and low-frequency components from one AR to another suggests that East Asian summer ARs represent a continuum of extratropical and monsoonal moisture plumes. [ABSTRACT FROM AUTHOR]

This study identifies diverse synoptic weather patterns of warm-season heavy rainfall events (HREs) in South Korea. The HREs not directly connected to tropical cyclones (TCs) (81.1%) are typically associated with a midlatitude cyclone from eastern China, the expanded North Pacific high, and strong southwesterly moisture transport in between. They are frequent both in the first (early summer) and second rainy periods (late summer) with impacts on the south coast and west of the mountainous region. In contrast, the HREs resulting from TCs (18.9%) are caused by the synergetic interaction between the TC and meandering midlatitude flow, especially in the second rainy period. The strong south-southeasterly moisture transport makes the southern and eastern coastal regions prone to the TC-driven HREs. By applying a self-organizing map algorithm to the non-TC HREs, their surface weather patterns are further classified into six clusters. Clusters 1 and 3 exhibit a frontal boundary between the low and high with differing relative strengths. Clusters 2 and 5 feature an extratropical cyclone migrating from eastern China under different background sea level pressure patterns. Cluster 4 is characterized by the expanded North Pacific high with no organized negative sea level pressure anomaly, and cluster 6 displays a development of a moisture pathway between the continental and oceanic highs. Each cluster exhibits a distinct spatiotemporal occurrence distribution. The result provides useful guidance for HRE prediction by depicting important factors to be differently considered depending on their synoptic categorization. [ABSTRACT FROM AUTHOR]

In the summer of 2020, South Korea experienced record-breaking rainfall due to 15 consecutive heavy rainfall events (HREs) from mid-June to early September. Among them, 11 HREs occurred in late June to mid-August with distinct synoptic characteristics depending on the occurrence period. All HREs from 29 June to 27 July (P1) were triggered by extratropical cyclones, while those from 28 July to 15 August (P2) mainly occurred along the monsoon rainband. We argue that their transition is associated with atmospheric teleconnections. During P1, the western North Pacific subtropical high (WNPSH) anomalously extended westward, but its northward expansion was hindered by the meridional wave train from the suppressed convection over the South China Sea. This condition prevented a northward migration of the monsoon rainband but allowed more extratropical cyclones to pass over the Korean Peninsula, resulting in four HREs. During P2, the South China Sea convection was enhanced, and its circulation response prompted an abrupt northward expansion of the WNPSH with a large pressure gradient along its northern boundary. With intensified southwesterly moisture transport, a monsoon rainband was activated over the Korean Peninsula, producing six HREs. The opposite phases of the summer North Atlantic Oscillation, i.e., negative in P1 but positive in P2, further contributed to the anomalous monsoon circulation by modulating the midlatitude circulation response to the South China Sea convection. This study demonstrates that the nature of summertime HREs in East Asia can be strongly modulated by remote forcings. [ABSTRACT FROM AUTHOR]

Atmospheric rivers (ARs) are an important concern in regional water management; however, little is known about the AR impacts on hydrology in East Asia (EA). This study analyzes the characteristics of storms, precipitation (P), streamflow (Q), and runoff coefficient (R) in the Namgang‐dam basin in Korea related to ARs for 2000–2013, as well as the sensitivity of the analysis results to AR detection methods, using observed P, Q, and three different AR inventories. The basin experiences 37.3 storms annually, of which 54% are AR storms that provide over 60% of the annual P and Q. The AR (non‐AR) storms are dominant in storm frequency and the storm‐total P and Q for January–July (August–December) with peaks in July (August). The monthly AR frequency varies closely with the seasonal variations in the EA monsoon and the North Pacific storm track which modulate the number of extratropical cyclones. The AR storms produce most of the extreme events; they also generate larger storm‐mean P and Q than the non‐AR storms for all months. The seasonal variations in R are related to the total (AR‐ and non‐AR storms combined) P through the seasonal soil water variations, making the AR effects on R unclear. Considering 95% confidence intervals, the AR storms are distinguished well from the non‐AR storms in storm frequency and the storm‐total and storm‐mean P and Q. The sensitivity to AR inventories is not critical in quantifying the AR‐storm characteristics and their impacts on hydrologic variables except for R. Key Points: Atmospheric river storms provide over 60% of the annual rainfall/runoff to become the primary source of water resources and extreme hydrologic events for the basinThe seasonal cycles of runoff/runoff coefficients are primarily shaped by the seasonal cycle of soil water content in response to seasonal precipitationDifferences among multiple atmospheric rivers (ARs) inventories are a minor concern for the uncertainties in assessing the AR effects on the hydrology in the basin [ABSTRACT FROM AUTHOR]

Atmospheric river (AR) and its impact on monsoon rainfall in East Asia are investigated by considering their month‐to‐month variations during the East Asian summer monsoon (EASM). The AR in the EASM, defined as an anomalously enhanced plume‐like water vapor transport, frequently forms over eastern China, Korea and western Japan. However, its characteristics vary from the early (June‐July) to the late (August‐September) period of the EASM. In the early EASM, AR is typically characterized by a quasi‐stationary monsoon southwesterly along the northern boundary of the western North Pacific subtropical high (WNPSH), which is further intensified by a migrating extratropical cyclone in the north. In contrast, the late‐EASM AR, which is less frequent than the early EASM AR, is primarily organized by a migrating extratropical cyclone. The quasi‐stationary monsoon southwesterly is less influential as the northern boundary of the WNPSH shifts northward, being decoupled from the subtropical ocean. Both the early‐ and late‐EASM ARs contribute substantially to monsoon rainfall, especially to heavy rainfall events. In the early EASM, 35%–70% of total rainfall amount and 60%–80% of heavy rainfall events in eastern China, Korea and western Japan are associated with AR. Although weakened, AR‐related rainfall is still significant in the late EASM in Korea and western Japan. These results indicate that AR is a key ingredient of EASM precipitation and its subseasonal variations should be taken into account to better understand and predict AR‐related extreme precipitation in East Asia. Key Points: Atmospheric river (AR) in the early East Asian summer monsoon (EASM) is characterized by an anomalously enhanced monsoon southwesterlyThe late‐EASM AR is dominated by a migrating extratropical cyclone under the weakened monsoon southwesterlyThe AR is largely responsible for monsoon rainfall, especially that associated with heavy rainfall event [ABSTRACT FROM AUTHOR]