Multiscale Nature of Atmospheric Rivers

Abstract

This study provides evidence for the multiscale nature of atmospheric rivers (ARs) by differentiating them based on high- (HF) and low-frequency (LF) moisture transports. The HF-dominant ARs exhibit migratory behavior as they are typically accompanied by extratropical cyclones. Their spatial distribution is seasonally synchronized with midlatitude storm activity. On the other hand, the LF-dominant ARs stay in place as they are associated with quasi-stationary circulation. They prevail in the subtropical monsoon regions in the summer hemisphere. The ARs are often jointly affected by HF and LF processes. Such intermediate ARs are frequently observed along the poleward boundary of subtropical highs. The latter two AR types are locally more persistent than the HF-dominant ARs, implying the importance of LF dynamics in long-lasting AR impacts. We suggest that multiscale analysis of ARs will offer valuable insights into their diversity and hydrological impacts.Plain Language Summary Atmospheric rivers (ARs) are narrow corridors of intense water vapor transport often concurrent with extratropical cyclones. Through case studies and climatological analyses, however, this study shows that ARs are not only associated with high-frequency (HF) moisture transport such as those driven by extratropical cyclones but also related to low-frequency (LF) moisture transport driven by quasi-stationary circulation. The HF-dominant ARs are observed along the midlatitude storm tracks, while the LF-dominant ARs appear in summer monsoon regions in low latitudes. There are also a non-negligible number of ARs jointly influenced by HF and LF processes, particularly along the poleward flank of subtropical highs. Noticeably, ARs strongly influenced by LF processes have a longer local duration than others. It implies that LF dynamics, which have received relatively less attention, need to be incorporated for a better understanding and prediction of the long-lasting AR impacts. This study highlights that ARs have diverse characteristics by HF and LF processes. Taking their multiscale nature into account thus will be helpful when delineating the dynamics and hydrological impacts of ARs.