Serial Clustering of Atmospheric Rivers: Climatology, Relevance for Precipitation, and Synoptic–Dynamic Characteristics

Atmospheric rivers (ARs) play a key role in the global water cycle and can contribute to precipitation extremes and flooding. In some situations, multiple ARs can impact a particular region in rapid succession over a period of several days to several weeks, resulting in repeated incursions of strong water vapor flux that can support large precipitation accumulations. This process, referred to as serial clustering, can, thereby, lead to high-impact flooding and substantial contributions to annual water resources.

In this study, episodes of serial AR clustering are identified over the Northern Hemisphere in the ECMWF ERA5 reanalysis for 1981–2022, and the characteristics of these episodes are examined. ARs are first detected every 6 h as elongated (>2000 km) areas of grid points with strong (> 90th percentile of climatology) column-integrated water vapor transport, and AR episodes are identified at each grid point as successive 6-h AR occurrences that are separated by ≤ 5 days. Serial clustering episodes are then identified as episodes that last for ≥ 7 days and exhibit an AR at ≥ 25% of the 6-h time steps. The climatological characteristics of the serial clustering episodes are elucidated, and their relevance for total precipitation and for precipitation extremes on daily to subseasonal timescales is quantified, with a focus on the United States. Composite analysis is then performed for episodes in selected regions over the western and central U.S. in order to characterize the large-scale atmospheric conditions and synoptic-dynamic processes that can result in serial clustering of ARs. Results demonstrate that AR clustering tends to occur in persistent anomalous planetary-scale circulation regimes that promote recurrent synoptic-scale Rossby wave patterns and repeated extratropical cyclone development. Factors that may contribute to the persistence of these circulation regimes are discussed.