Atmospheric Blocking and Other Large‐Scale Precursor Patterns of Landfalling Atmospheric Rivers in the North Pacific: A CESM2 Study
Atmospheric rivers are transient, filamentary plumes of enhanced water vapor transport that have profound socioeconomic impacts on many regions around the globe, yet an adequate understanding of their formation and maintenance processes remains elusive. An accurate representation of ARs in Earth system models is essential because ARs directly affect flood risks, agriculture, and water management in regions that are extremely reliant on water availability, such as along the west coast of North America. Around half of the annual precipitation in the western United States is delivered by ARs, typically concentrated in the winter season.
This study uses a climate model to investigate the large-scale phenomena that influence atmospheric river landfall statistics. Atmospheric rivers are detected using an objective algorithm applied to a reanalysis (as a proxy for observations) and the CESM2 in a pre-industrial fully-coupled configuration. The model is shown to produce a reasonable depiction of AR frequency of occurrence. Using both reanalysis and CESM2, an analysis of AR precursor patterns is undertaken. Previous studies have attempted to link AR statistics to major modes of climate variability, like the PNA pattern. Evidence presented in this study downplays such connections. Instead, it appears that landfalling ARs are more closely connected to the development of a high-latitude ridge and blocking events. These feature occur more than a week ahead of the ARs, which tend to be associated with cold surges off of Asia and moist plumes that span the north Pacific. The CESM2 captures some of these features, though some details are biased compared to the reanalysis.
Atmospheric rivers have a profound socioeconomic impact on the western USA. There has been increasing interest in understanding ARs, especially to understand whether they might change with a changing climate. This study shows that a standard-resolution climate model can capture not only a rough semblance of AR activity but a rather compelling climatological depiction. Further, the model appears to have similar precursor features of ARs as are detected in reanalysis, lending some credibility to their projection under future climate scenarios. This study also shows that ARs form through a chain of interacting events that lend to their predictability. The results imply that the presence of a high-latitude ridge and related atmospheric blocking are particularly important for AR formation, while the relationship to modes of variability appears relatively weak.
This analysis shows that when a large area of high pressure forms over Alaska and far eastern Siberia, the preferred path of storms crossing the North Pacific is diverted toward the south. The southward‐shifted storms, some of which develop into atmospheric rivers, guide moist air toward the U.S. west coast and can produce extreme precipitation. The climate model performs reasonably well at representing atmospheric rivers and their associated precursor weather patterns. These results highlight important connections between atmospheric rivers and the large‐scale weather patterns than can precede them by more than a week.