Atmospheric rivers cause the majority of precipitation-induced landslides in Western North America

Atmospheric rivers (ARs) are transient channels of intense horizontal water vapor transport in the lower atmosphere. Accounting for over 90% of the poleward moisture transport from the tropics to the mid-latitudes, ARs play a critical role in freshwater supply but are also responsible for extreme weather, natural disasters, large environmental and societal damages, and great economic loss. In Western North America, a hotspot of AR and landslide activity, precipitation from land-falling ARs has been identified as the trigger of some major landslide events. However, the spatio-temporal attribution and quantification of this causal relation remains unassessed. In this study, we developed a methodological approach rooted in stochastic climate theory and time series analysis to quantify the strength, directionality, and significance of the non-linear association between land-falling ARs and precipitation induced-landslides (PILs). Applying our methods in Western North America from 1996 to 2018, we find that precipitation attributable to land-falling ARs preceded more than 80% of days with PILs. Implementing randomization experiments, we discover that this causal relation is significant along the West Coast. To further characterize the dynamics of landslide-triggering ARs, we use an AR-strength scale to classify them in terms of intensity and persistence. We demonstrate that strong ARs are the main precursors of PILs. Differentiating between individual AR events and sequences of ARs making landfall in close succession, known as AR families, we discover that both scenarios contribute equally to landslide activity along the West Coast. Our findings offer quantitative insights into land-atmosphere coupling and advance the understanding and potential predictability of landslide hazards in Western North America. Our methodological approach, extendable to further regional and global analysis, is a robust tool to investigate where landslides (or any other precipitation-induced hazard) are caused by land-falling ARs (or any other atmospheric driver). Our contribution constitutes a fundamental and necessary step towards improved landslide forecasting and mitigation efforts.