Atmospheric rivers as triggers of compound flooding: quantifying extreme joint events in Western North America under climate change

The article investigates how atmospheric rivers (ARs)—narrow corridors of intense moisture transport—act as key triggers of compound inland flooding along Western North America. Using a large ensemble of regional climate simulations (CanRCM4-LE) and validating AR detection against ERA5 reanalysis, the authors quantify how often extreme flooding mechanisms coincide with AR conditions. They focus on two compound flood pathways: Rain-on-Snow (ROS) (heavy rain + rapid snowmelt) and Saturation Excess Flooding (SEF) (heavy rain + already-saturated soils), and they compare present-day conditions with future warming levels up to +4°C global warming.

The findings show ARs are dominant drivers of extreme compound flooding, especially in coastal and mountainous regions (notably the Pacific Northwest), and their contribution generally increases under warming, though with important nuances. AR-related SEF and extreme-precipitation runoff intensify more consistently across warming scenarios, while ROS tends to increase in the near term but can weaken later as snowpack declines—shifting ROS risk toward higher elevations. A major takeaway is that the most damaging AR-flood outcomes depend strongly on antecedent conditions (snowpack and soil moisture), not just extreme rainfall, and that internal climate variability adds substantial uncertainty—meaning projections of exactly where/when AR-driven compound floods intensify can be noisy even if the overall risk trend is upward.