Atmospheric Rivers and Compound Inland Flooding under Climate Change: Quantifying Contributions and Uncertainties

Atmospheric rivers (ARs) are narrow bands of concentrated moisture that account for approximately 90% of poleward water vapor transport from the tropics. Landfalling ARs generate intense precipitation that contributes to coastal water supply but can also lead to severe flooding. ARs can trigger and intensify other natural processes such as snowmelt and soil moisture, leading to Compound inland flooding (CIF) events. These include rain on snow (ROS) events, when rain causes significant snowmelt amplifying runoff, and saturation excess flooding (SEF) events, when rain falls on saturated soil and the decreased infiltration amplifies runoff. In a nonstationary climate, AR events are projected to increase in both frequency and intensity. Despite implications for compound events, ARs are often studied in a univariate context, which risks underestimating the severity of AR events and hinders hazard mitigation efforts. This study characterizes AR relationships with other hazards to investigate how they contribute to CIF events, specifically ROS and SEF events. Using the CanRCM4 single model initial-condition large ensemble dataset, the frequency and seasonality of AR-driven CIF events is determined for Western North American coastal areas in historical and future periods, while focusing on how ARs interact with additional factors such as snowpack and soil moisture. Internal variability is quantified and assessed using the signal to noise ratio to determine the relative strength of projected trends. ARs were found to contribute up to 90% of CIF events in coastal areas and some orographic areas. Additionally, this relationship is expected to grow stronger in warmer climates, especially for ROS events. The findings underscore the importance of considering environmental factors in AR-related flooding risks for flood management strategies and infrastructure design to adapt to a changing climate.