Amplification of Chinook Heatwaves in a Warming Climate

Abstract

A category of dynamically forced warm-season heat events are associated with Chinook (or foehn) flows in the vicinity of substantial topography. Latent warming of such flows exhibits exponential dependence on temperature, which for warm-season events, can result in extreme heat. The June 2021 Western North American heatwave event featured air trajectories that experienced latent warming over the Gulf of Alaska and during orographic ascent in coastal regions of Southeast Alaska. Thermodynamic analysis of air trajectories demonstrates that a Chinook-type mechanism can explain the extreme lower-tropospheric warmth in coastal regions of the Pacific Northwest and British Columbia. Using projected end-of-century warming values for the trajectory source region, thermodynamic analysis and model simulations indicate that Chinook warming amplification of up to 75% would occur. This additional warming is driven by exponential growth of latent warming, a formidable amplifier of climate warming for this class of topographically forced heat events.