Projected changes in atmospheric pathways of Western North American heatwaves simulated from high-resolution coupled model simulations

Western North America (WNA) is a regional hotspot for summer heat extremes. However, our understanding of the atmospheric processes driving WNA heatwaves remains largely based on a few case studies. In this study, we investigate the general characteristics of atmospheric pathways associated with WNA heatwaves using a 30-member high-resolution coupled model simulation. Synthesizing the WNA heatwave events across the large ensemble, we reinforce the view that WNA heatwaves are systematically driven by: (1) a Rossby wave train originating from the western North Pacific, (2) poleward moisture transport toward the Gulf of Alaska, occasionally via atmospheric rivers, and (3) downstream ridge amplification over WNA. Although these features also appear in the late twenty-first-century projections, notable changes include farther poleward moisture transport and broader ridge development in the future. Under the anomaly-based heatwave definition used in this study, which removes the influence of mean temperature change, the frequency of WNA heatwaves is projected to decrease. Our findings suggest that mechanisms identified in case studies, including upstream Rossby wave packets and subsequent moist processes, are broadly applicable to understanding WNA heatwaves over recent decades and their projected changes.