Projected Changes in Atmospheric Pathways of Western North American Heatwaves from High-Resolution Coupled Model Simulations

Abstract

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. The occurrences of these heatwaves reveal the following common features: (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 21st-century projections, notable changes include farther poleward moisture transport and broader ridge development in the future. 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.