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. 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.
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Data availability
All data used in this study are publicly available. NOAA-CPC global daily maximum/minimum temperature and precipitation data can be downloaded from https://psl.noaa.gov/data/gridded/index.html. SPEAR Large ensemble data can be downloaded from https://www.gfdl.noaa.gov/spear_large_ensembles/. The CMIP6 model output used in this study is available at https://aims2.llnl.gov/search/cmip6/. The ERA5 reanalysis hourly data used in the Supplementary Information can be downloaded from https://cds.climate.copernicus.eu/datasets/reanalysis-era5-pressure-levels?tab=overview for pressure levels.
Code availability
The feature tracking algorithm code used in this study is a Python open-source package, CONTRACK, which is accessible from https://github.com/steidani/ConTrack52. Other custom scripts directly implement the statistical methods and techniques described in the “Methods” section.
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Acknowledgements
The authors thank Drs. Liwei Jia and Donghyuck Yoon for helpful comments on an earlier version of the manuscript. M.P. acknowledges funding under award NA18OAR4320123 and NA22OAR4050663D from the National Oceanic and Atmospheric Administration, U.S. Department of Commerce. The statements, findings, conclusions, and recommendations are those of the authors and do not necessarily reflect the views of the National Oceanic and Atmospheric Administration or the U.S. Department of Commerce.
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M.P. conceived the study, conducted the analysis, and drafted the manuscript. N.C.J. contributed to the interpretation of the results, provided critical feedback throughout the development of the project, and participated in writing the manuscript.
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Park, M., Johnson, N.C. Projected changes in atmospheric pathways of Western North American heatwaves simulated from high-resolution coupled model simulations. npj Clim Atmos Sci (2026). https://doi.org/10.1038/s41612-025-01319-y
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DOI: https://doi.org/10.1038/s41612-025-01319-y


