Multi-decadal change in western US nighttime vapor pressure deficit

Amid reports from western US wildland fire managers that, compared to when many started their careers, fires are burning longer throughout the day before reducing in intensity overnight, we examined decadal changes in nighttime vapor pressure deficits over the western US with a focus on the summer fire season. We calculated changes using a recently updated observation-assimilating reanalysis (ERA5) available at hourly resolution over the 1980-2019 period. Analysis identifies the proximate cause (atmospheric temperature v. moisture content) of the observed changes and the extent to which they have been captured in climate model simulations. Increases in nighttime vapor pressure deficits of >50% in 40 years are evident over foothills of mountains ranges adjacent to arid plateaus. The largest observed increases greatly exceed the forced climate-model response. Correlation analysis reveals a broad link between the variability of western US summer-nighttime vapor pressure deficit and the Pacific Decadal Oscillation.