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 United States 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 vs. 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.

Automated summary

The study found significant increases in nighttime vapor pressure deficits during the summer fire season in the western United States over the past 40 years, particularly in foothills of mountain ranges adjacent to arid plateaus. These increases greatly exceed climate model predictions and show a broad connection to the Pacific Decadal Oscillation.