FY 2021

Multi-decadal change in western US nighttime vapor pressure deficit

Chiodi, A.M., B.E. Potter, and N.K. Larkin

Geophys. Res. Lett., 48(15), e2021GL092830, doi: 10.1029/2021GL092830, View online (open access) (2021)

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.

Plain Language Summary. Western US wildland fire managers have reported that fires are burning longer into the night and increasing in intensity earlier in the morning compared to when many started their careers. Increasing nighttime vapor pressure deficit—a measure of the drying power of air—is widely suspected to be responsible for these perceived changes in fire behavior. It is also suspected that human-caused increases in nighttime temperatures are responsible. We used a recently released observation-based data set to quantify the extent to which nighttime vapor pressure deficits have changed over the last 40 years and determine the proximate cause for the observed changes (particularly whether temperature effects on their own can explain them). We also explored how well the observed changes are captured in climate model simulations. Results highlight large increases in nighttime vapor pressure deficit over the foothills of mountain ranges next to arid plateaus, where the observation-based increases greatly exceed the climate model projections, and both temperature and humidity play a role in driving the observed changes.