Soil moisture-driven increases in hot and dry days in the American West

An increasingly large body of work has focused on the observed and projected uptick in compound high heat and high humidity extremes, which pose major hazards to human health. However, in arid and semiarid regions, a greater concern is hot and dry extremes that can lead to wildfire spread and increased evaporation of surface water resources. To analyze changes in humidity conditional on temperature, we use a recently developed quantile regression-based model that can flexibly describe the temperature-humidity relationship, as well as how it is changing in response to increasing global temperatures. We identify a dramatic decrease in dew point during hot and dry days in the semiarid Interior West of the United States that is amplified as a function of temperature. On average across the region from 1973-2018, we have observed a 2.5°C decrease in the 5th percentile of dew point on days with temperatures at the 90th percentile; the magnitude of this decrease increases to 3.0°C and 3.9°C on days in the 95th and 99th percentile of temperature, respectively. The observed decrease in dew points on hot days is linked to decreases in soil moisture during the prior winter and spring, which is a significant predictor of the probability of having unusually dry conditions during hot weather -- although not of the hot weather itself. Reductions in soil moisture have driven the large uptick in hot and dry events since 2000, and are jointly caused by internal variability and anthropogenic influence. Model projections from CMIP5 suggest that soil moisture in the region will continue to decrease through 2100, suggesting that the desiccation of the American West will continue.