The Influence of Drought on Heat Wave Intensity, Duration, and Exposure

Abstract The societal risks posed by heat waves can be reinforced and magnified during drought conditions. To explore this further in the United States, heat waves and drought events were defined at U.S. Climate Reference Network (USCRN) stations using daily maximum air temperature and weekly time series of the U.S. Drought Monitor (USDM). Standardized air temperature and heat index anomalies were evaluated to determine how droughts influenced heat wave duration, intensity, and heat exposure. Overall, compound heat waves had higher levels of heat exposure than noncompound heat waves, particularly during the late afternoon and early evening hours. On average, compound events had 46.9°C more cumulative extreme heat degree hours (EHDHs; degrees Celsius above the 90th percentile) of exposure than noncompound heat waves with a third of USCRN stations having 50°C EHDHs or more. This reduced to 16.4°C EHDHs when heat index was used to define exposure. Increases in compound heat wave exposure were primarily driven by droughts lengthening heat waves (12 h–2 days longer) than by higher air temperature intensity (0.2 standard deviations warmer). Given that 5-cm standardized soil moisture anomalies were found to both vary (wet and dry conditions) within drought events and strongly impact both air temperature and heat index intensity, the muted intensity response to drought may be related to the USDM’s limited response to short-term precipitation events that are sufficient to moisten the soil and impact heat waves, but not ameliorate drought. This highlights the importance of evaluating heat waves at subdaily scales and explicitly accounting for the role of soil moisture in influencing heat wave intensity and exposure. Significance Statement Droughts were found to increase the risk of heat exposure when comparing compound to noncompound heat waves, particularly during the late afternoon to early evening periods. The increased exposure was driven more by droughts lengthening the duration of heat waves rather than increasing their intensity. Soil moisture anomalies provided further clarity on the compounding effect as a more direct measure of deficits important to heat wave evolution than the all-drought-type approach of the U.S. Drought Monitor.