Quantifying the Drivers of Compound Heatwaves and Drought Events in The Contiguous United States

Over the years, several studies have examined heatwaves and drought events within the CONUS as singular extreme events, but few studies considered these two weather extremes simultaneously as compound or concurrent extreme events. However, profound implications of concurrent arid and high-temperature events exist, including reduced crop productivity, stresses on energy infrastructure, and mortality. This research quantifies the major drivers of compound heat and drought events during the summer months (June – August) within the contiguous United States (CONUS). We used several variables from ERA5 (e.g., daily 2m temperature, geopotential heights, soil moisture), and precipitation from observationally-informed Livneh and PRISM datasets over the period 1950 - 2022. We first create a database of compound heatwave and drought events and compute associated trends, frequencies and intensities of these events across the CONUS. We consider several percentiles of temperatures (e.g., 85^th, 90^th, and 95^th) for heatwave events whilst using common drought indices like the Standardized Precipitation Index (SPI) to define drought events. These multiple thresholds and indices enable us to present a robust and comprehensive analysis of the compound dry, hot extreme events as a function of space. Furthermore, the evolution in the degree of land-atmosphere interactions such as soil-moisture deficits, reduced vegetation and evapotranspiration at different scales are also evaluated for each event in our database. Results indicate that coupled with known increases in heatwaves and drought occurrences individually in recent years, compound hot, dry extreme events are also on the rise across regions such as the Southwest US, Southern Great Plains, and the Corn Belt. The results from the study would be beneficial in advising the agricultural sector on the potential of compound heat and drought events to enable them to minimize losses. Moreso, the results will also aid socio-economic planning, especially in the aspect of energy management, on how to plan for and mitigate the effects of compound hot and dry events during its season and in the future years going forward.