How Climate Change Is Intensifying U.S. Heatwaves during Droughts in CESM1

Although the link between droughts and heatwaves is widely acknowledged, how climate change affects this link remains uncertain. Here we assess human-induced changes in U.S. summertime drought-heatwave interaction that may yield more such compound events using a coupled climate model simulation (CESM1), subjected to respective Year-1850 and Year-2000 radiative forcings. Our results reveal a strong dependency of heatwaves on soil moisture variability in water-limited regions of the Southern Plans and Southwest U.S.; while in energy-limited regions of the North/Northeast U.S., heatwaves are insensitive to drought severity. Applying a statistical model based on pair copula constructions, we find anthropogenic warming leads to enhanced soil moisture–temperature coupling in moisture-limited areas, and consequently amplified intensity and increased likelihood of heatwaves during severe droughts over the South and West U.S.. This strengthened coupling can explain a substantial fraction of rising temperature extremes related to the long-term climate change, thereby highlighting an increased role of land–atmosphere feedback in the development of hotter droughts in a warmer climate. To contrast, coupling effects remain weak and largely unchanged in energy-limited regions, thereby yielding no appreciable contribution to heatwave intensification over the North/Northeast U.S., apart from the long-term warming effects.