Revisiting the link between soil moisture deficits and heatwaves

Severe heatwaves tend to strike during drought conditions, primarily considered a consequence of persistent, often quasi-stationary anticyclonic circulation. A key mechanism for heatwave intensification is the positive feedback between rapidly desiccating soils through elevated atmospheric evaporative demand and the associated enhanced surface sensible heating. The effect of such enhanced sensible heating is often quantified by comparing the evolution of heatwaves in climate model simulations with freely evolving soil water to additional simulations in which soil moisture is kept at climatological levels, and can reach up to several degrees Celsius. With this approach, one can gauge the effect of deviations from present-day average soil moisture, but this becomes increasingly hypothetical as we shift away from climatological norms and toward a future marked by widespread projected increases in agro-ecological drought during summer months. In such a climate change context, a general key question to address is: How does heatwave intensity depend on the initial state of soil moisture? To investigate this, we re-simulate historical heatwaves using CESM2, a state-of-the-art global Earth System Model, and examine how these events would have unfolded under different land surface conditions. We also explore the long-noted — yet never fully quantified — effect of soil drought on anticyclonic circulation itself.