Heat waves are broadly expected to increase in severity and frequency under climate change. Case studies highlight a number of physical mechanisms that play a role in present-day heat waves, which typically occur during a coalescence of anomalous atmospheric and land surface conditions. However, a unified model of heat wave physics is lacking, primarily owing to difficulty in disentangling the forcing versus feedback roles of soil moisture and atmospheric variability. Here, we provide observational evidence that soil moisture modulation of heat waves is a generic feature of midlatitude continental climates, and develop a theoretical framework to understand this modulation. Using separation of timescales we derive a diagnostic equation for the nonlinear response of temperature to soil moisture variations, and a dynamical Hasselmann-like model for the soil moisture variations themselves. We find that soil moisture fluctuations control the frequency and intensity of temperature extremes by slowly altering the background state on which rapid atmospheric variability acts, rather than by altering atmospheric variability itself. We also find the slow soil moisture variations are well-approximated as being primarily driven by stochastic precipitation variability. Our framework provides a first-principles understanding of soil moisture’s role in midlatitude temperature extremes.
