Assessing the role of thermodynamic drivers underlying extreme heat in a warming climate

In recent years, heatwaves around the globe have eclipsed long–term maximum temperature records. While a stronger warming over land than sea is both expected and observed, regional hot extremes are warming at an even faster pace. Modeling studies have suggested that soil moisture–temperature feedbacks drive this amplification in climate projections of the ongoing century, and there is solid observational evidence of a link between high temperatures and desiccating soils for individual events: dry soils invoke a shift in the surface energy partitioning toward sensible heating, thereby promoting higher air temperatures. A particularly notable heatwave unfolded in late June 2021 in the Pacific Northwest, baffling the scientific community with its high intensity. Using a combination of reanalysis data and factorial Earth System Model simulations, we show that heat released from condensation over the North Pacific and local soil moisture deficits strongly contributed to the extreme heat where the temperatures were most anomalous. Mediated by desiccating soils, our analysis also points to complex land–atmosphere interactions beyond intensified surface sensible heating. Since it remains unclear to what extent an enhanced thermodynamic potential — such as epitomized by this remarkable “black swan event” — is responsible for the observed exacerbation of heatwaves in recent decades, we we also investigate the link between summertime soil drought and hot extremes in our changing climate.