How Dry Soil Moisture Extremes Exacerbate Heatwaves over the Contiguous United States

Increased heatwave frequency across the United States has led to the need for improved predictability of heatwave events. A detailed understanding of land-atmosphere interactions and the relationship between soil moisture and temperature extremes could provide useful information for prediction. This study identifies for many locations a threshold of soil moisture below which there is an increase in the sensitivity of the atmosphere to soil moisture. This shift to a hypersensitive regime causes the atmosphere to be in prime condition to be susceptible to atmospherically driven heatwave conditions. The breakpoint where the regime shift occurs is estimated using segmented regression applied to ERA5 reanalysis data. It is shown that as the soil gets drier, there is an associated decrease in latent heat and increase in sensible heat which leads to a strong positive feedback of increased air temperature at the surface, which further dries out the soil. Central, southwestern and southeastern parts of the US seem to have regions of clear regime shifts while the eastern part of the US generally does not get dry enough to reveal significant breakpoints. The drop in latent heat flux appears to be the major driver of this increased sensitivity, which then affects the sensible heat flux. An investigation of flux tower sites at specific locations is carried out to verify these breakpoint behaviors. Accurate estimation of these breakpoints would lead to improved heatwave prediction. This analysis could also add to other methods of evaluating forecast models.