Environmental Science and Engineering Seminar

Heatwaves cause great harm to societies, especially in midlatitude regions that are not adapted to high temperatures. An accurate projection for future heat extremes requires a fundamental understanding of the physical mechanisms. Several mechanistic drivers of heatwaves, such as atmospheric blocking and soil moisture-atmosphere feedback, are well-known for their ability to raise surface air temperature. However, what limits the maximum surface air temperature in heatwaves remains unclear, and this became evident during recent Northern Hemisphere heatwaves which achieved temperatures far beyond the upper tail of the observed statistical distribution.

Here, we present the hypothesis with corroborating evidence that convective instability limits annual maximum surface air temperatures over midlatitude land. We provide a theory for the upper bound of midlatitude temperatures, which accurately describes the observed relationship between temperatures at the surface and in the mid-troposphere. We also interpret recent extreme heatwaves using this theory and discuss how continental extreme temperatures will change with global warming.