Abstract
Heatwaves damage societies globally and are intensifying with global
warming. 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 unknown; 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 (TXx) 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. Known heatwave drivers shift the
position of the atmospheric state in the phase space described by the
theory, changing its proximity to the upper bound.Our theory suggests
that the upper bound for midlatitude TXx should increase 1.9 times as
fast as 500-hPa temperatures. Using empirical 500-hPa warming, we
project that the upper bound of TXx over Northern Hemisphere midlatitude
land (40°N-65°N) will increase about twice as fast as global mean
surface air temperature, and TXx will increase faster than this bound
over regions that dry on the hottest days.