An upper bound for extreme temperatures over midlatitude land.

Area of Science:

• Atmospheric Science
• Climate Science
• Meteorology

Background:

• Heatwaves are intensifying globally due to climate change.
• Known drivers like atmospheric blocking and soil moisture feedbacks increase heatwave intensity.
• The upper limit of maximum surface air temperature during heatwaves remains poorly understood.

Purpose of the Study:

• To investigate the physical mechanisms limiting maximum surface air temperatures (TXx) during heatwaves over midlatitude land.
• To develop a theoretical framework for the upper bound of midlatitude temperatures.

Main Methods:

• Formulated a theory based on convective instability to describe the upper bound of midlatitude temperatures.
• Analyzed the relationship between surface and mid-tropospheric temperatures.
• Utilized empirical data on 500-hPa warming to project future temperature bounds.

Main Results:

• Evidence suggests convective instability limits annual maximum surface air temperatures (TXx) over midlatitude land.
• The proposed theory accurately describes the observed relationship between surface and mid-tropospheric temperatures.
• Heatwave drivers alter proximity to this temperature upper bound.
• The upper bound for midlatitude TXx is projected to increase 1.9 times faster than 500-hPa temperatures.

Conclusions:

• Convective instability acts as a critical factor in setting the upper limit for heatwave temperatures in midlatitudes.
• The theoretical framework provides a new understanding of heatwave temperature extremes.
• Projected increases in the upper bound of TXx over Northern Hemisphere midlatitudes are substantial, rising twice as fast as global mean surface air temperature.
• Certain regions may experience TXx increases exceeding this projected bound, particularly those that dry out on the hottest days.