Quantifying the physical processes leading to atmospheric hot extremes at a global scale

Heat waves are among the deadliest climate hazards. Yet the relative importance of the physical processes causing their near-surface temperature anomalies (T ')-advection of air from climatologically warmer regions, adiabatic warming in subsiding air and diabatic heating-is still a matter of debate. Here we quantify the importance of these processes by evaluating the T ' budget along air-parcel backward trajectories. We first show that the extreme near-surface T ' during the June 2021 heat wave in western North America was produced primarily by diabatic heating and, to a smaller extent, by adiabatic warming. Systematically decomposing T ' during the hottest days of each year (TX1day events) in 1979-2020 globally, we find strong geographical variations with a dominance of advection over mid-latitude oceans, adiabatic warming near mountain ranges and diabatic heating over tropical and subtropical land masses. In many regions, however, TX1day events arise from a combination of these processes. In the global mean, TX1day anomalies form along trajectories over roughly 60 h and 1,000 km, although with large regional variability. This study thus reveals inherently non-local and regionally distinct formation pathways of hot extremes, quantifies the crucial factors determining their magnitude and enables new quantitative ways of climate model evaluation regarding hot extremes.

Automated summary

This study quantifies the relative importance of physical processes causing heat waves by evaluating the T' budget along air-parcel trajectories. It finds that diabatic heating and to a lesser extent adiabatic warming were the primary causes of extreme near-surface temperature anomalies during the 2021 heat wave in western North America. The study also reveals geographical variations in the dominance of advection, adiabatic warming, and diabatic heating in forming hot extremes.