Understanding the vertical temperature structure of recent record-shattering heatwaves

Abstract

Extreme heatwaves are one of the most impactful natural hazards, posing risks to human health, infrastructure, and ecosystems. Recent theoretical and observational studies have suggested that the vertical temperature structure during heatwaves limits the magnitude of near-surface heat through convective instability. In this study, we thus examine in detail the vertical temperature structure during three recent record-shattering heatwaves, the Pacific Northwest (PNW) heatwave in 2021, the western Russian (RU) heatwave in 2010, and the western European and UK (UK) heatwave in 2022, by decomposing temperature anomalies (T′) in the entire tropospheric column above the surface into contributions from advection, adiabatic warming and cooling, and diabatic processes. All three heatwaves exhibited bottom-heavy yet vertically deep positive T′ extending throughout the troposphere. Importantly, though, the T′ magnitude and the underlying physical processes varied greatly in the vertical within each heatwave, as well as across distinct heatwaves, reflecting the diverse synoptic storylines of these events. The PNW heatwave was strongly influenced by an upstream cyclone and an associated warm conveyor belt, which amplified an extreme quasi-stationary ridge and generated substantial mid- to upper-tropospheric positive T′ through advection and diabatic heating. In some contrast, positive upper-tropospheric T′ during the RU heatwave was caused by advection, while during the UK heatwave, it exhibited modest positive diabatic contributions from upstream latent heating only during the early phase of the respective ridge. Adiabatic warming notably contributed positively to lower-tropospheric T′ in all three heatwaves, but only in the lowermost 200–300 hPa. Near the surface, all three processes contributed positively to T′ in the PNW and RU heatwaves, while near-surface diabatic T′ was negligible during the UK heatwave. Moreover, there is clear evidence of an amplification and downward propagation of adiabatic T′ during the PNW and UK heatwaves, whereby the maximum near-surface T′ coincided with the arrival of maximum adiabatic T′ in the boundary layer. Additionally, the widespread ageing of near-surface T′ over the course of these events is fully consistent with the notion of heat domes, within which air recirculates and accumulates heat. Our results for the first time document the four-dimensional functioning of anticyclone–heatwave couplets in terms of advection, adiabatic cooling or warming, and diabatic processes and suggest that a complex interplay between large-scale dynamics, moist convection, and boundary layer processes ultimately determines near-surface temperatures during heatwaves.