Global characterisation of the vertical temperature anomaly structure of heat extremes over land in ERA5

The formation of surface heat extremes is usually described in terms of surface processes and upper-level dynamics. However, their full vertical temperature profile contains additional essential information about the involved processes and dynamics. So far, it remains unclear whether heat extremes are associated with characteristic vertical temperature anomaly profiles and how they vary across the globe.
In this study, we globally and systematically classify vertical temperature anomaly profiles during annual maximum 2-m temperatures, so-called TXx events, using a k-means clustering approach. After a suitable normalisation and scaling of the anomaly profiles, we find three clusters, whose global distribution closely follows the polar, mid-latitude, and tropical climate zones. The three clusters capture key structural differences of heat extremes. Within the tropical cluster, positive temperature anomalies during TXx events are confined to the (often deep) boundary layer and intensify progressively in the days leading up to the event, while the upper troposphere is not deviating from its climatological mean. The mid-latitude cluster also exhibits bottom-heavy temperature anomalies, which, however, extend throughout the full troposphere, showing a strong vertical coupling during heat extremes. In the polar cluster, heat extremes are characterised by deep tropospheric warm anomalies, accompanied by the erosion of the near-surface inversion layer, resulting in a shallow layer of particularly strong temperature anomalies near the ground.
These results show that while multiple physical mechanisms can generate a heat extreme, at first order, temperature anomaly profiles during heat extremes are very similar to each other within a given climate zone. The variability between TXx events is much larger than the variability between the median profile of different grid points in the same cluster. Besides, the temperature profiles of the most extreme events are more similar to those of their cluster than the more moderate events, suggesting a typical dynamics of the most extreme heat events.