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

Abstract. 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. So far, it is an open question whether heat extremes are associated with characteristic vertical temperature anomaly profiles and, if they exist, how they vary across the globe. In this study, we globally and systematically classify vertical temperature anomaly profiles during annual maximum 2-m temperature events (TXx) using a k-means clustering approach. After normalising and scaling 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 vertically 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 TXx events. In the polar cluster, the events 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, the normalised and scaled temperature anomaly profiles during heat extremes are very similar to each other within a given climate zone. Deviations from the cluster median during individual TXx events mainly come from the variability between TXx events rather than the variability between the median profiles of different grid points. Finally, the normalised and scaled temperature anomaly profiles of the most extreme TXx events are particularly well represented by the grid point's median profile for all TXx events, suggesting a typical dynamics of the most extreme heat events.