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

Abstract

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.

A Global Quantification of the Physical Processes Leading to Near‐Surface Cold Extremes

“…Then, the terms on the r.h.s. of Equation are evaluated by integrating them along $$(\boldsymbol{x}(t),t)$$, from $${t}_{g}$$ to $${t}_{X}$$, as detailed in Röthlisberger and Papritz (2023).…”

“…The second numerical residual, $$\mathrm{r}\mathrm{e}\mathrm{s}2$$, arises from numerical inaccuracies in the computation of derivatives in Equation and is negligible everywhere (Figure S1d in Supporting Information ). As in Röthlisberger and Papritz (2023), we compute an overall residual $$\mathrm{r}\mathrm{e}\mathrm{s}$$ as the sum of the seasonality $${T}^{\prime }$$ and the two numerical residuals, which is presented in Figure 1e.…”

“…of Equation denote from left to right, $${T}^{\prime }$$ due to changes in the temperature climatology over time, $${T}^{\prime }$$ arising from horizontal advection of the air parcel in the direction of the climatological temperature gradient, $${T}^{\prime }$$ resulting from vertical motion, and $${T}^{\prime }$$ arising from diabatic processes along the trajectory. As in Röthlisberger and Papritz (2023), these terms are hereafter referred to as seasonality $${T}^{\prime }$$, advective $${T}^{\prime }$$, adiabatic $${T}^{\prime }$$ and diabatic $${T}^{\prime }$$, respectively.…”

“…We address this research gap by employing a novel Lagrangian temperature anomaly decomposition introduced by Röthlisberger and Papritz (2023), who quantified the contributions of advection, adiabatic warming and diabatic heating to temperature anomalies during near‐surface hot extremes. Here, we apply their methodology to near surface temperature anomalies during the coldest days of each year (hereafter TN1day events) globally in the period 1979–2020 using ERA5 reanalyzes (Hersbach et al., 2020, see below).…”

“…In the following, we briefly introduce the concept of the temperature anomaly decomposition followed by a description of its technical implementation in Section 2. For a more in‐depth description the reader is referred to Röthlisberger and Papritz (2023).…”