A new global Lagrangian analysis of near-surface temperature extremes

Abstract

Temperature extremes strongly affect the society and the environment, yet a complete physical understanding of their formation mechanisms is still lacking. Specifically, the relative importance of the three key processes—horizontal advection, subsidence accompanied by adiabatic warming, and diabatic heating—remains controversial. This paper presents a global quantification of the contributions from these processes to near-surface temperature extremes using the Lagrangian framework. Two Lagrangian potential temperature anomaly decompositions are applied: one based on the full fields of the respective terms, and the other one based on the anomaly fields of the respective terms (i.e., deviations from their corresponding climatologies). The results from the decomposition based on full fields mostly align with those of a previous study, while the decomposition based on anomaly fields offers a different assessment of the roles of the different processes. Most importantly, horizontal transport is attributed the primary role for both extremes globally.

Plain Language Summary: Temperature extreme events can have serious impacts on the society and the environment. However, a full understanding of how temperature extremes form in the Earth's atmosphere is still lacking. What remains especially unclear is whether temperature extremes result from horizontal or vertical transport of air masses or from the heating and cooling of these air masses on their way. This study takes a global approach to assess the roles of these processes in the formation of warm and cold extremes. By tracing the evolution of air masses, the study evaluates the contribution of each process. Following a previous approach, the analysis suggests that warm and cold extremes form quite differently: While horizontal transport appears to dominate the formation of cold extremes globally, warm extremes seem to result from a variety of processes. However, the current study then reveals that, when considering deviations from climatological conditions for each process, anomalies in the horizontal transport dominate the formation of both warm and cold extremes. Thus, globally, horizontal transport appears to be the key factor in the formation of both extremes.