Extreme temperature changes from one day to another that are associated with either warming or cooling can have a significant impact on health, the environment, and society. Previous studies have shown that such day-to-day temperature (DTDT) changes are typically more pronounced in the extratropics than the tropics. However, the underlying physical processes and the relationship between extreme events and the large-scale atmospheric circulation remain poorly understood. Here, these processes are investigated for different locations around the globe based on ERA5 reanalysis data and Lagrangian backward-trajectory calculations. We show that extreme DTDT changes in the extratropics are generally associated with changes in air mass transport, particularly shifts from warmer to colder air parcels or vice versa that are linked to regionally specific synoptic-scale circulation anomalies (ridge or trough patterns). These dominant effects of advection are modulated by changes in adiabatic and diabatic processes in the transported air parcels, which either amplify or dampen DTDT decreases (cooling events) and increases (warming events) depending on the region and season. In contrast, extreme DTDT changes during December–February in the tropics are controlled by local processes rather than changes in advection. For instance, the most significant DTDT decreases are associated with a shift from less cloudy to more cloudy conditions, highlighting the crucial role of solar radiative heating. The mechanistic insights into the extreme DTDT changes obtained in this study can help improve the prediction of such events and anticipate future changes in their occurrence frequency and intensity, which will be investigated in part 2 of this study.
