Physical Processes Leading to Extreme day-to-day Temperatures Changes, Part I: Present-day Climate

Abstract. Extreme temperature changes from one day to another, either associated with warming or cooling, can have a significant impact on health, environment, and society. Previous studies have quantified that such day-to-day temperature (DTDT) variations are typically more pronounced in the extratropics compared to tropical zones. 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 observation, 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, in particular shifts from warm to cold air advection or vice versa, linked to regionally specific synoptic-scale circulation anomalies (ridge or through patterns). These dominant effects of advection are modulated by changes in adiabatic and diabatic processes in the transported air parcels, which tend to either amplify or dampen DTDT decreases (cold events) and increases (warm events) depending on the region and season. In contrast, DTDT extremes 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 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 II of this study.