Tracing climate changes in the Diurnal Temperature Cycle via their differential signatures on amplitude and phase

Abstract:

The diurnal cycle of surface temperature plays a major role in Earth’s climate, from atmosphere-land interactions over ecosystem changes to human well-being, yet it remains poorly understood. Historically, the amplitude of the diurnal cycle narrowed over much of the 20th century, yet several competing mechanisms have been proposed as explanations. Similarly, global climate models (GCMs) still widely disagree on the sign and magnitude of possible changes in the diurnal temperature cycle under global warming. More work is thus needed to understand the mechanisms that govern the diurnal cycle as well as to increase our confidence in future projections.

Here we present an idealized model of the diurnal temperature cycle, derived from surface energy balance. Our model points to three mechanisms that jointly control the diurnal temperature cycle at a given location: (a) the daily amplitude of shortwave forcing, (b) the mean temperature, and (c) the surface’s mean hydrology, which governs evaporation. To validate the model we compare its predictions against the diurnal temperature cycle in ERA5 reanalysis. In doing so we take care to disentangle the fact that the diurnal temperature cycle at a single location is forced both by the local day-night cycle as well as non-local advection. Our model is able to predict the spatial pattern of diurnal temperature range (DTR) as well as the seasonal cycle of DTR, lending confidence to our approach.

Importantly, our results show that historical timeseries of DTR by themselves are not sufficient to link observed changes in DTR to a mechanism. However, we also find that the phase of the diurnal cycle responds differently than its amplitude to different climatic changes. As such, joint consideration of DTR as well as its phase could make it feasible to attribute past changes in the diurnal cycle, and to build confidence in future projections.