Processes controlling the seasonal cycle of surface temperature: theory and idealised simulations

From the perspective of the annual harmonic, the role of heat capacity in controlling the seasonal cycle of surface temperature is readily apparent: a larger heat capacity means a greater phase delay between solar insolation and surface temperature, as well as a reduced amplitude. But how other processes, including latent and sensible heat fluxes, influence surface energy budget and thereby the seasonal cycle of temperature is not well understood. Here we use a linearisation of the surface energy budget to isolate how a range of processes influence the seasonal cycle of surface temperature. The theory highlights how surface wind speed and relative humidity can induce phase delays in surface temperature, analogous to the effect of heat capacity. The framework also quantifies how these variables can modify asymmetry in the seasonal cycle of surface temperature (i.e., differing lengths of warming and cooling seasons) from that expected from insolation alone. In addition to the linearisation approach, we perform simulations with an idealised climate model (“Isca”) to quantify the role of these processes in setting the overall phase and amplitude of the seasonal cycle of surface temperature. Implications of the theory and idealised simulations for understanding variations in the seasonal cycle of temperature across latitude, across surface types (e.g., land vs ocean), and across climate states are discussed.