Identifying regional drivers shaping daily maximum temperatures and their extremes

Daily maximum air temperatures (T_max) are shaped by radiation, advection, atmospheric circulation, and land-surface processes, all interacting through complex feedbacks but essentially reflecting changes in the local surface energy budget. Here, we use a land-atmosphere systems approach to derive an analytical expression for daily maximum temperatures that depends solely on observed radiative and surface-evaporative conditions, requiring no additional parameters. We do this by accounting for the surface energy balance, heat storage variations within the lower atmosphere and explicitly constrain vertical turbulent exchange using the thermodynamic limit of maximum power. This approach reproduces observations very well with residual errors comparable to the reanalysis data. We then applied it to understand variations in T_max and found that its day-to-day variability is predominantly shaped by shortwave cloud radiative effects and longwave water-vapor emissivity in the humid tropics, while heat advection and storage effects are the primary contributors in drier subtropics and high latitudes. Hot extremes, however, are mostly shaped by anomalies in land-surface characteristics including soil water stress and turbulent fluxes, with secondary contributions from heat advection and radiative effects. Both variability and extremes in the tropics were linked to changes in moisture, while the heat-storage and advective effects dominate in dry subtropics and high-latitude regions. These findings reveal the regional radiative and hydrological drivers of temperature variations within the thermodynamic energy budget and provide a baseline for understanding biases and inter-model variability in climate models. It can further help in assessing first-order changes in daily maximum temperatures due to various aspects of global change.