Physical constraints for temperature biases in climate models

Abstract

In general, biases of climate models depend upon the climate state (i.e., are nonstationary). Recent studies have shown that the adoption of a stationary temperature bias can lead to an overestimation of projected summer warming in southern Europe. It has also been proposed to use a bias correction that increases linearly with temperature. While such an assumption is well‐justified for near‐term projections, one wonders whether and at what temperature this relation levels off if it does. Here we show, using regional climate model simulations of the ENSEMBLES project and from a single‐model perturbed physics ensemble, that the linear bias assumption breaks down at high model temperatures, followed by a transition to a constant bias relation. This transition is apparent in strongly biased model simulations and supported using a pseudo‐reality approach. We show that soil moisture scarcity explains a large degree of summer temperature biases across both ensembles and that the limits of soil moisture depletion are responsible for the transition. A linear temperature bias correction therefore potentially over‐corrects summer warming, and implicitly assumes unphysical relations between soil moisture and temperature, in particular when considering high‐emission scenarios. We conclude that a physically consistent and time‐dependent temperature bias correction considering the state of the soil would increase the robustness of bias correction and reduce the uncertainty of 21st century summer warming.