Evaluation of Quasi‐Equilibrium Criteria for Coupled Climate Model Simulations

Abstract We evaluate five commonly‐applied criteria to validate that a climate model is in so‐called “quasi‐equilibrium,” using a suite of five simulations with CO 2 concentrations between 1× and 16× Pre‐Industrial values. We find that major changes in ocean circulation can occur after common thermal equilibrium criteria are reached, such as a small Top of Atmosphere radiative flux imbalance, or weak trends in surface air temperature, sea surface temperature, and deep ocean temperature. Ocean circulation change, in turn, impact high‐latitude SAT, sea ice, and the Inter‐tropical Convergence Zone position. For future modeling studies and intercomparison projects aiming for an ocean in quasi‐equilibrium, we suggest that time series of key meridional overturning circulation (MOC) metrics in the Atlantic, Pacific, and Southern Ocean are saved, and that MOC trends are less than 1 Sv/1000 years, and DOT trends less than 0.1°C/century for the final 1000 years of the simulations.