Fast and slow timescales in the tropical low-cloud response to increasing CO₂ in two climate models

To obtain physical insights into the response and feedback of low clouds (C_l) to global warming, ensemble 4 × CO₂ experiments were carried out with two climate models, the Model for Interdisciplinary Research on Climate (MIROC) versions 3. 2 and 5. For quadrupling CO₂, tropical-mean C_l decreases, and hence, acts as positive feedback in MIROC3, whereas it increases and serves as negative feedback in MIROC5. Three time scales of tropical-mean C_l change were identified-an initial adjustment without change in the global-mean surface air temperature, a slow response emerging after 10-20 years, and a fast response in between. The two models share common features for the former two changes in which C_l decreases. The slow response reflects the variability of C_l associated with the El Niño-Southern Oscillation in the control integration, and may therefore be constrained by observations. However, the fast response is opposite in the two models and dominates the total response of C_l. Its sign is determined by a subtle residual of the C_l increase and decrease over the ascending and subsidence regions, respectively. The regional C_l increase is consistent with a more frequent occurrence of a stable condition, and vice versa, as measured by lower-tropospheric stability (LTS). The above frequency change in LTS is similarly found in six other climate models despite a large difference in both the mean and the changes in the low-cloud fraction for a given LTS. This suggests that the response of the thermodynamic constraint for C_l to increasing CO₂ concentrations is a robust part of the climate change.