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

Abstract

To obtain physical insights into the response and feedback of low clouds (Cl) 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 Cl 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 Cl 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 Cl decreases. The slow response reflects the variability of Cl 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 Cl. Its sign is determined by a subtle residual of the Cl increase and decrease over the ascending and subsidence regions, respectively. The regional Cl 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 Cl to increasing CO2 concentrations is a robust part of the climate change. © 2011 The Author(s).