, Volume 39, Issue 7-8, pp 1627-1641,
Open Access This content is freely available online to anyone, anywhere at any time.
Date: 17 Sep 2011

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.