Cloud-climate feedbacks between precipitation, radiation, circulation strength, atmospheric temperature and moisture, and ocean temperature are studied with an idealized model of the Walker circulation in a nonrotating atmosphere coupled to an ocean mixed layer. The model atmospheric dynamics and physics are idealized from an intermediate-complexity GCM, the Quasiequilibrium Tropical Circulation Model of Neelin and Zeng. The vertical atmospheric structure is reduced to a single baroclinic mode to which the Weak Temperature Gradient (WTG) approximation is applied. Two cloud types, high, convective anvils and low, non-precipitating stratus are included and coupled to the large scale dynamics. The atmosphere is coupled to an ocean mixed layer via a consistent surface energy budget. Analytic approximations with a simplified radiation scheme are derived and used to explain numerical results with a more realistic radiation scheme. The model simplicity allows interactions between different parts of the ocean/atmosphere system to be cleanly elucidated, yet also allows the areal extent of deep convection and the horizontal structure of the Walker circulation to be internally determined by the model.

Due to their strong top-of-atmosphere radiative cancellation, high clouds are found to have little overall effect on the circulation strength and convective area fraction. Instead, to leading order, these are set by the clear sky radiative fluxes. Low clouds are found to cool both the ocean and atmosphere, to slightly increase the circulation strength, and to shrink the convective area rather significantly. The climate is found to be more sensitive to doubled greenhouse gas experiments with low clouds than without.