A simplified model of the Walker circulation with an interactive ocean mixed layer and cloud-radiative feedbacks

Abstract

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. This study has two main purposes: 1) to formulate a conceptual framework that includes the dominant feedbacks between clouds and a large-scale divergent circulation; and 2) to use this framework to investigate the sensitivity of the climate system to these interactions. Two cloud types - high, convective anvils and low, nonprecipitating 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. Because of 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 horizontally varying ocean heat transport and 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 significantly. The climate is found to be less sensitive to doubled greenhouse gas experiments with low clouds than without. © 2005 American Meteorological Society.