Convection, cloud-radiative feedbacks and thermodynamic ocean coupling in simple models of the walker circulation

Abstract

The authors consider a set of simple models for the divergent component of the tropical atmospheric circulation, and the associated precipitation field. A number of strong simplifying assumptions are made, leaving deep convection and radiation (both simply parameterized) as the key processes in the models. The first case considered is that of fixed sea surface temperature (SST), with an SST gradient imposed across the domain, in the limit of zero convective time scale or strict quasi-equilibrium (SQE). Steady solutions are found for sufficiently weak cloud-radiative feedback. As the parameter controlling the strength of the cloud-radiative feedback is increased, the region of nonzero precipitation shrinks in size and the precipitation grows stronger there. For cloud-radiative feedback stronger than a particular value, steady solutions cannot be found, and numerically obtained time-dependent solutions blow up. In the next case considered, the slab ocean lower boundary condition is used. In this case the behavior is dramatically different. In the steady solutions, the strength of the radiative feedback now has little effect on the size of the precipitating region. The finite convective time scale changes the stability criterion, and when the steady solutions become unstable, rather than blowing up, well-behaved radiative-convective oscillations set in. The mechanism for these oscillations is essentially local radiative-convective or surface flux convective instability, so they can be studied at a single point. Their frequencies are intraseasonal, and some inferences can be drawn from them about how ocean coupling affects the Madden-Julian oscillation. The simplicity of the models allows a relatively complete understanding of their behavior. The sensitivity of the solutions to the key parameters, especially the convective time scale and the cloud radiative feedback parameter, are discussed in some detail.