Moist Hadley circulation: constraints and the role of wave-convection coupling in an aqua-planet AGCM

Basic climate dynamics of tropical circulation is studied by using an aqua-planet atmospheric general circulation model. Under an idealized globally uniform solar insolation, a number of numerical experiments have been conducted by varying the meridional structure of sea-surface temperature (SST). The model produced overturning meridional circulation like the Hadley cells even under globally uniform SST as suggested in previous studies. Its strength varied from being faint to strong somewhat comparable to the actual Hadley circulation, depending on a tunable parameter of the convective scheme of the model. The parameter thresholds the occurrence of deep convection and thereby affects its temporal variability. With additional experiments, it is suggested that the strength of the "Hadley circulation", or the "ITCZ", in the model is controlled by the convectively-coupled equatorial waves, which introduces favors in meridional distribution of convection occurrence. They also affect the meridional extent of the circulation. If meridional structures are introduced in SST, convection occurs favorably above SST maxima as expected. However, the transition is gradual, and the control through wave-convection coupling remains. Discussion is made on thermodynamic constraints and angular momentum balances.