Roles of the Moisture and Wave Feedbacks in Shaping the Madden-Julian Oscillation

In this study, we investigate the moisture and wave feedbacks in the Madden-Julian Oscillation (MJO) dynamics via applying the general trio-interaction theoretical model. The trio-interaction model can reproduce observed large-scale characteristics of the MJO in terms of horizontal quadrupole-vortex structure, vertically tilted structure led by planetary boundary layer (PBL) convergence, slow eastward propagation with a period of 30-90 days, and planetary-scale circulation. We can identify the moisture feedback effects in this model by using diagnostic thermodynamic and momentum equations, and investigate the wave feedback effects by using a diagnostic moisture equation. The moisture feedback is found to be responsible for producing the MJO dispersive modes when the convective adjustment process is slow. The moisture feedback mainly acts to reduce the frequency and growth rate of the short waves, while leaving the planetary waves less affected; so neglecting the moisture feedback is a good approximation for the wavenumber-one MJO. The wave feedback is shown to slow down the eastward propagation and increase the growth rate of the planetary waves. The wave feedback becomes weak when the convective adjustment time increases; thus neglecting the wave feedback is a good approximation for the MJO dynamics during a slow adjustment process. Sensitivities of these two feedbacks to other parameters are also discussed. These theoretical findings suggest that the two feedback processes, thus the behaviors of the simulated MJO mode, should be sensitive to the parameters used in cumulus parameterizations.