Key Physics of the Madden-Julian Oscillation Based on Multi-model Simulations (Invited Presentation)

In this presentation, by analyzing 27 climate models that participated in the recent MJO Task Force and GEWEX GASS MJO global model comparison project, key processes responsible for realistic MJO simulations, including its amplitude and propagation, are explored under the “moisture mode” framework. Supporting evidences for the hypothesis of the “moisture mode” for the MJO will be presented. While surface flux and radiative heating anomalies are considered important for amplifying the MJO, their strength per unit MJO precipitation anomaly is found to be negatively correlated to MJO amplitude across these multi-model simulations. However, model MJO amplitude is found to be closely tied to a model’s convective moisture adjustment time scale, a measure of how rapidly precipitation must increase to remove excess column water vapor, or alternately the efficiency of surface precipitation generation per unit column water vapor anomaly. Models with stronger MJO amplitude are characterized by a shorter convective time scale, weaker ascending motion, smaller cloud fraction, and more bottom-heavy vertical velocity profile per unit MJO precipitation compared to models with a longer convective time scale. On the other hand, horizontal advection of moist static energy (MSE) is found to play a crucial role for realistic eastward propagation of the MJO in GCM simulations. Due to model deficiencies in simulating both the MJO circulation and spatial distribution of background MSE, the horizontal advection of MSE is greatly underestimated in the poor MJO models, and largely offset by effects from radiative and surface fluxes, leading to rather weak eastward or even westward propagation of MJO convection in those models. Finally, a diagnostic metric for the eastward propagation of the MJO by using mean lower-tropospheric specific humidity will be presented.