Fundamental Causes of Propagating and Non-propagating MJOs in MJOTF/GASS models
Dr. WANG Lu
University of Hawaii
This study investigates the fundamental causes of differences in the Madden-Julian oscillation (MJO) eastward propagation among models that participated in a recent model inter-comparison project. These models are categorized into good and poor groups characterized by prominent eastward propagation and non-propagation, respectively. Column integrated moist static energy (MSE) budgets are diagnosed for the good and the poor models. It is found that a zonal asymmetry in the MSE tendency, characteristic of eastward MJO propagation, occurs in the good group, while such an asymmetry does not exist in the poor group. The difference arises mainly from anomalous vertical and horizontal MSE advection. The former is attributed to the zonal asymmetry of upper-middle tropospheric vertical velocity anomalies acting on background MSE vertical gradient; the latter is mainly attributed to the asymmetric zonal distribution of low-tropospheric meridional wind anomalies advecting background MSE/moisture field. Based on the diagnosis above, a new mechanism for MJO eastward propagation that emphasizes the second-baroclinic-mode vertical velocity is proposed.
A set of atmospheric general circulation model experiments with prescribed diabatic heating profiles were conducted to investigate the causes of different anomalous circulations between the good and the poor models. The numerical experiments reveal that the presence of a stratiform heating at the rear of MJO convection is responsible for the zonal asymmetry of vertical velocity anomaly and is important to strengthening lower-tropospheric poleward flows to the east of MJO convection. Thus, a key to improve the poor models is to correctly reproduce the stratiform heating. The roles of Rossby and Kelvin wave components in MJO propagation are particularly discussed.