MJO sensitivity to basic state and parameterization in an atmospheric general circulation model

The Madden-Julian oscillation is investigated using a modified version of the National Center for Atmospheric Research Community Atmospheric Model 3. The model MJO exhibits a discharge-recharge character in which column-integrated moist static energy (MSE) builds before peak MJO convection, and is discharged during and after the MJO convective event. It is shown that horizontal advection and latent heat flux anomalies are the leading terms in the vertically-integrated MSE budget. Anomalous horizontal advection helps to moisten the atmosphere in advance of MJO convection, and synoptic-scale disturbances are a key regulator of horizontal MSE advection.

The RAS convection parameterization employed in the modified version of CAM has a minimum entrainment threshold that may help explain the tendency for MSE to build in advance of model MJO convection. Sensitivity tests explore the amplitude and character of intraseasonal variability as a function of minimum entrainment rate. Amplitude is shown to increase monotonically with minimum entrainment rate. Advanced diagnostics explore convective behavior (e.g. cloud height, mass flux) as a function of MJO phase as minimum entrainment parameters are varied. The mean state is also analyzed.

Given the strong contribution of latent heat flux to the MSE budget in this model, experimentation is conducted with the model parameterizations of surface latent and sensible heat flux in an aquaplanet framework. Basic state wind corrections (westerly or easterly) are added to the wind vector that is input into the flux parameterizations, along with a background wind speed correction to retain a similar wind speed (and flux) climatology. It is demonstrated that the basic state wind strongly influences the propagation speed and amplitude of model MJO variability through the surface latent and sensible heat flux parameterizations.