Investigating the thermodynamic control of the Madden-Julian oscillation using weak temperature gradient approximation

The Madden-Julian oscillation (MJO) is a dominant mode of tropical intraseasonal variability in the atmosphere. It is characterised by eastward-propagating tropical convective anomalies and associated circulation anomalies, yet the controlling thermodynamic and dynamic mechanisms are still not well understood.

The weak temperature gradient (WTG) approximation may be used to investigate the relative importance of the thermodynamic control to the overall control of convection in the MJO. The WTG approximation is based on the observation that horizontal temperature gradients are small in the tropical atmosphere, and can be used as a parametrization of the large scale in limited domain simulations. We use a cloud resolving model (CRM) which implements the WTG approximation by relaxing the domain averaged potential temperature to a reference profile that represents the ambient environment.

In order to investigate the MJO using WTG, we impose a time series of perturbations in the potential temperature and moisture profiles. These perturbations were derived from data taken during an MJO event in TOGA COARE field program. Since convection is sensitive to the reference profile, imposing changes in the profile consistent with observations will provide a way to compare modeled convection to observations. The extent to which the modeled convection represents observations will provide information as to how important thermodynamic mechanisms are to the overall control of the MJO.