A Dynamical Transition between Rossby Modes and the MJO in an Idealized GCM

Abstract The Madden–Julian oscillation (MJO) produced by an idealized general circulation model (GCM) is studied. The model is a spectral dynamical core with a simple treatment of the hydrological cycle; MJO-like variability is induced by forcing zonal sea surface temperature gradients in the tropics using a prescribed ocean heat flux. Surface friction in the model is perturbed by altering the roughness length for momentum. The MJO weakens and eventually disappears entirely as the roughness length is decreased from its control value. The MJO is found to be sensitive to the roughness length in both the tropics and extratropics. Composite disturbances are constructed; the MJO has an associated vortex dipole straddling the equator. A similar vortex dipole exists when the roughness length is small, where disturbances have a closer connection to Rossby modes than to the MJO. In the lower- and midtroposphere, the main difference in the vorticity budgets of these two modes is in the rotational advection of vorticity, though the rotational terms are not dominant in the upper troposphere where the absolute vorticity is small. It is argued that greater nonlinearity relative to the mean rotational flow when the roughness length is large causes the transition of Rossby modes into the MJO. Connections to theories for the MJO which focus on the role of nonlinear Rossby wave dynamics are explored. Despite the strong influence on MJO variability, the statistics of the leading principal components of the velocity potential and the structure of the corresponding empirical orthogonal functions are insensitive to changes in the roughness length.