The instabilities produced by a linear model of the tropical atmosphere coupled to a prognostic equation for water vapour are investigated. For parameter regimes relevant to the Indo-Pacific warm pool, the long-time asymptotic behaviour of the unstable waves is found to be absolutely unstable, so that the amplitude of disturbances will grow in time at every point in the domain. Other limits of the system do not produce this same behaviour at these length and time scales. It is shown that the resultant long-time behaviour of the instability is characterized by roughly equal roles for temperature and moisture fluctuations in setting the thermodynamic tendency of the waves. Under the assumption of a zonally varying flow, it is shown analytically that localized regions of instability may be formed, again using parameter choices relevant to the warm pool. The dynamics and thermodynamics of these local instabilities show some correspondence with the observed development of the Madden-Julian Oscillation as it propagates through the warm pool.
The tropical intraseasonal variability in an idealized moist general circulation model (GCM) which has a simple moist convection scheme and realistic radiative transfer, but no parameterization of cloud processes is investigated. In a zonally symmetric aquaplanet state, variability is dominated by westward-propagating Rossby waves. Enforcing zonal asymmetry through the application of a prescribed heat flux in the slab ocean bottom boundary leads to the development of a slow, eastward propagating mode which bears some of the characteristics of the observed Madden-Julian Oscillation (MJO). When the ocean heat flux is made stronger, high frequency Kelvin waves exist alongside the MJO mode. The spatial distribution of precipitation anomalies in the disturbances most resemble the MJO when very shallow slab ocean depths (1 m) are used, but the mode still exists at deeper slabs. Sensitivity experiments to the parameters of the convection scheme suggest that the simulated MJO mode couples to convection in a way that is distinct from both Kelvin and Rossby waves generated by the model. Analysis of the column moist static energy (CMSE) budget of the MJO mode suggests that radiative heating plays only a weak role in destabilizing the mode, in contrast to many previous idealized modelling studies of the MJO. Instead, the CMSE budget highlights the importance of the lifecycle of vertical advection for the destabilization and propagation of the MJO. Synergies between the generated MJO mode and linear theories of the MJO are discussed as well.