The dynamics of the Madden-Julian Oscillation

The dynamics of the Madden-Julian Oscillation (MJO) are explored using a three-layer nonlinear shallow water numerical model with simple physical parameterizations. The model is formulated on an equatorial beta-plane in an atmosphere initially in radiative-convective equilibrium and at rest.

Observations suggest that the tropical atmosphere is only marginally unstable to moist convection. Accordingly, the convective parameterization in the model is based on the assumption of boundary layer quasi-equilibrium. The essential idea is that convective clouds consume the convective available potential energy (CAPE) at almost the same rate as it is generated by large-scale processes, such as radiative cooling, and the surface fluxes of latent and sensible heat. The convective parameterization allows for both shallow non-precipitating and deep precipitating clouds.

The convection is driven by a fixed, localized sea surface temperature anomaly, and the resulting development shows many of the characteristic features of the MJO. Shallow convection moistens the middle troposphere and provides favorable conditions for further deep convection. The developing stage is followed by radiative cooling and drying of the middle troposphere by precipitation, which in turn suppresses deep convection. These processes form the basis for modulating deep convection over the west Pacific warm pool and for generating the propagating disturbances which characterize the MJO.