Investigations of the Convectively Coupled Equatorial Waves and the Madden-Julian Oscillation

Abstract

The Madden-Julian Oscillation (MJO) and the Convectively Coupled Equatorial Waves (CCEW) are coherent structures of convection and various large-scale fields. These phenomena are not well understood, despite their importance to the tropical climate. A toy model of the CCEW consisting of a pair of shallow water wave modes coupled by a simple convective parameterization is considered. The linear behavior of the system is analyzed, showing a growth spectrum that is similar to the spectrum that is observed. To explore the processes involved in propagation and maintenance of the MJO disturbance, we analyze the MSE budget of the disturbance within a numerical model. In an idealized experiment, the column-integrated long-wave heating is the only significant source of column-integrated MSE acting to maintain the MJO-like anomaly balanced against the combination of column-integrated horizontal and vertical advection of MSE and Latent Heat Flux. Eastward propagation of the MJO-like disturbance is associated with MSE generated by both column-integrated horizontal and vertical advection of MSE, with the column long-wave heating generating MSE that retards the propagation. The contribution to the eastward propagation by the column-integrated horizontal advection term is dominated by meridional advection of MSE by anomalous synoptic eddies caused by the suppression of eddy activity ahead of the MJO convection. This suppression is linked to the barotropic conversion mechanism; with the gradients of the low frequency wind experienced by the synoptic eddies within the MJO envelope acting to modulate the Eddy Kinetic Energy. The meridional eddy advection’s contribution to poleward propagation is dominated by the mean state’s (meridionally varying) eddy activity acting on the anomalous MSE gradients associated with the MJO. In a follow-up experiment, the variations in the propagation speed of MJO with variations in the imposed SST distribution are seen to be driven by the variations in meridional advection of the mean MSE profile by the MJO-related winds, which are in turn dominated by the variations in the mean MSE profile due to the variations of the SST. A brief investigation of the MSE budget for a more realistic case shows an increase in the MSE sink due to meridional advection as the MJO progresses from genesis over the Indian Ocean to decay in the central Pacific. The increase in this sink appears to be the cause of MJO’s demise.