THE LIFE-CYCLE OF THE MADDEN-JULIAN OSCILLATION

A composite life cycle of the Madden-Julian oscillation (MJO) is const ructed from the cross covariance between outgoing longwave radiation ( OLR), wind, and temperature. To focus on the role of convection, the c omposite is based on episodes when a discrete signal in OLR is present . The composite convective anomaly possesses a predominantly zonal wav enumber 2 structure that is confined to the eastern hemisphere. There, it propagates eastward at about 5 m s-1 and evolves through a systema tic cycle of amplification and decay. Unlike the convective anomaly, t he circulation anomaly is not confined to the eastern hemisphere. The circulation anomaly displays characteristics of both a forced response , coupled to the convective anomaly as it propagates across the easter n hemisphere, and a radiating response, which propagates away from the convective anomaly into the western hemisphere at about 10 m s-1. The forced response appears as a coupled Rossby-Kelvin wave while the rad iating response displays predominantly Kelvin wave features. When it i s amplifying, the convective anomaly is positively correlated to the t emperature perturbation, which implies production of eddy available po tential energy (EAPE). A similar correlation between upper-tropospheri c divergence and temperature implies conversion of EAPE to eddy kineti c energy during this time. When it is decaying, temperature has shifte d nearly into quadrature with convection, so their correlation and pro duction of EAPE are then small. The same correspondence to the amplifi cation and decay of the disturbance is mirrored in the phase relations hip between surface convergence and anomalous convection. The correspo ndence of surface convergence to the amplification and decay of the co nvective anomaly suggests that frictional wave-CISK plays a key role i n generating the MJO.