The onset of convection in the Madden-Julian oscillation

An observational study of the onset of convection in the Madden- Julian osc illation (MJO) was performed. Composites of radiosonde data from the Compre hensive Aerological Reference Data Set were constructed for an ensemble of tropical stations located in the Indian Ocean, Maritime Continent, and west ern Pacific Ocean. The composites suggest that for the off- equatorial stations used in this s tudy, the MJO period may be set by the buildup and discharge of the low- le vel moist static energy. This result supports the discharge- recharge hypot hesis of Blade' and Hartmann. MJO events appear to begin when the off- equa torial atmosphere has been destabilized through a combination of low- level moist static energy buildup and concurrent drying of the middle tropospher e by subsidence in the wake of the previous cycle of MJO convection. The lo w- level moist static energy buildup is controlled by a corresponding incre ase in low- level moisture. The increase in low- level moisture is not caus ed by the 1000- mb convergence. For the stations examined here, the converg ence lags the moist static energy buildup and is instead in phase with the convection.