The Onset of Convection in the Madden–Julian Oscillation

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Abstract
An observational study of the onset of convection in the Madden–Julian oscillation (MJO) was performed. Composites of radiosonde data from the Comprehensive Aerological Reference Data Set were constructed for an ensemble of tropical stations located in the Indian Ocean, Maritime Continent, and western Pacific Ocean. The composites suggest that for the off-equatorial stations used in this study, the MJO period may be set by the buildup and discharge of the low-level moist static energy. This result supports the discharge–recharge hypothesis of Bladé and Hartmann. MJO events appear to begin when the off-equatorial atmosphere has been destabilized through a combination of low-level moist static energy buildup and concurrent drying of the middle troposphere by subsidence in the wake of the previous cycle of MJO convection. The low-level moist static energy buildup is controlled by a corresponding increase in low-level moisture. The increase in low-level moisture is not caused by the 1000-mb convergence. For the stations examined here, the convergence lags the moist static energy buildup and is instead in phase with the convection.

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