How the Occurrence of a Low-Level Moisture Anomaly Drives MJO Precipitation

Composite analysis of radiosonde data and weak temperature gradient (WTG) simulations of the Madden-Julian Oscillation strongly suggest that the associated precipitation anomaly is driven by the confluence of low-level moisture ahead of the active phase. This, in turn lowers the convective inhibition such that stochastic mechanical lifting can trigger convection. In contrast to convectively-coupled Kelvin waves, convective inhibition during the MJO cycle is modulated by the boundary layer moist entropy rather than by the free-tropospheric temperature. Composite MJO behavior at discrete locations spanning the Indian Ocean and West Pacific accord with observations from the DYNAMO field project in many respects. Cloud-resolving model simulations forced by thermodynamics of the composite MJO envelope show that a boundary layer moisture anomaly is a sufficient condition for MJO precipitation in the presence of interactive radiation.