AbstractMoisture mode instability is thought to occur in the tropical oceanic atmosphere when precipitation is a strongly increasing function of saturation fraction (precipitable water divided by saturated precipitable water) and when convection acts to increase the saturation fraction. A highly simplified model of the interaction between convection and large-scale flows in the tropics suggests that there are two types of convectively coupled disturbances: the moisture mode instability described above and another unstable mode dependent on fluctuations in the convective inhibition. The latter is associated with rapidly moving disturbances such as the equatorially coupled Kelvin wave.A toy aquaplanet beta-plane model with realistic sea surface temperatures produces a robust Madden–Julian oscillation–like disturbance that resembles the observed phenomenon in many ways. Convection in this model exhibits a strong dependence of precipitation on saturation fraction and does indeed act to increase this parameter in situations of weak environmental ventilation of disturbances, thus satisfying the criteria for moisture mode instability. In contrast, NCEP’s closely related Global Forecast System (GFS) and Climate Forecast System (CFS) models do not produce a realistic MJO. Investigation of moist entropy transport in NCEP’s final analysis (FNL), the data assimilation system feeding the GFS, indicates that convection tends to decrease the saturation fraction in these models, precluding moisture mode instability in most circumstances. Thus, evidence from a variety of sources suggests that the MJO is driven at least in part by moisture mode instability.