Is the Madden-Julian Oscillation a Moisture Mode?

The governing thermodynamics of the Madden-Julian Oscillation (MJO) is examined using sounding and reanalysis data. On the basis of four objective criteria, results suggest that the MJO behaves like a moisture mode-a system whose thermodynamics is governed by moisture-only over the Indian Ocean. Over this basin, the MJO shows a slow convective adjustment timescale, its zonal scale is smaller, and it exhibits slow propagation, allowing moisture modes to exist. Elsewhere, the faster-propagating wavenumber 1-2 components are more prominent preventing weak temperature gradient (WTG) balance to be established. As a result, temperature and moisture play similar roles in the MJO's thermodynamics outside the Indian Ocean. Plain Language Summary The Madden-Julian Oscillation is one of the most important phenomena that occur at the subseasonal to seasonal timescale and is a source of weather predictability at this timescale. Despite its importance, many features of Madden-Julian Oscillation (MJO) remain elusive, and many theories have been proposed to understand its behavior. Arguably, the most recent popular MJO theory is the moisture mode theory. The theory posits that moisture governs the evolution of the MJO. Here, we show that this theory is applicable only over the Indian Ocean, where MJO's small zonal scale and slow propagation allow moisture modes to exist at the scale of the MJO. Elsewhere, temperature fluctuations in the MJO become as important as moisture, a feature that is inconsistent with the moisture mode theory.