Moisture Mode Theory's Contribution to Advances in our Understanding of the Madden-Julian Oscillation and Other Tropical Disturbances

Purpose of Review Our understanding of the Madden-Julian Oscillation (MJO) and other tropical motion systems has significantly improved in recent years. This article reviews the contribution of moisture mode theory to this progress. Recent Findings Two realizations have contributed significantly to our understanding of the MJO: (1) Free tropospheric water vapor plays an important role in the occurrence and organization of tropical deep convection. (2) The latent heat released in convection is quickly transported around the tropics by gravity waves, the physical mechanism underpinning the weak temperature gradient (WTG) approximation. Simple models of the tropics that include (1) and (2) revealed the existence of moisture modes, waves in which water vapor plays a dominant role in their evolution. It was soon recognized that the MJO exhibits properties of moisture modes. The ensuing development and application of the so-called moisture mode theory of the MJO have led to the recognition that horizontal and vertical moisture advections are central to the propagation of the MJO, and that cloud-radiative heating is at least partially responsible for its maintenance. Moisture mode theory has also been applied to understand the MJO's seasonality, Maritime Continent transit, and response to increasing CO2. Recent work suggests that moisture mode theory can be extended beyond the MJO in order to explain the observed diversity of tropical motion systems. A mounting body of evidence indicates that the MJO has properties of moisture modes. Extension of the theory beyond the MJO may help us further understand the processes that drive large-scale tropical circulations.