The Co-Dependence of Moist Convection and the MJO

Dynamically, moist convection and the MJO are inseparable. Without the large-scale circulation of the MJO, widespread deep convection could not develop. However, without shallower convection to first moisten the troposphere, the circulation pattern of the MJO cannot persist. The relationship between cloud moistening and MJO convective onset was proposed a few years prior to TOGA COARE, during which such a relationship was observed. About 20 years later during DYNAMO, such a relationship was confirmed over the Indian Ocean as well. When column integrated relative humidity reaches 60–70% after a period of moistening driven by cloud detrainment and evaporation, widespread deep convection can form more easily, and an active MJO can occur.

Three modes of convection are observed over the course of an MJO suppressed and active phase pair: shallow boundary layer clouds, moderately deep congestus clouds, and then deep convection, which often takes on the form of mesoscale convective systems (MCSs). The ability of convection to more frequently grow upscale into MCSs during an active MJO is important to the maintenance and propagation of the MJO. First, MCSs generate diabatic heating through a deep layer and over a wide area. Latent heat release occurs in narrow convective updrafts and more broadly within stratiform regions. In the upper portions of MCSs, the latent heating is reinforced by radiative heating in stratiform and non-precipitating anvil regions. The resulting deep heating profile is important for producing the Gill-type circulation response that is observed during a mature, convectively active MJO event. Second, MCSs produce regions of anvil clouds that do not precipitate (or do so only very lightly) but extend laterally for hundreds of kilometers or more from precipitating regions. Such large cloud regions produce a cloud-radiative feedback, the magnitude of which is important for determining the propagation speed of the MJO convective envelope when it is present over the warm pool.