Updating the paradigm of Convectively Coupled Equatorial Waves and the MJO

Most discrepancies between observations and the linear Matsuno-Gill theory of tropical waves are currently attributed to the neglect of coupling between the waves and moist tropical convection. Although these quantities are undoubtedly related, the extent to which this reflects true coupling versus forcing of one by the other has not been adequately established. Our new theoretical and observational analysis indicates that the neglect of spatial base state variations is the primary limitation of the Matsuno-Gill theory, whereas the neglect of the convective coupling is of secondary importance. This suggests an updated paradigm in which models that include both effects, but are still linear, would continue to provide a useful framework for interpreting tropical variations. Such models would retain non-dispersive Generalized Kelvin waves, and also zonally symmetric oscillations, as important eigenmodes of base states with meridional and vertical structure.

The observational analysis is based on Linear Inverse Modeling (LIM), which deduces the linear evolution operator for tropical anomalies using the time-lag covariances and cross-covariances of the circulation and humidity fields in the ERA-Interim dataset. The eigenmodes of this operator are highly seasonally dependent, consistent with the strong modification of the wave dynamics by the seasonally varying base state. The eigenmodes are also not mutually orthogonal, as they are in the Matsuno-Gill or CCEW paradigm, and this is important for the predictable growth and decay of anomalies. We also show that although the circulation-humidity coupling is overall of secondary importance, it does significantly affect the evolution of some modes and the MJO.