Dynamic core of the MJO

Abstract:

Substantial progress has been made in understanding the dynamics of the MJO during the past decades. However, there is no consensus on the underlying mechanism for the MJO. Most current ideas of MJO dynamics involves atmospheric deep convection and its interaction with moisture. These theories explain the eastward propagation of the MJO through moisture enhancement to the east of large-scale convection. Here, using a simple analytical approach with equatorial shallow water equations, we revisit the view on the MJO as slowed-down Kelvin waves due to damping. We show that the dynamic core of the MJO can be described in terms of a harmonic oscillator that can be excited by stochastic forcing. The mechanism for selecting MJO scales comes from momentum damping, and the mode propagates eastward because of exactly the same reason for Kelvin waves to propagate eastward due to the Coriolis effect. Our unified theory of fast Kelvin waves at synoptic scales and the slow MJO at planetary scales supports the notion of a distinction and continuous transition between equatorial Kelvin waves and the MJO. We also discuss the role of feedback mechanisms that help sustain an MJO event longer.