The Mechanism of the Madden-Julian Oscillation: A New Perspective
Prof. Geoffrey Vallis
University of Exeter, UK
I describe a mechanism whereby convective aggregation and eastward propagating equatorial disturbances, similar to the Madden--Julian oscillation, arise. We construct an idealized mode consisting only of the shallow water equations plus a humidity variable. For a broad range of parameters the system is ‘excitable' and self-sustaining, even if linearly stable, with condensation producing convergence and gravity waves that, acting together, trigger more condensation. On the equatorial beta-plane the convection first aggregates near the equator, generating patterns related to those in the Matsuno--Gill problem. However, the pattern is unsteady and more convection is triggered on its eastern edge, leading to a precipitating disturbance that progresses eastward.