Numerical Simulations of the Madden and Julian Oscillation

Abstract

An integration of a general circulation model, with an ocean covered globe (or ?aqua planet?), exhibits disturbances that are similar to observed eastward propagating waves of period 30 to 60 days (which we refer to as the Madden and Julian oscillation). The structure of the disturbances resembles a Kelvin wave, although the speed of propagation is slower than anticipated from theory as applied to a dry atmosphere. However, a simple model of the tropical atmosphere demonstrates that the wave speed is sensitive to moisture effects. This notion is confirmed by two further general circulation model experiments in which the latent beat release is increased; in both cases the intrinsic speed of the wave is reduced in inverse proportion to the vertical gradient of equivalent potential temperature. The time-mean circulation of the basic aqua-planet integration exhibits some unusual features; for example a double Hadley cell, with wending branches displaced some 15° either side of the equate. Dynamical reasons for the maintenance of the aqua-planet circulations are discussed since these shed some light on the general circulation of the earth's atmosphere.