The Madden-Julian Oscillation and the West Pacific Warm Pool

The period of the Madden-Julian Oscillation (MJO) ranges from 30 to 60 days, which corresponds to a mean propagation speed around the earth of roughly 8-16 m/s. However, over the West Pacific warm pool the active convective phase moves to the east at approximately 5 m/s. The reason for this slowdown is not known, but may be due to the strong effect of convection on the MJO. In this talk, we propose a simpler hypothesis. The warm pool will certainly induce its own stationary circulation independent of the MJO, which is likely to take the form of a Gill-type response to an extended equatorial heat source. We hypothesize that the combination of circulations and the associated rainfall due to the MJO and the warm pool gives rise to a pattern of precipitation that propagates more slowly to the east across the warm pool than the global-average propagation speed of the MJO. There are likely to be non-linear interactions between the MJO and the warm pool circulation. However, we propose that a simple linear superposition of these circulations captures the fundamental mechanism of this retardation. We use the minimal model of Fuchs and Raymond to represent the MJO. The warm pool circulation is represented by the linearized flow response to the warm sea surface temperature anomaly. Both elements treat convection using a moisture mode parameterization with the wind induced surface heat exchange (WISHE) also acting. The results are compared with a recent alternative scheme by Adames and Kim.