Stability of the Walker Circulation

Abstract

A stability analysis is made for the east-west Walker circulation in the tropics by means of eigenfrequency calculations of a small perturbation. The characteristic equations are formulated based on the assumption that the vertical scale of the perturbation is small compared to the depth of the basic Walker circulation, whereby the perturbation analysis is applied for a middle layer of the Walker circulation using the equatorial ?-plane approximation. The analysis result shows that the Walker circulation is unstable against the small perturbation when the basic zonal mean wind is weak easterly, at which the Doppler-shifted frequencies of the mixed Rossby-gravity and the inertial-gravity modes in the wave perturbation both match closely the frequency of inertial gravitational oscillation of the zonal mean perturbation. The e-folding time of the perturbation was found to he ?1?2 weeks for the typical moan Walker circulation.