The Walker Circulation with Observed Zonal Winds, a Mean Hadley Cell, and Cumulus Friction

Abstract

The term Walker Circulation is used to refer to the zonal overturning across the equatorial Pacific driven by enhanced convection over the Indonesian region. In this work, an attempt is made to simulate the Walker Circulation using a linear model that includes a cumulus friction parameterization. The work of Geisler is extended by including a realistic mean zonal wind field obtained from the FGGE dataset and a prescribed mean Hadley cell that is computed from an analytical streamfunction. The model is forced by a stationary tropical heat source. The sensitivity of the model circulation to changes in the basic state is examined. Model results show that the inclusion of a nonzero mean zonal wind field tends to enhance the extratropical response in the winter hemisphere. Including a cumulus friction parameterization tends to damp the zonal wind response near the heating center and also lower the level of zero zonal wind in the model Walker Circulation. Including a mean Hadley cell in the basic state has the greatest effect on the model circulation in the tropics. It acts to raise the level of zero wind which makes the model circulation better resemble the observed Walker Circulation. Advection by the mean vertical velocity field is found to be a major term in the u-momentum equation and is of opposite sign from the largest cumulus friction term. Results indicate that when cumulus friction is included in a linear model calculation, a mean vertical velocity field should also be included. When the effects of the zonal mean winds and the Hadley Cell/cumulus friction terms are included the model response resembles the observed tropical and subtropical responses to the El Niño ocean temperature anomaly.