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A Linear Model of the Walker Circulation

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  • 1 Rosenstiel School of Marine and Atmospheric Sciences, University of Miami, Miami, FL 33149
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Abstract

In this paper we present results of a model study of the Walker Circulation defined as the steady response of a stratified atmosphere to an isolated equatorial heat source. The model equations are linearized with respect to a basic state of no motion and constant static stability. Cumulus friction provides the momentum damping. In the absence of cumulus friction, the center of the overturning in the equatorial plane is near 500 mb. With the addition of cumulus friction in an amount consistent with a zonally averaged precipitation of 2 m year−1, the center is lowered to 600 mb. Cumulus friction in this amount also seems to provide reasonable amplitudes in the response. It is found that the branch of the overturning in the equatorial plane east of the heating is weaker than and has a larger longitudinal scale than the branch west of the heating. This asymmetry becomes more pronounced with a narrowing of the longitudinal width of the heat source. The mass flux in meridional overturnings is found to be comparable to that in the zonal overturnings. The pair of meridional cells to the west of the center of the heat source rotate in the same sense as the Hadley circulation, while the pair to the east of the center of the heat source rotate in the sense opposite to the Hadley circulation. It is suggested that the superposition of the eastern pair of cells and the Hadley cell should produce zones of low-level convergence parallel to the equator over the Pacific.

Abstract

In this paper we present results of a model study of the Walker Circulation defined as the steady response of a stratified atmosphere to an isolated equatorial heat source. The model equations are linearized with respect to a basic state of no motion and constant static stability. Cumulus friction provides the momentum damping. In the absence of cumulus friction, the center of the overturning in the equatorial plane is near 500 mb. With the addition of cumulus friction in an amount consistent with a zonally averaged precipitation of 2 m year−1, the center is lowered to 600 mb. Cumulus friction in this amount also seems to provide reasonable amplitudes in the response. It is found that the branch of the overturning in the equatorial plane east of the heating is weaker than and has a larger longitudinal scale than the branch west of the heating. This asymmetry becomes more pronounced with a narrowing of the longitudinal width of the heat source. The mass flux in meridional overturnings is found to be comparable to that in the zonal overturnings. The pair of meridional cells to the west of the center of the heat source rotate in the same sense as the Hadley circulation, while the pair to the east of the center of the heat source rotate in the sense opposite to the Hadley circulation. It is suggested that the superposition of the eastern pair of cells and the Hadley cell should produce zones of low-level convergence parallel to the equator over the Pacific.

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