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The dynamics of variability in the zonal index are studied using a simple circulation model. Results from simulations with differing drag suggest that variability in the zonal index involves a positive feedback from transient eddies but that this feedback, which involves the action of surface drag in increasing the baroclinicity of zonal flows, operates only in the presence of strong drag. Model experiments are conducted in which the barotropic component of the zonal flow is held fixed, so that eddy fluxes of heat and momentum cannot be organized by variability in the barotropic zonal flow. The eddy fluxes in these “FBU” experiments do not, for strong drag, display nearly the same persistence as do the fluxes in the control runs. These results clearly demonstrate the presence of a positive eddy feedback for the zonal index but show also that the existence of this feedback is parameter dependent.
Walter A. Robinson (Sun,) studied this question.