We thank J. M. Wallace for his insightful comments and his suggestion of the EPWI time series that we use in this study. We also thank T. Birner, E. D. Maloney, W. J. Randel, R. Ueyama, and two anonymous reviewers for helpful comments and suggestions during the preparation of this manuscript. KMG and DWJT were supported by National Science Foundation Contract AGS-0936255.
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