The author would like to thank Prof. J. M. Wallace and Dr. Roberta Quadrelli for helpful discussions. The author also thanks Dr. John Fyfe and two anonymous reviewers for their useful comments. Most of this work was conducted during a stay as visiting scientist at the Joint Institute for Study of Atmosphere and Ocean of the University of Washington. The numerical model experiments were conducted at the Institut de Recherches en Informatique Scientifique (IDRIS).
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