We are grateful to Alan O’Neill, Lesley Gray, Paul Kushner, Marvin Geller, Darryn Waugh, Bill Randel, and Gavin Esler for useful suggestions, and particularly to Matthew Wittman for substantial contributions to the discussion, ideas, and techniques developed in this study. The NCEP––NCAR reanalyses were obtained from the IRI Ingrid data server at the Lamont-Doherty Earth Observatory of Columbia University. The ERA-40 data were obtained from the ECMWF data server. This research was funded, in part, by an award to the Cooperative Institute for Climate Applications and Research (CICAR) from the U.S. National Oceanic and Atmospheric Administration and, in part, from a grant to Columbia University from the U.S. National Science Foundation.
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