We are grateful to James Holton, Dennis Hartmann, Walter Robinson, and Murry Salby for their comments on an early draft of this paper. We thank Mark Schoeberl and two anonymous reviewers for their constructive suggestions, Mark Baldwin for providing us with the data of the NAM index, and Katie Coughlin for helpful discussion. NCEP reanalysis data is obtained from the NOAA–CIRES Climate Diagnostics Center, Boulder, Colorado, from their Web site at http://www.cdc.noaa.gov/. This work is supported by the National Science Foundation, Division of Atmospheric Sciences, Climate Dynamics, under Grant ATM 9813770.
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