This research was supported by both the National Science Foundation through Grant ATM-9712834 and the National Aeronautics and Space Administration through Grant No. NAG5-6207. I would like to thank both Dr. Sukyoung Lee and an anonymous reviewer for their helpful comments on this paper. I would also like to thank the NOAA Climate Diagnostics Center for providing me with the NCEP–NCAR reanalysis dataset.
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