The authors thank David Karoly, Jorgen Frederiksen, and Kathy Harris for helpful discussions and comments, and Brian Hoskins and Jake Badger for advice on balancing the initial value perturbations. The comments of two anonymous reviewers helped to improve the manuscript. The OLR and NCEP–NCAR reanalysis data were provided through the NOAA Climate Diagnostics Center (http://www.cdc.noaa.gov). This research was supported in part through the Australian Government Cooperative Research Centres Program and in part by the Pan-American Climate Studies Program, under Project GC95-820, sponsored by NOAA’s Office of Global Programs.
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