The authors thank Jim McWilliams and Jeff Whittaker for their insights into the subtleties of balance. DJM wishes to acknowledge the meteorological and social hospitality of the MMM group at NCAR Boulder over the past three summers. Additional thanks from DJM to the 1997 Woods Hole GFD Summer School for encouragement and many discussions related to the presentation of this work, especially Ed Spiegel, Rupert Ford, Joe Pedlosky, Rick Salmon, and Jack Whitehead. In this research, DJM has been supported by NSF DMR-9404374, DOE DE-FG02-88ER25053, and the Alfred P. Sloan Foundation.
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