The authors are grateful to the late Y. Hayashi, B. Mapes, B. Wang, P. J. Webster, and M. Wheeler for their interest in this work and their useful comments. They also benefited from comments from R. H. Johnson and an anonymous reviewer and discussions with their colleagues, A. Arakawa, C. Lin, J. W.-B. Lin, J. D. Neelin, and N. Zeng. They also thank J. K. Roskovensky for his careful review of the manuscript. This work was supported by the National Oceanic and Atmospheric Administration under Grant NA66GP0218 and by the National Science Foundation under Grants ATM-9500338 and ATM-9902838.
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