This work was supported by grants from the NOAA Office of Global Programs to DSB and ESS and to the Hayes Center. JJB was supported by a grant from the Climate Dynamics Division of the National Science Foundation (ATM-9313383). We thank the reviewers whose comments make this paper a large improvement over the first manuscript. The work described in this paper began as a superproblem for the 1993 Friday Harbor summer school, organized by Professor Peter Rhines. Our thanks, therefore, to Professor Rhines for facilitating the early stages of this work.
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