This work was supported by National Science Foundation Grant ATM-9312888 and by National Oceanographic and Atmospheric Administration Grant GC95773. The author is grateful to Mark Cane for his detailed and constructive comments on the manuscript to bring this paper into shape. He enjoyed the stimulating discussions with Bin Wang and Roger Lukas. The author thanks Eli Tziperman and Edward Schneider for their critiques that led to the improvement of the paper, Thomas Schroeder and Diane Henderson for their careful reading and editing of the manuscript, and Xiaowei Sun for producing some of the figures. This is SOEST Contribution Number 4171.
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