We would like to thank and H. Berbery, G. Cazes-Boezio, and K. Mo for helpful discussions, and the two reviewers whose comments led to a substantial improvement of the manuscript. Many of the computations were performed by G. Kerneis and A. Loireau, two summer student visitors from the French Ecole Polytechnique. This work was supported by NOAA Grants NA06GPO427 and NA06GPO511. The NCEP–NCAR reanalysis data were provided through the NOAA Climate Diagnostics Center (information online at http://www.cdc.noaa.gov/).
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