We thank E. Roeckner and F.-F. Jin for helpful discussions, which stimulated this study, and three anonymous reviewers for their valuable comments that helped to improve the manuscript. All hindcast experiments were carried out on the Earth Simulator. The atmosphere model (ECHAM4) was provided by Max Planck Institute for Meteorology, Hamburg; the ocean model (OPA) by Laboratoire d'Ocèanographie Dynamique et Climatologie, Paris; and the coupler (OASIS) by CERFACS, Toulouse.
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