1. Climate Research Division, Scripps Institution of Oceanography, 0224, La Jolla, CA 92037, USA.
2. Program for Climate Model Diagnoses and Intercomparison/Lawrence Livermore National Laboratory, Post Office Box 808, Livermore, CA 94550, USA.
3. UK Met Office Hadley Centre and University of Reading, Reading RG6 6BB, UK.
4. National Center for Atmospheric Research, Post Office Box 3000, Boulder, CO 80307, USA.