1. School of Meteorology, University of Oklahoma, Norman, OK 73019, USA.
2. School of Mathematical Sciences, Monash University, Clayton, Victoria 3800, Australia.
3. Hadley Centre, MetOffice, Bracknell RG12 2SY, UK.
4. National Center for Atmospheric Research (NCAR), Boulder, CO 80307, USA.
5. Geophysical Fluid Dynamics Laboratory, National Oceanic and Atmospheric Administration, Princeton, NJ 08542, USA.