1. M. P. Baldwin is at NorthWest Research Associates, Bellevue, WA 98007, USA.
2. D. W. J. Thompson is in the Department of Atmospheric Science, Colorado State University, Fort Collins, CO 80523, USA.
3. E. F. Shuckburgh is in the Department of Applied Mathematics and Theoretical Physics, University of Cambridge, Cambridge CB3 0WA, UK.
4. W. A. Norton is in the Department of Atmospheric, Oceanic, and Planetary Physics, University of Oxford, Oxford OX1 3PU, UK.
5. N. P. Gillett is in the School of Earth and Ocean Sciences, University of Victoria, Victoria, BC V8W 3P6, Canada.