The L19 and L30 AMIPII GCM integrations were performed at the Hadley Centre by Alison Pamment. Figure 1 was provided by Alison Pamment. The authors thank two anonymous reviewers for comments which led to improvements in the text. P. Inness, J. Slingo, S. Woolnough and R. Neale acknowledge support through the NERC-funded UK Universities Global Atmospheric Modelling Programme.