Gabriel A Vecchi1, Thomas L Delworth2, Ben Booth3. 1. Department of Geosciences and the Princeton Environmental Institute, Princeton University, Princeton, New Jersey 08544, USA. 2. National Oceanic and Atmospheric Administration's Geophysical Fluid Dynamics Laboratory, Princeton, New Jersey 08542, USA. 3. Met Office Hadley Centre, Exeter EX1 3PB, UK.