This work has benefited from discussions with David Andrews, Lesley Gray, Steven Hardiman, Paul Kushner, Adam Monahan, and Ted Shepherd. We especially thank Jahanshah Davoudi for many helpful and stimulating discussions. Two anonymous reviewers provided helpful comments. TB is funded in part through the Natural Sciences and Engineering Research Council and the Canadian Foundation for Climate and Atmospheric Sciences. PDW is funded through a Fellowship from the UK Natural Environment Research Council (reference: NE/D009138/1) and received a Rupert Ford Travel Award from the Royal Meteorological Society to support this study.
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