Water vapor and the dynamics of climate changes
Water vapor is not only Earth's dominant greenhouse gas. Through the release of latent heat when it condenses, it also plays an active role in dynamic processes that shape the global circulation of the atmosphere and thus climate. Here we present an overview of how latent heat release affects atmosphere dynamics in a broad range of climates, ranging from extremely cold to extremely warm. Contrary to widely held beliefs, atmospheric circulation statistics can change nonmonotonically with global-mean surface temperature, in part because of dynamic effects of water vapor. For example, the strengths of the tropical Hadley circulation and of zonally asymmetric tropical circulations, as well as the kinetic energy of extratropical baroclinic eddies, can be lower than they presently are both in much warmer climates and in much colder climates. We discuss how latent heat release is implicated in such circulation changes, particularly through its effect on the atmospheric static stability, and we illustrate the circulation changes through simulations with an idealized general circulation model. This allows us to explore a continuum of climates, to constrain macroscopic laws governing this climatic continuum, and to place past and possible future climate changes in a broader context.
© 2010 American Geophysical Union. Received 29 June 2009; accepted 3 December 2009; published 2 July 2010. We are grateful for support by the Davidow Discovery Fund, the National Science Foundation (grant ATM‐0450059), and a David and Lucile Packard Fellowship. The simulations shown were performed on Caltech's Division of Geological and Planetary Sciences Dell cluster. Portions of section 2 were presented previously by Schneider and O'Gorman  at the 15th 'Aha Huliko'a Hawaiian Winter Workshop. We thank Ian Eisenman, Yohai Kaspi, Tim Merlis, Steven Sherwood, and two reviewers for helpful comments on a draft of this paper. The Editor responsible for this paper was Gerald North. He thanks Robert Korty and an additional anonymous reviewer.
Published - Schneider2010p10860Rev_Geophys.pdf