Response of the Hadley Circulation to Climate Change in an Aquaplanet GCM Coupled to a Simple Representation of Ocean Heat Transport
It is unclear how the width and strength of the Hadley circulation are controlled and how they respond to climate changes. Simulations of global warming scenarios with comprehensive climate models suggest the Hadley circulation may widen and weaken as the climate warms. But these changes are not quantitatively consistent among models, and how they come about is not understood. Here, a wide range of climates is simulated with an idealized moist general circulation model (GCM) coupled to a simple representation of ocean heat transport, in order to place past and possible future changes in the Hadley circulation into a broader context and to investigate the mechanisms responsible for them. By comparison of simulations with and without ocean heat transport, it is shown that it is essential to take low-latitude ocean heat transport and its coupling to wind stress into account to obtain Hadley circulations in a dynamical regime resembling Earth's, particularly in climates resembling present-day Earth's and colder. As the optical thickness of an idealized longwave absorber in the simulations is increased and the climate warms, the Hadley circulation strengthens in colder climates and weakens in warmer climates; it has maximum strength in a climate close to present-day Earth's. In climates resembling present-day Earth's and colder, the Hadley circulation strength is largely controlled by the divergence of angular momentum fluxes associated with eddies of midlatitude origin; the latter scale with the mean available potential energy in midlatitudes. The importance of these eddy momentum fluxes for the Hadley circulation strength gradually diminishes as the climate warms. The Hadley circulation generally widens as the climate warms, but at a modest rate that depends sensitively on how it is determined.
© 2011 American Meteorological Society. Manuscript received 18 May 2010, in final form 5 November 2010). We thank Paul O'Gorman for performing the simulations without ocean heat transport and for helpful comments and discussions. We are grateful for support by the National Science Foundation (Grants ATM-0450059 and AGS-1019211), the Davidow Discovery Fund, and a David and Lucile Packard Fellowship. The simulations were performed on the Division of Geological and Planetary Sciences's Dell cluster at the California Institute of Technology. The program code for the simulations described in this paper, and the simulation results themselves, are available from the authors upon request.
Published - Levine2011p13662J_Atmos_Sci.pdf