In simulations of 21st-century climate change, the tropical Hadley
circulation expands and weakens as the global mean surface temperature
increases. However, the reasons for such changes in the Hadley
circulation are not completely understood.
With simulations with an idealized GCM, we investigate dynamical mechanisms responsible for changes in the Hadley circulation over a wide range of climates, spanning those that are likely to occur in the future and some that may have occurred in the past. The GCM includes a hydrologic cycle and is coupled to a simple ocean model that accounts for dynamically varying ocean heat transport in low latitudes. We generate climate changes in the GCM by varying the optical thickness of an idealized longwave absorber. The Hadley circulation contracts weakly as the optical thickness and, with it, the global-mean surface temperature increases. The response of the strength of the Hadley circulation is more complex and non-monotonic: The strength of the Hadley circulation is maximal for a climate similar to that of present-day Earth and is smaller for much colder and much warmer climates.
Large-scale eddy fluxes of midlatitude origin are found to influence the strength of the Hadley circulation over the range of climates simulated; however, their relative importance decreases as the climate warms and the Hadley circulation begins to respond more directly to changes in thermal driving. We discuss the effects of latent heat release on the energy balance of the tropical atmosphere and on the strength of the Hadley circulation. The mechanisms responsible for changes in the width of the Hadley circulation are currently unclear.
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