Response of the Hadley Circulation to Climate Change in an Aquaplanet GCM Coupled to Ocean Heat Transport

Simulations of climate change scenarios predict a weakening and

poleward expansion of the Hadley circulation for the 21^st

century. Why these changes in the Hadley circulation occur is not

well understood. Both thermodynamic changes in low latitudes and

changes in eddy fluxes in the subtropics are likely to play a

role. Here we use an idealized atmosphere GCM coupled to an

aquaplanet surface with a simple representation of ocean heat

transport to investigate mechanisms responsible for changes in the

Hadley circulation. The atmosphere absorbs radiation like a gray

body; we vary the climate by varying the optical thickness of the

longwave absorber to represent varying greenhouse gas

concentrations. The ocean heat transport in the model is confined to

low latitudes and responds to changes in the temperature and wind

stress at the surface. Having a representation of low-latitude ocean

heat transport is critical to obtain a Hadley circulation that

resembles Earth's in the present climate and to obtain dynamically

consistent responses of the Hadley circulation to climate changes.

In the GCM simulations, the strength of the Hadley circulation

changes non-monotonically with surface temperature. The strength

peaks and is relatively constant over a broad range of climates with

global-mean surface temperatures between about 285º K and

300º K, that is, including climates resembling the

present. The strength is lower in much colder and much warmer

climates. Eddy momentum fluxes strongly influence the strength of

the Hadley circulation, peaking in strength at a climate with a

global-mean surface temperature of about 280º K. On the other

hand, the relative impact of the eddies compared with the mean flow

in advecting angular momentum steadily decreases with increasing

surface temperature, with very warm climates being characterized by

close to angular momentum-conserving upper branches of the Hadley

circulation. The width of the Hadley circulation does not change

substantially as the climate warms relative to the present, but it

decreases as the climate cools substantially. Theoretical arguments

are proposed to explain some of the behavior seen in the simulations.