JP1.17
Response of the Hadley Circulation to Climate Change in an Aquaplanet GCM Coupled to Ocean Heat Transport
Xavier J. Levine, Caltech, Pasadena, CA; and T. Schneider
Simulations of climate change scenarios predict a weakening and
poleward expansion of the Hadley circulation for the 21st
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.
Joint Poster Session 1, Theory
Monday, 8 June 2009, 4:30 PM-6:00 PM, Stowe Room
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