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  • Lu, J., G. Chen, D. Frierson, 2008: Response of the zonal mean atmospheric circulation to El Nino versus global warming. J. Climate. In press.

The change in the zonal mean atmospheric circulation under global warming is studied in
comparison with the response to El Nino forcing, by examining the model simulations conducted for
the Fourth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC). In contrast
to the strengthening and contraction of the Hadley cell and the equatorward shift of the
tropospheric zonal jets in response to El Nino, the Hadley cell weakens and expands poleward, and
the jets move poleward under a warmed climate, despite the ~SEl Nino-like~T enhanced warming over
equatorial central and eastern Pacific.
Two feasible mechanisms are proposed for the zonal mean circulation response to global warming:
(1) The increase in static stability of the subtropical and mid-latitude troposphere, a result of
the quasi-moist adiabatic adjustment to the surface warming, tends to stabilize the eddy growth
on the equatorward side of the storm track and push the eddy activity and the associated
eddy-driven wind and subsidence poleward, leading to the poleward expansion of the Hadley cell
and the shift of jet; (2) the strengthening of the mid-latitude wind at the upper-troposphere and
lower-stratosphere, arguably a consequence of the rise in the height of the tropopause and the
associated increase in the meridional temperature gradient, can increase the phase speed of the
eddies emanating from the mid-latitudes, and thus the critical latitudes (where the eddy phase
speed matches the background zonal wind, and where the eddies break and extract angular momentum
from the thermally driven wind) displace poleward and carry the eddy-driven circulation with it.
Both mechanisms are somewhat, if not completely, distinct from those in response to the El Nino
condition. The hydrological impacts of global warming also exhibit distinct patterns over the
subtropics and mid-latitudes in comparison to the El Nino.
Since both mechanisms do not essentially depend on the details of the SST warming in the
equatorial oceans and the variety of model physical parameterizations, these extratropical
responses to global warming may be of increased credence due to the fact that they are
dynamically determined.

Full Article: http://www.cgd.ucar.edu/cas/Staff/Jianlu/lcf08_final.pdf

Last Updated: 2008-08-25

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