Journal article

The equilibrium response to idealized thermal forcings in a comprehensive GCM: Implications for recent tropical expansion

Allen R, Sherwood S, Norris J, Zender C ...see all

Atmospheric Chemistry and Physics, vol. 12, issue 10 (2012) pp. 4795-4816

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Several recent studies have shown the width of the tropical belt has
increased over the last several decades. The mechanisms driving tropical
expansion are not well known and the recent expansion is underpredicted
by state-of-the art GCMs. We use the CAM3 GCM to investigate how
tropical width responds to idealized atmospheric heat sources, focusing
on zonal displacement of the tropospheric jets. The heat sources
include global and zonally restricted lower-tropospheric warmings
and stratospheric coolings, which coarsely represent possible impacts
of ozone or aerosol changes. Similar to prior studies with simplified
GCMs, we find that stratospheric cooling – particularly at high-latitudes
– shifts jets poleward and excites Northern and Southern Annular
Mode (NAM/SAM)-type responses. We also find, however, that modest
heating of the midlatitude boundary layer drives a similar response;
heating at high latitudes provokes a weaker, equatorward shift and
tropical heating produces no shift. Over 70 % of the variance in
annual mean jet displacements across 27 experiments is accounted
for by a newly proposed "Expansion Index", which compares mid-latitude
tropospheric warming to that at other latitudes. We find that previously
proposed factors, including tropopause height and tropospheric stability,
do not fully explain the results. Results suggest recently observed
tropical expansion could have been driven not only by stratospheric
cooling, but also by mid-latitude heating sources due for example
to ozone or aerosol changes.

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  • R. J. Allen

  • S. C. Sherwood

  • J. R. Norris

  • C. S. Zender

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