Weakened QBO Due to Stronger Mean Tropical Upwelling

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Weakened QBO Due to Stronger Mean Tropical
Upwelling
Y. Kawatani1 and K. Hamilton2 1JAMSTEC
Research Institute for Global Change,
2IPRC Nature, 497, 478-481 (2013)
Zonal wind QBO amplitude at 70 hPa near the
equator. Top Observed values from radiosonde
winds during 19532012. Bottom Results from the
MPI-ESM-MR global coupled model driven with
observed concentrations of greenhouse gases from
1900 to 2005 and then with projected increases
through 2100.
The quasi-biennial oscillation (QBO) dominates
the circulation in the tropical stratosphere and
impacts the large-scale seasonal mean climate in
both the stratosphere and the troposphere.
Evidence for long-term trends in the QBO have
until now been equivocal, raising questions about
the extent of stratospheric circulation changes
in a global warming context. A new analysis of
near-equatorial radiosonde observations for
19532012 reveals a long-term decrease in the
zonal wind QBO amplitudes, which is particularly
notable at the 70 hPa level where the QBO
amplitudes dropped by roughly one-third over the
period. This trend is also apparent in the global
warming simulations of the four models in CMIP5
that realistically simulate the QBO. In the
lowermost stratosphere, the vertical structure of
the QBO is strongly linked to the mean upwelling,
which itself is a key factor in determining
stratospheric composition. The reduced QBO
amplitudes are likely due to stronger mean flow
of air from the tropical troposphere into the
stratosphere.
Timeheight section of the amplitude of the
simulated equatorial QBO in four global coupled
models run with observed concentrations of
greenhouse gases from 1900 to 2005 and then with
projected increases through 2100. (a) MPI-ESM-MR,
(b) HadGEM2-CC, (c) MIROC-ESM-CHEM, (d)
MIROC-ESM. Contour labels in m/s.