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Ann. Geophys., 25, 1071-1092, 2007
www.ann-geophys.net/25/1071/2007/
© European Geosciences Union 2007
The QBO as potential amplifier and conduit to lower altitudes of solar cycle influence
H. G. Mayr1, J. G. Mengel2, C. L. Wolff1, F. T. Huang3, and H. S. Porter4
1Goddard Space Flight Center, Greenbelt, MD, USA
2Science Systems & Applications, Lanham, MD, USA
3Creative Computing Solutions Inc. Rockville, MD, USA
4Furman University, Greenville, SC, USA
Abstract. In several papers, the solar cycle (SC) effect in the
lower atmosphere has been linked observationally to the Quasi-biennial
Oscillation (QBO) of the zonal circulation. Salby and Callaghan (2000) in
particular analyzed the QBO wind measurements, covering more than 40 years,
and discovered that they contain a large SC signature at 20 km. We present
here the results from a study with our 3-D Numerical Spectral Model (NSM),
which relies primarily on parameterized gravity waves (GW) to describe the
QBO. In our model, the period of the SC is taken to be 10 years, and the relative
amplitude of radiative forcing varies exponentially with height, i.e., 0.2%
at the surface, 2% at 50 km, and 20% at 100 km and above. Applying spectral
analysis to identify the SC signature, the model generates a relatively
large modulation of the QBO, which reproduces the observations
qualitatively. The numerical results demonstrate that the QBO modulation,
closely tracking the phase of the SC, is robust and persists at least for 70
years. The question is what causes the SC effect, and our analysis shows
that four interlocking processes are involved: (1) In the mesosphere at
around 60 km, the solar UV variations generate in the zonal winds a SC
modulation of the 12-month annual oscillation, which is hemispherically
symmetric and confined to equatorial latitudes like the QBO. (2) Although
the amplitude of this equatorial annual oscillation (EAO) is relatively
small, its SC modulation is large and extends into the lower stratosphere
under the influence of, and amplified by, wave forcing. (3) The amplitude
modulations of both EAO and QBO are essentially in phase with the imposed SC
heating for the entire time span of the model simulation. This indicates
that, due to positive feedback in the wave mechanism, the EAO apparently
provides the pathway and pacemaker for the SC modulation of the QBO. (4) Our
analysis demonstrates that the SC modulations of the QBO and EAO are
amplified by tapping the momentum from the upward propagating gravity waves.
Influenced and amplified by wave processes, the QBO thus acts as conduit to
transfer to lower altitudes the larger SC variations in the UV absorbed in
the mesosphere. Our model produces in the temperature variations of the QBO
and EAO measurable SC modulations at polar latitudes near the tropopause.
The effects are apparently generated by the meridional circulation, and
planetary waves presumably, which redistribute the energy from the
equatorial region where the waves are very effective in amplifying the SC
influence.
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