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| Titel |
The stratospheric wintertime response to applied extratropical torques and its relationship with the annular mode |
| VerfasserIn |
Peter Watson, Lesley Gray |
| Konferenz |
EGU General Assembly 2015
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 17 (2015) |
| Datensatznummer |
250106823
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| Publikation (Nr.) |
 EGU/EGU2015-6500.pdf |
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| Zusammenfassung |
| The response of the wintertime Northern Hemisphere (NH) stratosphere to applied
extratropical zonally symmetric zonal torques, simulated by a primitive equation model of the
middle atmosphere, is presented. This is relevant to understanding the effect of gravity wave
drag (GWD) in models and the influence of natural forcings such as the quasi-biennial
oscillation (QBO), El Ninõ-Southern Oscillation (ENSO), solar cycle and volcanic
eruptions on the polar vortex. There is a strong feedback due to planetary waves, which
approximately cancels the direct effect of the torque on the zonal acceleration in
the steady state and leads to an EP flux convergence response above the torque’s
location. The residual circulation response is very different to that predicted assuming
wave feedbacks are negligible. The results are consistent with the predictions of ray
theory, with applied westerly torques increasing the meridional potential vorticity
gradient, thus encouraging greater upward planetary wave propagation into the
stratosphere.
The steady state circulation response to torques applied at high latitudes closely
resembles the Northern annular mode (NAM) in perpetual January simulations. This
behaviour is analogous to that shown by the Lorenz system and tropospheric models.
Imposed westerly high-latitude torques lead counter-intuitively to an easterly zonal mean
zonal wind (u) response at high latitudes, due to the wave feedbacks. However, in simulations
with a seasonal cycle, the feedbacks are qualitatively similar but weaker, and the
long-term response is less NAM-like and no longer easterly at high latitudes. This is
consistent with ray theory and differences in climatological u between the two types of
simulations. The response to a a tropospheric wave forcing perturbation is also NAM-like.
These results suggest that dynamical feedbacks tend to make the long-term NH
extratropical stratospheric response to arbitrary external forcings NAM-like, but only if the
feedbacks are sufficiently strong. This may explain why the observed polar vortex
responses to natural forcings such as the QBO and ENSO are NAM-like. The results
imply that wave feedbacks must be understood and accurately modelled in order to
understand and predict the influence of GWD and other external forcings on the polar
vortex, and that biases in a model’s climatology will cause biases in these feedbacks. |
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