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| Titel |
Driving of the SAO by gravity waves as observed from satellite |
| VerfasserIn |
M. Ern, P. Preusse, M. Riese |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
0992-7689
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| Digitales Dokument |
URL |
| Erschienen |
In: Annales Geophysicae ; 33, no. 4 ; Nr. 33, no. 4 (2015-04-29), S.483-504 |
| Datensatznummer |
250121191
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| Publikation (Nr.) |
 copernicus.org/angeo-33-483-2015.pdf |
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| Zusammenfassung |
| It is known that atmospheric dynamics in the tropical stratosphere have an
influence on higher altitudes and latitudes as well as on surface weather
and climate. In the tropics, the dynamics are governed by an interplay of the
quasi-biennial oscillation (QBO) and semiannual oscillation (SAO) of the
zonal wind. The QBO is dominant in the lower and middle stratosphere, and the
SAO in the upper stratosphere/lower mesosphere. For both QBO and SAO the
driving by atmospheric waves plays an important role. In particular, the role
of gravity waves is still not well understood.
In our study we use observations of the High Resolution
Dynamics Limb Sounder (HIRDLS) satellite instrument to derive gravity wave momentum fluxes and
gravity wave drag in order to investigate the interaction of gravity waves
with the SAO. These observations are compared with the ERA-Interim
reanalysis. Usually, QBO westward winds are much stronger than QBO eastward
winds. Therefore, mainly gravity waves with westward-directed phase speeds
are filtered out through critical-level filtering already below the
stratopause region. Accordingly, HIRDLS observations show that gravity waves
contribute to the SAO momentum budget mainly during eastward wind shear, and
not much during westward wind shear. These findings confirm theoretical
expectations and are qualitatively in good agreement with ERA-Interim and
other modeling studies. In ERA-Interim most of the westward SAO driving is
due to planetary waves, likely of extratropical origin. Still, we find in
both observations and ERA-Interim that sometimes westward-propagating gravity
waves may contribute to the westward driving of the SAO. Four characteristic
cases of atmospheric background conditions are identified. The forcings of
the SAO in these cases are discussed in detail, supported by gravity wave
spectra observed by HIRDLS. In particular, we find that the gravity wave
forcing of the SAO cannot be explained by critical-level filtering alone;
gravity wave saturation without critical levels being reached is also
important. |
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