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@ARTICLE{Ern:280095,
      author       = {Ern, Manfred and Preusse, Peter and Riese, Martin},
      title        = {{D}riving of the {SAO} by gravity waves as observed from
                      satellite},
      journal      = {Annales geophysicae},
      volume       = {33},
      number       = {4},
      issn         = {0992-7689},
      address      = {Katlenburg, Lindau},
      publisher    = {Copernicus},
      reportid     = {FZJ-2015-07842},
      pages        = {483-504},
      year         = {2015},
      abstract     = {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.},
      cin          = {IEK-7},
      ddc          = {550},
      cid          = {I:(DE-Juel1)IEK-7-20101013},
      pnm          = {244 - Composition and dynamics of the upper troposphere and
                      middle atmosphere (POF3-244)},
      pid          = {G:(DE-HGF)POF3-244},
      typ          = {PUB:(DE-HGF)16},
      UT           = {WOS:000353840000007},
      doi          = {10.5194/angeo-33-483-2015},
      url          = {https://juser.fz-juelich.de/record/280095},
}