Preprints
https://doi.org/10.5194/acp-2021-190
https://doi.org/10.5194/acp-2021-190

  22 Mar 2021

22 Mar 2021

Review status: a revised version of this preprint is currently under review for the journal ACP.

The semiannual oscillation (SAO) in the tropical middle atmosphere and its gravity wave driving in reanalyses and satellite observations

Manfred Ern1, Mohamadou Diallo1, Peter Preusse1, Martin G. Mlynczak2, Michael J. Schwartz3, Qian Wu4, and Martin Riese1 Manfred Ern et al.
  • 1Institut für Energie- und Klimaforschung – Stratosphäre (IEK–7), Forschungszentrum Jülich GmbH, 52425 Jülich, Germany
  • 2NASA Langley Research Center, Hampton, Virginia, USA
  • 3Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California, USA
  • 4National Center for Atmospheric Research, High Altitude Observatory, Boulder, Colorado, USA

Abstract. Gravity waves play a significant role in driving the semiannual oscillation (SAO) of the zonal wind in the tropics. However, detailed knowledge of this forcing is missing, and direct estimates from global observations of gravity waves are sparse. For the period 2002–2018, we investigate the SAO in four different reanalyses: ERA-Interim, JRA-55, ERA-5, and MERRA-2. Comparison with the SPARC zonal wind climatology and quasi-geostrophic winds derived from Microwave Limb Sounder (MLS) and Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) satellite observations show that the reanalyses reproduce some basic features of the SAO. However, there are also large differences, depending on the model setup. Particularly, MERRA-2 seems to benefit from dedicated tuning of the gravity wave drag parameterization and assimilation of MLS observations. To study the interaction of gravity waves with the background wind, absolute values of gravity wave momentum fluxes and drag derived from SABER satellite observations are compared with different wind data sets: the SPARC wind climatology, data sets combining ERA-Interim at low altitudes and MLS or SABER quasi-geostrophic winds at high altitudes, as well as data sets that combine ERA-Interim, SABER quasi-geostrophic winds, and direct wind observations by the TIMED Doppler Interferometer (TIDI). In the lower and middle mesosphere SABER absolute gravity wave drag correlates well with positive vertical gradients of the background wind, indicating that gravity waves contribute mainly to the driving of the SAO eastward wind phases and their downward propagation with time. At altitudes 75–85 km, SABER absolute gravity wave drag correlates better with absolute values of the background wind, suggesting a more direct forcing of the SAO winds by gravity wave amplitude saturation. Above about 80 km SABER gravity wave drag is mainly governed by tides rather than by the SAO. The reanalyses reproduce some basic features of the SAO gravity wave driving: All reanalyses show stronger gravity wave driving of the SAO eastward phase in the stratopause region. For the higher-top models ERA-5 and MERRA-2 this is also the case in the lower mesosphere. However, all reanalyses are limited by model-inherent damping in the upper model levels, leading to unrealistic features near the model top. Our analysis of the SABER and reanalysis gravity wave drag suggests that the magnitude of SAO gravity wave forcing is often too weak in the free-running general circulation models, therefore, a more realistic representation is needed.

Manfred Ern et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • CC1: 'Comment on acp-2021-190 -- tidal forces', Paul PUKITE, 28 Mar 2021
    • AC3: 'Reply on CC1', Manfred Ern, 13 Jul 2021
  • RC1: 'Comment on acp-2021-190', Anonymous Referee #1, 21 Apr 2021
  • RC2: 'Comment on acp-2021-190', Anonymous Referee #2, 02 May 2021

Manfred Ern et al.

Manfred Ern et al.

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Short summary
Details of the driving of the semiannual oscillation (SAO) of the tropical winds in the middle atmosphere are still not known. We investigate the SAO and its driving by small-scale gravity waves (GWs) using satellite data and different reanalyses. In a large altitude range GWs mainly drive the SAO westerlies, but in the upper mesosphere GWs seem to drive both SAO easterlies and westerlies. Reanalyses reproduce some features of the SAO, but are limited by model-inherent damping at upper levels.
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