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
a proxy for absolute gravity wave 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; and 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
the SABER absolute gravity wave drag proxy 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, the SABER
absolute gravity wave drag proxy 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.
The Stratopause Semiannual Oscillation in the Berlin Troposphere–Stratosphere–Mesosphere GCM
