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. Gravity waves play a significant role in driving the semiannual oscillation(SAO) of the zonal wind in the tropics. However, detailed knowledge of thisforcing is missing, and direct estimates from global observations of gravitywaves are sparse. For the period 2002–2018, we investigate the SAO in fourdifferent reanalyses: ERA-Interim, JRA-55, ERA-5, and MERRA-2. Comparisonwith the SPARC zonal wind climatology and quasi-geostrophic winds derived fromMicrowave Limb Sounder (MLS) and Sounding of the Atmosphere using BroadbandEmission Radiometry (SABER) satellite observations show that the reanalysesreproduce some basic features of the SAO. However, there are also largedifferences, depending on the model setup. Particularly, MERRA-2 seems tobenefit from dedicated tuning of the gravity wave drag parameterization andassimilation of MLS observations. To study the interaction of gravity waveswith the background wind, absolute values of gravity wave momentum fluxes anda proxy for absolute gravity wave drag derived from SABER satelliteobservations are compared with different wind data sets: the SPARC windclimatology; data sets combining ERA-Interim at low altitudes and MLS or SABERquasi-geostrophic winds at high altitudes; and data sets that combineERA-Interim, SABER quasi-geostrophic winds, and direct wind observations bythe TIMED Doppler Interferometer (TIDI). In the lower and middle mesospherethe SABER absolute gravity wave drag proxy correlates well with positivevertical gradients of the background wind, indicating that gravity wavescontribute mainly to the driving of the SAO eastward wind phases and theirdownward propagation with time. At altitudes 75–85 km, the SABERabsolute gravity wave drag proxy correlates better with absolute values of thebackground wind, suggesting a more direct forcing of the SAO winds by gravitywave amplitude saturation. Above about 80 km SABER gravity wave dragis mainly governed by tides rather than by the SAO. The reanalyses reproducesome basic features of the SAO gravity wave driving: all reanalyses showstronger gravity wave driving of the SAO eastward phase in the stratopauseregion. For the higher-top models ERA-5 and MERRA-2, this is also the case inthe lower mesosphere. However, all reanalyses are limited by model-inherentdamping in the upper model levels, leading to unrealistic features near themodel top. Our analysis of the SABER and reanalysis gravity wave dragsuggests that the magnitude of SAO gravity wave forcing is often too weak inthe free-running general circulation models; therefore, a more realisticrepresentation is needed.

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