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

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