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.

中文翻译：

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热带中层大气半年振荡（SAO）及其在再分析和卫星观测中的重力波驱动

重力波在驱动热带地区纬向风的半年振荡 (SAO) 方面发挥着重要作用。然而，缺乏关于这种强迫的详细知识，并且对重力波的全球观测的直接估计很少。在 2002 年至 2018 年期间，我们在四种不同的再分析中调查了 SAO：ERA-Interim、JRA-55、ERA-5 和 MERRA-2。与来自微波​​肢体探空仪 (MLS) 和使用宽带发射辐射测量 (SABER) 卫星观测的大气探测得到的 SPARC 纬向风气候学和准地转风的比较表明，再分析再现了 SAO 的一些基本特征。但是，也存在很大差异，具体取决于模型设置。特别，MERRA-2 似乎受益于对重力波阻力参数化和 MLS 观测同化的专门调整。为了研究重力波与背景风的相互作用，将来自 SABER 卫星观测的重力波动量通量的绝对值和绝对重力波阻力代理与不同的风数据集进行比较：SPARC 风气候学；结合低空 ERA-Interim 和高空 MLS 或 SABRE 准地转风的数据集；以及结合 ERA-Interim、SABRE 准地转风和 TIMED Doppler Interferometer (TIDI) 直接风观测的数据集。在中低层和中层，SABRE 绝对重力波阻力代理与背景风的正垂直梯度有很好的相关性，表明重力波主要有助于 SAO 东风相的驱动及其随时间向下传播。在海拔 75–85 km，SABRE绝对重力波阻力代理与背景风的绝对值更好地相关，表明重力波振幅饱和对SAO风的更直接强迫。80公里以上 SABRE 重力波阻力主要受潮汐控制，而不是受 SAO 控制。再分析再现了 SAO 重力波驱动的一些基本特征：所有再分析都显示了 SAO 在层顶区域向东相的更强重力波驱动。对于高层型号 ERA-5 和 MERRA-2，在较低的中间层也是如此。然而，所有再分析都受到模型上层模型固有阻尼的限制，导致模型顶部附近的特征不切实际。我们对 SABRE 和再分析重力波阻力的分析表明，在自由运行的大气环流模型中，SAO 重力波强迫的大小通常太弱；因此，需要更现实的表现形式。