The stratospheric meridional overturning circulation, also referred to as the Brewer–Dobson circulation (BDC), controls the composition of the stratosphere, which, in turn, affects radiation and climate. As the BDC cannot be directly measured, one has to infer its strength and trends indirectly. For instance, trace gas measurements allow the calculation of average transit times.Satellite measurements provide information on the distributions of trace gases for the entire stratosphere, with measurements of particularly long temporal and dense spatial coverage available for stratospheric water vapour (H₂O). Although chemical processes and boundary conditions confound interpretation, the influence of methane (CH₄) oxidation on H₂O in the stratosphere is relatively straightforward, and thus H₂O is an appealing tracer for transport analysis despite these caveats. In this work, we explore how mean age of air trends can be estimated from the combination of stratospheric H₂O and CH₄ data, by carrying out a proof of concept within the model environment of the Chemical Lagrangian Model of the Stratosphere (CLaMS). In particular, we assess the methodological uncertainties related to the two commonly used approximations of (i) instantaneous stratospheric entry mixing ratio propagation and (ii) constant correlation between mean age and the fractional release factor of CH₄. Performing various sensitivity studies with CLaMS, we test different methods of the mean age of air trend estimation, and we aim to provide simple and practical advice on the adjustment of the used approximations for obtaining more reliable mean age of air trends from the measurements of H₂O and CH₄.Our results show that the estimated mean age of air trends from the combination of stratospheric H₂O and CH₄ changes may be significantly affected by the assumed approximations. Depending on the investigated stratospheric region and the considered period, the error in estimated mean age of air trends can be large, especially in the lower stratosphere. For particular periods, the errors from the two approximations can lead to opposite effects, which may even cancel out. Finally, for a more reliable estimate of the mean age of air trends, we propose adjusting the approximation method by using an idealized age spectrum to propagate stratospheric entry mixing ratios. The findings of this work can be used for assessing the uncertainties in stratospheric BDC trend estimation from global satellite measurements.

中文翻译：

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如何根据平流层水蒸气和甲烷的变化来估计布鲁尔-多布森环流趋势？

平流层经向翻转环流，也称为布鲁尔-多布森环流 (BDC)，控制着平流层的组成，进而影响辐射和气候。由于 BDC 无法直接衡量，因此必须间接推断其强度和趋势。例如，微量气体测量允许计算平均传输时间。卫星测量提供了有关整个平流层微量气体分布的信息，可测量平流层水蒸气 (H ₂ O) 的特别长的时间和密集的空间覆盖范围。尽管化学过程和边界条件混淆了解释，但甲烷 (CH ₄ ) 氧化对 H _2的影响平流层中的 O 相对简单，因此尽管有这些警告，H ₂ O 仍是传输分析的有吸引力的示踪剂。在这项工作中，我们通过在平流层化学拉格朗日模型 (CLaMS) 的模型环境中进行概念验证，探索如何从平流层 H ₂ O 和 CH ₄数据的组合中估计空气趋势的平均年龄. _{特别是，我们评估了与 (i) 瞬时平流层进入混合比传播和 (ii) 平均年龄与 CH 4}的释放因子分数之间的恒定相关性的两种常用近似值相关的方法不确定性. 使用 CLaMS 进行各种敏感性研究，我们测试了空气趋势平均年龄估计的不同方法，我们的目标是为调整使用的近似值提供简单实用的建议，以便从 H 的测量中获得更可靠的空气趋势平均年龄₂ O 和 CH ₄。我们的结果表明，从平流层 H ₂ O 和 CH _{4的组合估计的平均空气年龄趋势}假设的近似值可能会显着影响变化。根据所调查的平流层区域和所考虑的时期，估计的空气趋势平均年龄的误差可能很大，尤其是在平流层较低层。对于特定时期，两个近似值的误差可能导致相反的效果，甚至可能抵消。最后，为了更可靠地估计空气趋势的平均年龄，我们建议通过使用理想化的年龄谱来传播平流层进入混合比来调整近似方法。这项工作的结果可用于评估来自全球卫星测量的平流层 BDC 趋势估计的不确定性。