How can Brewer–Dobson circulation trends be estimated from changes in stratospheric water vapour and methane?
<p>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 indire...
| Main Authors: | , , , , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2022-08-01
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| Series: | Atmospheric Chemistry and Physics |
| Online Access: | https://acp.copernicus.org/articles/22/9895/2022/acp-22-9895-2022.pdf |
| Summary: | <p>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.</p>
<p>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 (H<span class="inline-formula"><sub>2</sub></span>O). Although chemical processes and boundary conditions confound interpretation, the influence of methane (CH<span class="inline-formula"><sub>4</sub></span>) oxidation on H<span class="inline-formula"><sub>2</sub></span>O in the stratosphere is relatively straightforward, and thus H<span class="inline-formula"><sub>2</sub></span>O 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 H<span class="inline-formula"><sub>2</sub></span>O and CH<span class="inline-formula"><sub>4</sub></span> 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 CH<span class="inline-formula"><sub>4</sub></span>. 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 H<span class="inline-formula"><sub>2</sub></span>O and CH<span class="inline-formula"><sub>4</sub></span>.</p>
<p>Our results show that the estimated mean age of air trends from the combination of stratospheric H<span class="inline-formula"><sub>2</sub></span>O and CH<span class="inline-formula"><sub>4</sub></span> 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.</p> |
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| ISSN: | 1680-7316 1680-7324 |