The impact of sulfur hexafluoride (SF<sub>6</sub>) sinks on age of air climatologies and trends
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Published:2022-01-24
Issue:2
Volume:22
Page:1175-1193
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ISSN:1680-7324
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Container-title:Atmospheric Chemistry and Physics
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language:en
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Short-container-title:Atmos. Chem. Phys.
Author:
Loeffel SheenaORCID, Eichinger RolandORCID, Garny Hella, Reddmann Thomas, Fritsch FraukeORCID, Versick Stefan, Stiller GabrieleORCID, Haenel Florian
Abstract
Abstract. Mean age of air (AoA) is a common diagnostic for the strength of the stratospheric overturning circulation in both climate models and
observations. AoA climatologies and AoA trends over the recent decades of model simulations and proxies derived from observations of long-lived
tracers do not agree. Satellite observations show much older air than climate models, and while most models compute a clear decrease in AoA over the
last decades, a 30-year time series from measurements shows a statistically nonsignificant positive trend in the Northern Hemisphere extratropical middle
stratosphere. Measurement-based AoA derivations are often founded on observations of the trace gas sulfur
hexafluoride (SF6), a fairly long-lived gas with a
near-linear increase in emissions during recent decades. However, SF6 has chemical sinks in the mesosphere that are not considered
in most model studies. In this study, we explicitly compute the chemical SF6 sinks based on chemical processes in the global
chemistry climate model EMAC (ECHAM/MESSy Atmospheric Chemistry). We show that good agreement between stratospheric AoA in EMAC and MIPAS (Michelson Interferometer for Passive Atmospheric Sounding) is reached through the inclusion of chemical
SF6 sinks, as these sinks lead to a strong increase in the stratospheric AoA and, therefore, to a better agreement with MIPAS satellite
observations. Remaining larger differences at high latitudes are addressed, and possible reasons for these differences are discussed. Subsequently, we demonstrate that
the AoA trends are also strongly influenced by the chemical SF6 sinks. Under consideration of the SF6 sinks, the AoA trends over
the recent decades reverse sign from negative to positive. We conduct sensitivity simulations which reveal that this sign reversal does not result
from trends in the stratospheric circulation strength nor from changes in the strength of the SF6 sinks. We illustrate that even a
constant SF6 destruction rate causes a positive trend in the derived AoA, as the amount of depleted SF6 scales with
increasing SF6 abundance itself. In our simulations, this effect overcompensates for the impact of the accelerating stratospheric circulation
which naturally decreases AoA. Although various sources of uncertainties cannot be quantified in detail in this study, our results suggest that the
inclusion of SF6 depletion in models has the potential to reconcile the AoA trends of models and observations. We conclude the study with
a first approach towards a correction to account for SF6 loss and deduce that a linear correction might be applicable to values of AoA of
up to 4 years.
Funder
Grantová Agentura České Republiky Helmholtz-Gemeinschaft
Publisher
Copernicus GmbH
Subject
Atmospheric Science
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