Preprints
https://doi.org/10.5194/acp-2021-232
https://doi.org/10.5194/acp-2021-232

  23 Apr 2021

23 Apr 2021

Review status: this preprint is currently under review for the journal ACP.

The impact of SF6 sinks on age of air climatologies and trends

Sheena Loeffel1, Roland Eichinger2,1,5, Hella Garny1,2, Thomas Reddmann3, Frauke Fritsch1,2, Stefan Versick4, Gabriele Stiller3, and Florian Haenel3 Sheena Loeffel et al.
  • 1Deutsches Zentrum für Luft- und Raumfahrt (DLR), Institut für Physik der Atmosphäre, Oberpfaffenhofen, Germany
  • 2Ludwig Maximilians Universität, Institut für Meteorologie, Munich, Germany
  • 3Karlsruhe Institute of Technology, Institute for Meteorology and Climate Research, Karlsruhe, Germany
  • 4Karlsruhe Institute of Technology, Steinbuch Centre for Computing (SCC), Karlsruhe, Germany
  • 5Charles University, Department of Atmospheric Physics, Faculty of Mathematics and Physics, Prague, Czech Republic

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 its 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 of AoA over the last decades, a thirty-year timeseries from measurements shows a statistically non-significant positive trend. Measurement-based AoA derivations are often based on observations of the trace gas SF6, a fairly long-lived gas with a near-linear increase of emissions during the recent decades. However, SF6 has chemical sinks in the mesosphere, which 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. We show that good agreement of stratospheric AoA in EMAC and MIPAS is reached through the inclusion of chemical SF6 sinks, as those lead to a strong increase of the stratospheric AoA and thereby to a better agreement with MIPAS satellite observations. Remaining larger differences in high latitudes are addressed and possible reasons are discussed. Subsequently, we demonstrate that also the AoA trends are 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 results neither from trends of 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, since the amount of depleted SF6 scales with the increasing SF6 abundance itself. In our simulations, this effect overcompensates 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.

Sheena Loeffel et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on acp-2021-232', Eric Ray, 24 May 2021
  • RC2: 'Comment on acp-2021-232', Rostislav Kouznetsov, 28 May 2021

Sheena Loeffel et al.

Sheena Loeffel et al.

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Short summary
SF6-derived trends of stratospheric AoA from observations and from model simulations disagree in sign. SF6 experiences chemical degradation, which we here explicitly integrate in a global climate model. In our simulations, the AoA trend changes sign when SF6 sinks are considered, and thus the process has the potential to reconcile simulated with observed AoA trends. Further analyses reveal that the positive AoA trend is due to the SF6 sinks themselves.
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