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.
Highlights
The Brewer–Dobson circulation (BDC) describes the stratospheric transport circulation, consisting of the mean overturning circulation of air ascending in the tropical pipe, moving poleward and descending in the extratropics (Brewer, 1949; Dobson and Massey, 1956), as well as isentropic mixing
age of stratospheric air (AoA) from observations is mostly older than AoA from model simulations, and models simulate a decrease in AoA over recent decades, whereas trend estimates from observational data report a nonsignificant positive trend
For the first time, how longer-term trends are affected in a consistent manner, and we explore the different contributions from circulation changes, changes in abundance of reaction partners, and trends induced by constant destruction rates
Summary
The Brewer–Dobson circulation (BDC) describes the stratospheric transport circulation, consisting of the mean overturning circulation of air ascending in the tropical pipe, moving poleward and descending in the extratropics (Brewer, 1949; Dobson and Massey, 1956), as well as isentropic mixing. The same method can be applied to real long-lived tracers with a linear trend in tropospheric concentration, and AoA has been derived, for example, from balloon-borne in situ measurements of sulfur hexafluoride (SF6) (Andrews et al, 2001; Engel et al, 2009, 2017). This trace gas is suitable for these studies, as it is stable in the troposphere and stratosphere and its tropospheric concentrations have increased nearly linearly over recent decades. A near-global dataset of AoA covering 10 years from 2002 to 2012 was derived in Stiller et al (2012), Haenel et al (2015), and Stiller et al (2017), who retrieved SF6 distributions from MIPAS (Michelson Interferometer for Passive Atmospheric Sounding) satellite observations, but these cover a much shorter time period
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