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Discussion papers
https://doi.org/10.5194/acp-2018-1310
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/acp-2018-1310
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 11 Jan 2019

Research article | 11 Jan 2019

Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Atmospheric Chemistry and Physics (ACP).

Extratropical Age of Air trends and causative factors in climate projection simulations

Petr Šácha1,2, Roland Eichinger4,3, Hella Garny3,4, Petr Pišoft2, Simone Dietmüller3, Laura de la Torre1, David A. Plummer5, Patrick Jöckel3, Olaf Morgenstern6, Guang Zeng6, Neal Butchart7, and Juan A. Añel1 Petr Šácha et al.
  • 1EPhysLab, Faculty of Sciences, Universidade de Vigo, Ourense, Spain
  • 2Charles University in Prague, Faculty of Mathematics and Physics, Department of Atmospheric Physics, Prague, Czech Republic
  • 3Deutsches Zentrum für Luft- und Raumfahrt (DLR), Institut für Physik der Atmosphäre, Germany
  • 4Ludwig Maximilians Universität, Institut für Meteorologie, Munich, Germany
  • 5Environment and Climate Change Canada, Climate Research Division, Montréal, QC, Canada
  • 6National Institute of Water and Atmospheric Research (NIWA), Wellington, New Zealand
  • 7Met Office Hadley Centre, Exeter, UK

Abstract. Climate model simulations show a Brewer-Dobson circulation (BDC) acceleration in the course of climate change. While the mechanisms for the BDC strengthening are well understood, there are still open questions concerning its dynamical driving. Mean age of stratospheric air (AoA) is a useful transport diagnostic for accessing changes of the BDC. Analysing AoA from a subset of Chemistry Climate Model Initiative part 1 climate projection simulations, we find a remarkable agreement between most of the models in simulating the largest negative AoA trends in the extratropical lower to middle stratosphere of both hemispheres (approximately between 20 gpkm and 25 gpkm and 20°–50°N/S). We show that the occurrence of AoA trend minima in those regions is directly related to the climatological AoA distribution being sensitive to an upward shift of the circulation in response to a climate change. But also other factors like a reduction of aging by mixing (AbM) and residual circulation transit times (RCTTs) contribute to the AoA distribution changes by widening the AoA isolines. Furthermore we analyze the time evolution of AbM and RCTT trends in the extratropics and examine the connection to possible drivers like local residual circulation strength, net tropical upwelling and wave driving. However, after the correction for a vertical shift of pressure levels, we find only seasonally significant trends of residual circulation strength and zonal mean wave forcing (resolved and unresolved) without a clear relation between the trends of the analyzed quantities. This indicates that additional causative factors may influence the AoA, RCTT and AbM trends. Namely, we postulate a possible influence of stratospheric shrinkage on RCTT, AbM and therefore also on AoA trends. In this study, we postulate that the shrinkage of the stratosphere has the potential to influence the RCTT and AbM trends and thereby cause additional AoA changes over time.

Petr Šácha et al.
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Short summary
Climate models robustly project a Brewer-Dobson circulation (BDC) acceleration in the course of climate change. Analysing mean age of stratospheric air (AoA) from a subset of climate projection simulations, we find a remarkable agreement in simulating the largest AoA trends in the extratropical stratosphere. This is shown to be related with the upward shift of the circulation resulting in a so-called stratospheric shrinkage, which could be one of the so-far omitted BDC acceleration drivers.
Climate models robustly project a Brewer-Dobson circulation (BDC) acceleration in the course of...
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