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© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 25 Oct 2018

Research article | 25 Oct 2018

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

The influence of mixing on stratospheric circulation changes in the 21st century

Roland Eichinger1,2, Simone Dietmüller2, Hella Garny2,1, Petr Sacha3,4, Thomas Birner1, Harald Böhnisch5, Giovanni Pitari6, Daniele Visioni7,a, Andrea Stenke8, Eugene Rozanov8,9, Laura Revell10,11, David A. Plummer12, Patrick Jöckel2, Luke Oman13, Makoto Deushi14, Douglas E. Kinnison15, Rolando Garcia15, Olaf Morgenstern16, Guang Zeng16, Kane Adam Stone17,18,b, and Robyn Schofield17,18 Roland Eichinger et al.
  • 1Ludwig Maximilians Universität, Meteorological Institute Munich, Munich, Germany
  • 2Deutsches Zentrum für Luft- und Raumfahrt (DLR), Institut für Physik der Atmosphäre, Oberpfaffenhofen, Germany
  • 3Faculty of Sciences, EPhysLab, Universidade de Vigo, Ourense, Spain
  • 4Charles University Prague, Faculty of Mathematics and Physics, Department of Atmospheric Physics, Prague, Czech Republic
  • 5Karlsruhe Institute of Technology (KIT), Insitute of Meteorology and Climate Reasearch, Karlsruhe, Germany
  • 6Department of Physical and Chemical Sciences, Università dell’Aquila, L’Aquila, Italy
  • 7Department of Physical and Chemical Sciences and center of Excellence CETEMPS, Università dell’Aquila, L’Aquila, Italy
  • 8Institute for Atmospheric and Climate Science, ETH Zürich (ETHZ), Zürich, Switzerland
  • 9Physikalisch-Meteorologisches Observatorium Davos and World Radiation Center, Davos, Switzerland
  • 10Bodeker Scientific, Christchurch, New Zealand
  • 11School of Physical and Chemical Sciences, University of Canterbury, Christchurch, New Zealand
  • 12Environment and Climate Change Canada, Climate Research Division, Montréal, QC, Canada
  • 13National Aeronautics and Space Administration Goddard Space Flight Center (NASA GSFC), Greenbelt, Maryland, USA
  • 14Meteorological Research Institute (MRI), Tsukuba, Japan
  • 15National Center for Atmospheric Research (NCAR), Boulder, Colorado, US
  • 16National Institute of Water and Atmospheric Research (NIWA), Wellington, New Zealand
  • 17School of Earth Sciences, University of Melbourne, Melbourne, Australia
  • 18ARC Centre of Excellence for Climate System Science, Sydney, Australia
  • anow at: Mechanical and Aerospace Engineering, Cornell University, Ithaca, New York, USA
  • bnow at: Department of Earth Atmosphere and Planetary Science, Massachusetts Institute of Technology, Cambridge, Massachusetts, USA

Abstract. Climate models consistently predict an acceleration of the Brewer-Dobson circulation (BDC) due to climate change in the 21st century. However, the strength of this acceleration varies considerably among individual models, which constitutes a notable source of uncertainty for future climate projections. To shed more light upon the magnitude of this uncertainty and on its causes, we analyze the stratospheric mean age of air (AoA) of 10 climate projection simulations from the Chemistry Climate Model Initiative phase 1 (CCMI-I), covering the period between 1960 and 2100. In agreement with previous multi-model studies, we find a large model spread in the magnitude of the AoA trend over the simulation period. Differences between future and past AoA are found to be predominantly due to differences in mixing (reduced aging by mixing and recirculation) rather than differences in residual mean transport. We furthermore analyze the mixing efficiency, a measure of the relative strength of mixing for given residual mean transport, which was previously hypothesized to be a model constant. Here, the mixing efficiency is found to vary not only across models, but also over time in all models. Changes in mixing efficiency are shown to be closely related to changes in AoA and quantified to roughly contribute 10% to the long-term AoA decrease over the 21st century. Additionally, mixing efficiency variations are shown to considerably enhance model spread in AoA changes. To understand these mixing efficiency variations, we also present a consistent dynamical framework based on diffusive closure, which highlights the role of basic state potential vorticity gradients in controlling mixing efficiency and therefore aging by mixing.

Roland Eichinger et al.
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Status: open (until 20 Dec 2018)
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Roland Eichinger et al.
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Short summary
To shed more light upon the changes of the stratospheric circulation in the 21st century, climate projection simulations spanning from 1960 to 2100 of 10 state-of-the-art global climate models are analyzed. The study shows that in addition to changes in transport, also mixing plays an important role for the stratospheric circulation and that the properties of mixing vary over time. Furthermore, the influence of mixing is quantified and a dynamical framework is provided to understand the changes.
To shed more light upon the changes of the stratospheric circulation in the 21st century,...