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Volume 18, issue 8 | Copyright

Special issue: The Modular Earth Submodel System (MESSy) (ACP/GMD inter-journal...

Atmos. Chem. Phys., 18, 5655-5675, 2018
https://doi.org/10.5194/acp-18-5655-2018
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 3.0 License.

Research article 24 Apr 2018

Research article | 24 Apr 2018

Dynamics and composition of the Asian summer monsoon anticyclone

Klaus-Dirk Gottschaldt1, Hans Schlager1, Robert Baumann1, Duy Sinh Cai1, Veronika Eyring1, Phoebe Graf1, Volker Grewe1,2, Patrick Jöckel1, Tina Jurkat-Witschas1, Christiane Voigt1,3, Andreas Zahn4, and Helmut Ziereis1 Klaus-Dirk Gottschaldt et al.
  • 1Deutsches Zentrum für Luft- und Raumfahrt (DLR), Institut für Physik der Atmosphäre, Oberpfaffenhofen, Germany
  • 2Delft University of Technology, Aerospace Engineering, Delft, the Netherlands
  • 3Johannes Gutenberg-Universität, Institut für Physik der Atmosphäre, Mainz, Germany
  • 4Karlsruher Institut für Technologie (KIT), Institut für Meteorologie und Klimaforschung, Karlsruhe, Germany

Abstract. This study places HALO research aircraft observations in the upper-tropospheric Asian summer monsoon anticyclone (ASMA) into the context of regional, intra-annual variability by hindcasts with the ECHAM/MESSy Atmospheric Chemistry (EMAC) model. The observations were obtained during the Earth System Model Validation (ESMVal) campaign in September 2012. Observed and simulated tracer–tracer relations reflect photochemical O3 production as well as in-mixing from the lower troposphere and the tropopause layer. The simulations demonstrate that tropospheric trace gas profiles in the monsoon season are distinct from those in the rest of the year, and the measurements reflect the main processes acting throughout the monsoon season. Net photochemical O3 production is significantly enhanced in the ASMA, where uplifted precursors meet increased NOx, mainly produced by lightning. An analysis of multiple monsoon seasons in the simulation shows that stratospherically influenced tropopause layer air is regularly entrained at the eastern ASMA flank and then transported in the southern fringe around the interior region. Radial transport barriers of the circulation are effectively overcome by subseasonal dynamical instabilities of the anticyclone, which occur quite frequently and are of paramount importance for the trace gas composition of the ASMA. Both the isentropic entrainment of O3-rich air and the photochemical conversion of uplifted O3-poor air tend to increase O3 in the ASMA outflow.

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This study places aircraft trace gas measurements from within the Asian summer monsoon anticyclone into the context of regional, intra- and interannual variability. We find that the processes reflected in the measurements are present throughout multiple simulated monsoon seasons. Dynamical instabilities, photochemical ozone production, lightning and entrainments from the lower troposphere and from the tropopause region determine the distinct composition of the anticyclone and its outflow.
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