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Discussion papers | Copyright
https://doi.org/10.5194/acp-2018-688
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 06 Aug 2018

Research article | 06 Aug 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).

Structural changes in the shallow and transition branch of the Brewer–Dobson circulation induced by El Niño

Mohamadou Diallo1,2, Paul Konopka1, Michelle L. Santee3, Rolf Müller1, Mengchu Tao1, Kaley A. Walker4, Bernard Legras2, Martin Riese1, and Felix Ploeger1 Mohamadou Diallo et al.
  • 1Institute of Energy and Climate Research, Stratosphere (IEK-7), Forschungszentrum Jülich, 52425 Jülich, Germany
  • 2Laboratoire de Météorologie Dynamique, UMR8539, IPSL, UPMC/ENS/CNRS/Ecole Polytechnique, Paris, France
  • 3Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California, USA
  • 4Department of Physics, University of Toronto, Toronto, Ontario, Canada

Abstract. The stratospheric Brewer–Dobson circulation (BD-circulation) determines the transport and lifetime of key radiatively active trace gases and further impacts surface climate through downward coupling. Here, we quantify the variability in the lower stratospheric BD-circulation induced by the El Nino Southern Oscillation (ENSO), using satellite trace gas measurements and simulations with the Lagrangian chemistry transport model, CLaMS, driven by ERA-Interim and JRA-55 reanalyses. We show that despite discrepancies in the deseasonalised ozone (O3) mixing ratios between CLaMS simulations and satellite observations, the patterns of changes in the lower stratospheric O3 anomalies induced by ENSO agree remarkably well over the 2005–2016 period. Particularly during the most recent El Niño in 2015–2016, both satellite observations and CLaMS simulations show the largest negative tropical O3 anomaly in the record. Regression analysis of different metrics of the BD-circulation strength, including mean age of air, vertical velocity, residual circulation and age spectrum, shows clear evidence for structural changes of the BD-circulation in the lower stratosphere induced by El Niño, consistent with observed O3 anomalies. These structural changes during El Niño include a weakening of the transition branch of the BD-circulation between about 370–420K (∼100–70hPa) and equatorward of about 60° and, a strengthening of the shallow branch at the same latitudes and between about 420–500K (∼70–30hPa). The strengthening of the shallow branch induces negative tropical O3 anomalies due to enhanced tropical upwelling, while the weakening of the transition branch combined with enhanced downwelling due to the strengthening shallow branch leads to positive O3 anomalies in the extratropical upper troposphere-lower stratosphere (UTLS). Our results suggest that a shift of the ENSO basic state toward more frequent El Niño-like conditions in a warming future climate will substantially alter UTLS trace gas distributions due to these changes in the vertical structure of the stratospheric circulation.

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This paper investigates the structural changes in the shallow and transition branch of the BDC induced by El Nino using the Lagrangian model simulations driven by ERAi and JRA-55 combined with satellite observations. We found a clear evidence for structural changes of the BDC in the lower stratosphere induced by El Nino. El Nino includes a weakening of the transition branch between 370–420 K and equatorward of about 60 a strengthening of the shallow branch at the same latitudes between 420–500 K.
This paper investigates the structural changes in the shallow and transition branch of the BDC...
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