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Discussion papers
https://doi.org/10.5194/acp-2019-222
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/acp-2019-222
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 14 Mar 2019

Research article | 14 Mar 2019

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

Impact of El Niño Southern Oscillation on the interannual variability of methane and tropospheric ozone

Matthew J. Rowlinson1, Alexandru Rap1, Stephen R. Arnold1, Richard J. Pope1,2, Martyn P. Chipperfield1,2, Joe McNorton3, Piers Forster4, Hamish Gordon1, Kirsty J. Pringle1, Wuhu Feng1,5, Brian J. Kerridge6,7, Barry L. Latter6,7, and Richard Siddans6,7 Matthew J. Rowlinson et al.
  • 1Institute for Climate and Atmospheric Science, School of Earth and Environment, University of Leeds, Leeds, LS2 9JT, UK
  • 2National Centre for Earth Observation, University of Leeds, Leeds, LS2 9JT, UK
  • 3European Centre for Medium-Range Weather Forecasts, Reading, RG2 9AX, UK
  • 4Priestley International Centre for Climate, University of Leeds, LS2 9JT, Leeds, UK
  • 5National Centre for Atmospheric Science, University of Leeds, LS2 9JT, Leeds, UK
  • 6Remote Sensing Group, STFC Rutherford Appleton Laboratory, Harwell, Oxfordshire, UK
  • 7National Centre for Earth Observation, Harwell, Oxfordshire, UK

Abstract. The growth rate of global methane (CH4) concentrations has a strong interannual variability which is believed to be driven largely by fluctuations in CH4 emissions from wetlands and wildfires, as well as changes to the atmospheric sink. The El Niño Southern Oscillation (ENSO) is known to influence fire occurrence, wetland emission and atmospheric transport, but there are still important uncertainties associated with the exact mechanism and magnitude of this influence. Here we use a modelling approach to investigate how fires and meteorology control the interannual variability of global carbon monoxide (CO), CH4 and ozone (O3) concentrations, particularly during large El Niño events. Using a three-dimensional chemical transport model (TOMCAT) coupled to a sophisticated aerosol microphysics scheme (GLOMAP) we simulate changes to CO, hydroxyl radical (OH) and O3 for the period 1997–2014. We then use an offline radiative transfer model to quantify the impact of changes to atmospheric composition as a result of specific drivers.

During the El Niño event of 1997–1998, there were increased emissions from biomass burning globally. As a result, global CO concentrations increased by more than 40 %. This resulted in decreased global mass-weighted tropospheric OH concentrations of up to 9 % and a resulting 4 % increase in the CH4 atmospheric lifetime. The change in CH4 lifetime led to a 7.5 ppb yr−1 increase in global mean CH4 growth rate in 1998. Therefore biomass burning emission of CO could account for 72 % of the total effect of fire emissions on CH4 growth rate in 1998.

Our simulations indicate variations in fire emissions and meteorology associated with El Niño have opposing impacts on tropospheric O3 burden. El Niño-related atmospheric transport changes decrease global tropospheric O3 concentrations leading to a −0.03 Wm−2 change in O3 radiative effect (RE). However, enhanced fire emission of precursors such as nitrous oxides (NOx) and CO increase O3 RE by 0.03 Wm−2. While globally the two mechanisms nearly cancel out, causing only a small change in global mean O3 RE, the regional changes are large   up to −0.33 Wm−2 with potentially important consequences for atmospheric heating and dynamics.

Matthew J. Rowlinson et al.
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Short summary
Wildfires and meteorology have a large effect on atmospheric concentrations of important greenhouse gases such as methane and ozone. During the 1997 El Niño event, unusually large fire emissions caused large changes to global methane, particularly through CO emissions which affected the oxidation capacity of the atmosphere. There were also large regional effects on tropospheric ozone and its radiative forcing, but little global effect due to contrasting impacts of fire and meteorology.
Wildfires and meteorology have a large effect on atmospheric concentrations of important...
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