Impact of El Niño–Southern Oscillation on the interannual variability of methane and tropospheric ozone
The interannual variability of the greenhouse gases methane (CH4) and tropospheric ozone (O3) is largely driven by natural variations in global emissions and meteorology. The El Niño–Southern Oscillation (ENSO) is known to influence fire occurrence, wetland emission and atmospheric circulation, affe...
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Published in | Atmospheric chemistry and physics Vol. 19; no. 13; pp. 8669 - 8686 |
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Main Authors | , , , , , , , , , , , , |
Format | Journal Article |
Language | English |
Published |
Katlenburg-Lindau
Copernicus GmbH
09.07.2019
Copernicus Publications |
Subjects | |
Online Access | Get full text |
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Summary: | The interannual variability of the greenhouse gases methane
(CH4) and tropospheric ozone (O3) is largely driven by natural variations in global emissions and meteorology. The El Niño–Southern
Oscillation (ENSO) is known to influence fire occurrence, wetland emission
and atmospheric circulation, affecting sources and sinks of CH4 and
tropospheric O3, but there are still important uncertainties associated
with the exact mechanism and magnitude of this effect. Here we use a
modelling approach to investigate how fires and meteorology control the
interannual variability of global carbon monoxide (CO), CH4 and 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 climate 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, causing global CO concentrations to increase
by more than 40 %. This resulted in decreased global mass-weighted
tropospheric OH concentrations of up to 9 % and a consequent 4 %
increase in the CH4 atmospheric lifetime. The change in CH4
lifetime led to a 7.5 ppb yr−1 increase in the 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 that variations in fire emissions and meteorology
associated with El Niño have opposing impacts on tropospheric O3
burden. El Niño-related changes in atmospheric transport and humidity
decrease global tropospheric O3 concentrations leading to a −0.03 W m−2 change in the O3 radiative effect (RE). However, enhanced fire emission of precursors such as nitrogen oxides (NOx) and CO
increase O3 and lead to an O3 RE of 0.03 W m−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 W m−2 with potentially important consequences for atmospheric heating and
dynamics. |
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ISSN: | 1680-7324 1680-7316 1680-7324 |
DOI: | 10.5194/acp-19-8669-2019 |