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

Abstract

The interannual variability of the greenhouse gases methane (span classCombining double low line"inline-formula"CH4/span) and tropospheric ozone (span classCombining double low line"inline-formula"O3/span) 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 span classCombining double low line"inline-formula"CH4/span and tropospheric span classCombining double low line"inline-formula"O3/span, 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), span classCombining double low line"inline-formula"CH4/span and span classCombining double low line"inline-formula"O3/span 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 span classCombining double low line"inline-formula"O3/span 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./p 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 span classCombining double low line"inline-formula"CH4/span atmospheric lifetime. The change in span classCombining double low line"inline-formula"CH4/span lifetime led to a 7.5 ppb yrspan classCombining double low line"inline-formula"-1/span increase in the global mean span classCombining double low line"inline-formula"CH4/span growth rate in 1998. Therefore, biomass burning emission of CO could account for 72 % of the total effect of fire emissions on span classCombining double low line"inline-formula"CH4/span growth rate in 1998./p Our simulations indicate that variations in fire emissions and meteorology associated with El Niño have opposing impacts on tropospheric span classCombining double low line"inline-formula"O3/span burden. El Niño-related changes in atmospheric transport and humidity decrease global tropospheric span classCombining double low line"inline-formula"O3/span concentrations leading to a span classCombining double low line"inline-formula"-0.03/span W mspan classCombining double low line"inline-formula"-2/span change in the span classCombining double low line"inline-formula"O3/span radiative effect (RE). However, enhanced fire emission of precursors such as nitrogen oxides (span classCombining double low line"inline-formula"NOix/i/span) and CO increase span classCombining double low line"inline-formula"O3/span and lead to an span classCombining double low line"inline-formula"O3/span RE of 0.03 W mspan classCombining double low line"inline-formula"-2/span. While globally the two mechanisms nearly cancel out, causing only a small change in global mean span classCombining double low line"inline-formula"O3/span RE, the regional changes are large - up to span classCombining double low line"inline-formula"-0.33/span W mspan classCombining double low line"inline-formula"-2/span with potentially important consequences for atmospheric heating and dynamics.