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

Abstract. The growth rate of global methane (CH 4 ) concentrations has a strong interannual variability which is believed to be driven largely by fluctuations in CH 4 emissions from wetlands and wildfires, as well as changes to the atmospheric sink. The El Nino 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), CH 4 and ozone (O 3 ) concentrations, particularly during large El Ninoevents. 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 O 3 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 Ninoevent 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 CH 4 atmospheric lifetime. The change in CH 4 lifetime led to a 7.5 ppb yr −1 increase in global mean CH 4 growth rate in 1998. Therefore biomass burning emission of CO could account for 72 % of the total effect of fire emissions on CH 4 growth rate in 1998. Our simulations indicate variations in fire emissions and meteorology associated with El Ninohave opposing impacts on tropospheric O 3 burden. El Nino-related atmospheric transport changes decrease global tropospheric O 3 concentrations leading to a −0.03 Wm −2 change in O 3 radiative effect (RE). However, enhanced fire emission of precursors such as nitrous oxides (NO x ) and CO increase O 3 RE by 0.03 Wm −2 . While globally the two mechanisms nearly cancel out, causing only a small change in global mean O 3 RE, the regional changes are large up to −0.33 Wm −2 with potentially important consequences for atmospheric heating and dynamics.