The sensitivity of tropospheric methane to the interannual variability in stratospheric ozone CharlesD.Camp a, * ,MarkS.Roulston b ,AlbertF.C.Haldemann c ,YukL.Yung d a Department of Applied Mathematics, California Institute of Technology, Pasadena, CA 91125, USA b Pembroke College, Oxford, OX1 1DW, UK c Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91125, USA d Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125, USA Received 25 April 2000; received in revised form 8 August 2000; accepted 12 September 2000 Importance of this paper: Many processes in the stratosphere have important impacts upon tropospheric behavior. The forcing of tropospheric methane by stratospheric ozone is one such interaction for which we have long and accurate ob- servational records. We have empirically determined a quantitative measure of this interaction. This result provides the ®rst convincing demonstration in the data records of such a stratosphere±troposphere interaction. It can also be used to calibrate the modeling work being done in this area. Abstract The dominant processes aecting the concentration of tropospheric methane on interannual timescales are the biosphericandanthropogenicsourcesandchangesintheabundanceofthehydroxylradicalcausedbythechangesin theUV¯uxwhichresultfromchangesinstratosphericozoneabundance.Wehavecarriedoutanempiricalstudyofthe sensitivity of the methane to ¯uctuations in ozone column abundance. This analysis was carried out using monthly mean surface methane concentrations measured by the National Oceanic and Atmospheric Administration ± Climate Monitoring and Diagnostics Laboratory NOAA-CMDL) Global Cooperative Air Sampling Network from 1983 to 1998andozonecolumnabundancesobtainedbytheTotalOzoneMappingSpectrometerTOMS)andtheEPTOMS instrumentsoverthesametimeperiod.Wefocusedoninterannualvariabilitywithperiodsbetween15and60months, inwhichintervalthedominantozone¯uctuationisthequasi-biennialoscillationQBO),withaperiodofapproximately 29 months. In order to isolate the response of methane to ozone from the eects of variability in the sources and transportofmethane,werestrictedouranalysistodataatmid-latitudesinthesouthernhemisphere.Astatisticalstudy showsthatthesensitivityfactor a d lnCH 4 =d lnO 3   0:038  0:009.TheresponseofCH 4 lagsapproximately 6monthsbehindtheforcingbyO 3 .Asimplemodelwasusedtointerprettheempiricalresults.Ourresultscon®rmthat any mechanism that aects stratospheric ozone impacts the oxidizing potential of the troposphere. CH 4 ¯uctuations provide a quantitative measure of this important eect linking the upper and the lower atmosphere. Ó 2001 Elsevier Science Ltd. All rights reserved. Keywords: Interannual oscillation; QBO; Atmosphere; Stratosphere±troposphere interaction; Insolation 1. Introduction The most important mechanism for removing meth- ane from the atmosphere is its reaction with tropo- spheric OH radicals Khalil et al., 1993). Chemosphere ± Global Change Science 3 2001) 147±156 * Correspondingauthor.Tel.:+1-626-395-3992;fax:+1-626- 585-1917. E-mail address: cdc@gps.caltech.edu C.D. Camp). 1465-9972/01/$ - see front matter Ó 2001 Elsevier Science Ltd. All rights reserved. PII:S1465-997200)00053-2