Effects of the Stratospheric QBO on Long-Lived Greenhouse Gases in the Troposphere

An analysis is presented of results of an extended integration with a global general circulation model that includes treatment of a long-lived tropospheric trace constituent as well as a momentum source that forces a realistic quasi-biennial oscillation (QBO). It is shown that the dynamical QBO may modulate stratosphere-troposphere exchange in such a manner as to produce a QBO in global-mean tropospheric tracer mixing ratio. For the growth rate of tropsopehric methane, this transport-induced QBO is expected to have a peak-to-peak amplitude of 1-2 ppbv/year, which, while modest compared with the full range of observed variability in growth rate, is still significant. The observed methane growth rate time series during 1983-1999 is shown to be consistent with the predicted QBO effect, although the record is definitely dominated by other interannual variations.