Influence of tropospheric methane concentrations on stratospheric composition and circulation

(P. Kenzelmann, St. Füglistaler, E. Rozanov, T. Peter)

Changes in abundance of stratospheric tracers have both a direct radiative effect on Earth's climate and an indirect impact caused by changes in stratospheric and tropospheric circulation. Water vapour plays an important role in the stratosphere first because it is an important greenhouse gas and second because of the production of HOx, which serves as a catalyser for ozone destruction (Evans et al., 1998; Dvortsov and Solomon, 2001). The abundance of water vapour in the stratosphere is controlled by two mechanisms, first by the troposphere-stratosphere transport of air, primarily in the tropics, and second by the oxidation of methane in the stratosphere. Both sources have a different latitudinal impact on the stratosphere (Fueglistaler, 2005). Changes in the water vapour entry mixing ratios produce a spatially homogene change, whereas changes in methane results in larger water vapour changes over the poles. The methane is entering the stratosphere in the tropics and is transported by the Brewer-Dobson circulation [Brewer, 1949] towards the poles. During this transport methane is oxidized to water. The increase of methane in the stratosphere leads to a perturbation of ozone content in the stratosphere. This could affect the climate in the troposphere through an alteration of stratospheric and tropospheric circulation.

A set of steady state runs with different methane concentrations will be performed to investigate its impact of rising methane concentration on stratospheric climate.

Figure: Change in stratospheric water vapour by enhancing tropospheric methane concentrations from 1.7 ppm to 0.7 ppm. Contour lines show change in %.

References

Dvortsov, V. L., and S. Solomon (2001): Response of the stratospheric temperatures and ozone to past and future increases in stratospheric humidity, J. Geophys. Res., 106, 7505- 7514.

Evans, S. J., R. Toumi, J. E. Harries, M. P. Chipperfield, and J. M. Russell (1998): Trends in stratospheric humidity and the sensitivity of ozone to these trends, J. Geophys. Res., 103, 8715- 8725.

Fueglistaler, S., and P. H. Haynes (2005): Control of interannual and longer-term variability of stratospheric water vapor, J. Geophys. Res., 110, D24108, doi:10.1029/2005JD006019.