Tropospheric ozone variations governed by changes in stratospheric circulation

The downward transport of stratospheric ozone is an important natural source of tropospheric ozone, particularly in the upper troposphere, where changes in ozone have their largest radiative effect(1). Stratospheric circulation is projected to intensify over the coming century, which could lead to an increase in the flux of ozone from the stratosphere to the troposphere(2-4). However, large uncertainties in the stratospheric contribution to trends and variability in tropospheric ozone levels(5-7) make it difficult to reliably project future changes in tropospheric ozone(8). Here, we use satellite measurements of stratospheric water vapour and tropospheric ozone levels collected between 2005 and 2010 to assess the effect of changes in stratospheric circulation, driven by El Nino/Southern Oscillation and the stratospheric Quasi-Biennial Oscillation, on tropospheric ozone levels. Wefindthat interannual variations in the strength of the stratospheric circulation of around 40%-comparable to the mean change in stratospheric circulation projected this century(2)-lead to changes in tropospheric ozone levels in the northern mid-latitudes of around 2%, approximately half of the interannual variability. Assuming that the observed response of tropospheric ozone levels to interannual variations in circulation is a good predictor of its equilibrium response, we suggest that the projected intensification of the stratospheric circulation over the coming century could lead to small but important increases in tropospheric ozone levels.