Impacts of the Quasi‐Biennial Oscillation and the El Niño‐Southern Oscillation on Stratosphere‐to‐Troposphere Ozone Transport: Assessment With Chemistry‐Climate Models

Abstract Ozone transported from the stratosphere to the troposphere is a key component of the source of tropospheric ozone, and this transport is related to the residual circulation in the stratosphere. The impacts of stratospheric Quasi‐Biennial Oscillation (QBO) and tropospheric El Niño‐Southern Oscillation (ENSO) events on stratospheric circulation will affect the stratosphere‐to‐troposphere transport of ozone. In this study we use average of stratosphere‐tagged ozone tracer (O 3 S) from two chemistry‐climate models to quantify the downward transport of stratospheric ozone. We found that under the westerly QBO (WQBO) phase, less than the average stratospheric ozone is transported to the troposphere (2.5–6 ppbv O 3 S anomaly from 500 to 200 hPa in mid‐high latitudes). The opposite is true under the easterly QBO (EQBO) phase, accompanied by more stratospheric ozone transport to the troposphere (5–10 ppbv O 3 S anomaly from 500 to 200 hPa). The impact of ENSO events on the stratosphere‐to‐troposphere ozone transport shows a hemispheric asymmetry. The ozone transported into the troposphere decreases by about 2.5–6 ppbv from 500 to 200 hPa in the Northern Hemisphere under the warm ENSO phase, but shows almost the same amount of anomalous increase in the Southern Hemisphere. The situation is opposite during the cold ENSO phase. This variation in ozone transport in response to the QBO and ENSO is related to the stratospheric residual circulation variations. In a warming future climate, the frequency of El Niño events will increase while the amplitude of QBO is expected to decrease, thus affecting the stratospheric source of tropospheric ozone.