The Role of Eddies in Tropospheric Circulation Trends Associated with Stratospheric Ozone Reduction in the Boreal Summer

We examined the impact of long-term trends in stratospheric ozone on the troposphere by investigating radiative heating anomalies caused by stratospheric ozone changes and associated changes in dynamical fields. We applied a linear trend analysis to five reanalysis datasets and five chemistry climate models (CCM) for the boreal summer (June-July-August) during an ozone-depleting period (1981-2000). CCM simulations with depleting ozone and fixed GHGs and SSTs showed a poleward shift of the subtropical jet and Hadley cell expansion. Reanalysis data also showed this poleward shift. We inferred that the ozone anomaly induced negative potential vorticity (PV) anomalies near the tropopause by causing anomalous radiative cooling. To separate direct radiative impacts from indirect eddy forcing, we used a PV inversion technique. The PV-induced zonal wind anomalies showed a poleward shift of the subtropical jet, but magnitudes were small in the low to middle troposphere. We used a dry general circulation model to examine the modification of tropospheric eddies by the basic state change due to the PV anomalies. The result showed that anomalous wave forcing in the upper troposphere accelerates (decelerates) the zonal wind north (south) of the jet. The deceleration forcing south of the jet drives the anomalous residual mean circulation in the lower latitudes, explaining the Hadley cell expansion. The Coriolis force associated with the anomalous residual mean circulation extends the zonal wind anomaly to the lower troposphere. These results suggest that stratospheric ozone, by modifying eddy activity in the troposphere, plays an important role in tropospheric climate change.