Climatology, trends and ENSO impacts in the wave forcing of the stratospheric zonal-mean flow and ozone transport

The wave forcing of the stratospheric zonal-mean flow and ozone transport is investigated through the calculation of the momentum and ozone transport budgets in the Transformed Eulerian-Mean (TEM) framework using the European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA-40). The climatology of the wave forcing is first validated then the long-term trends and El Nino/Southern Oscillation (ENSO) impacts are analyzed. This study reveals that a significant decrease in planetary wave activity in February in the Northern Hemisphere and in November in the Southern Hemisphere leads to a delay in the polar vortex breakdown one month later. Since the trends in the winds follow that of the wave activity, this study disproves the theory whereby an intensification of the polar vortex caused by ozone depletion induces a reduction in the high latitudes wave activity. The analysis of the dynamical transport of ozone reveals the fundamental role of ozone eddy transport in the ozone hole recovery. As the ozone hole grows larger, so does the ozone eddy transport thus balancing the largest trends in the ozone chemical destruction. This study underlines the fact that without an intensification of the ozone eddy transport over the 1980-2001 time period, the ozone hole over Antarctica would be drastically more severe. Finally, the response of the stratospheric dynamics to ENSO proves to be complex, only moderately statistically significant, and to vary greatly from month to month. Its temporal evolution resembles that of a Stratospheric Sudden Warming. ENSO composite differences (warm minus cold ENSO) show an increase in the vertical propagation of stationary planetary waves in January results in a brief weakening of the polar vortex in February. This is followed by a recovery in March, due to less propagation of stationary and transient planetary waves into the stratosphere. The ozone response to ENSO presents a dipole pattern, with negative anomalies in the tropics and positive anomalies in the polar region. The budget analysis of the ozone transport equation does not provide useful information about the mechanism for the ozone response because it is noisy and lacks coherent patterns.