Climatology and Trends in the Forcing of the Stratospheric Zonal-Mean Flow

Abstract

The momentum budget of the Transformed Eulerian-Mean (TEM) equation is calculated using the European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA-40). This study outlines the considerable contribution of the dissipative forcing, identified as a gravity wave drag, to the forcing of the zonal-mean flow. A trend analysis shows that, in recent times, the onset and break down of the Northern Hemisphere (NH) stratospheric polar night jet occur later. This temporal shift is associated with long-term changes in the planetary wave activity that are mainly due to synoptic waves. In the Southern Hemisphere (SH), the polar vortex shows a tendency to persist further into the SH summertime. This is explained by a statistically significant decrease in the intensity of the stationary EP flux divergence over the 1980-2001 period. The prevailing theory explaining the long-term changes in the stratospheric polar vortex postulates that ozone depletion leads to a strengthening of westerly winds which in turn causes the reduction in wave activity in high latitudes. We show that the strongest component in the dynamical response to stratospheric ozone changes is in fact the feedback of planetary wave activity on the zonal wind. Finally, we identify long-term changes in the Brewer-Dobson circulation that are mainly caused by trends in the planetary wave activity during winter and by trends in the gravity wave body force otherwise.