Climatology and trends in the forcing of the stratospheric zonal-mean flow

摘要

The momentum budget of the Transformed Eulerian-Mean (TEM) equation iscalculated using the European Centre for Medium-Range Weather Forecasts(ECMWF) reanalysis (ERA-40) and the National Centers for EnvironmentalPrediction (NCEP) Reanalysis 2 (R-2). This study outlines the considerablecontribution of unresolved waves, deduced to be gravity waves, to the forcingof the zonal-mean flow. A trend analysis, from 1980 to 2001, shows that theonset and break down of the Northern Hemisphere (NH) stratospheric polarnight jet has a tendency to occur later in the season in the more recentyears. This temporal shift follows long-term changes in planetary waveactivity that are mainly due to synoptic waves, with a lag of one month. Inthe Southern Hemisphere (SH), the polar vortex shows a tendency to persistfurther into the SH summertime. This also follows a statistically significantdecrease in the intensity of the stationary EP flux divergence over the1980–2001 period. Ozone depletion is well known for strengthening the polarvortex through the thermal wind balance. However, the results of this workshow that the SH polar vortex does not experience any significant long-termchanges until the month of December, even though the intensification of theozone hole occurs mainly between September and November. This study suggeststhat the decrease in planetary wave activity in November provides animportant feedback to the zonal wind as it delays the breakdown of the polarvortex. In addition, the absence of strong eddy feedback before Novemberexplains the lack of significant trends in the polar vortex in the SH earlyspring. A long-term weakening in the Brewer-Dobson (B-D) circulation in thepolar region is identified in the NH winter and early spring and during theSH late spring and is likely driven by the decrease in planetary waveactivity previously mentioned. During the rest of the year, there are largediscrepancies in the representation of the B-D circulation and the unresolvedwaves between the two reanalyses, making trend analyses unreliable.