INTERANNUAL VARIABILITY OF STRATOSPHERIC ZONAL WIND FORCED BY THE NORTHERN LOWER-STRATOSPHERIC LARGE-SCALE WAVES

An interactive stratospheric model was run for 13 years while being fo rced with the observed daily varying Northern Hemisphere (NH) waves (n umbers 1 to 3) on the 368-K isentropic surface (which lies near 150 mb ) from 1980 to 1993. It reproduced much of the observed interannual va riability (IAV) in the high-latitude monthly mean stratospheric zonal winds during MI winter, especially in December. In the model, wavenumb er 1 played the major role in producing the high-latitude IAV. In addi tion, observations (from 1974 to 1995) revealed some strong anticorrel ations between the large-scale 150-mb stationary wave-1 amplitude and the 10-mb high-latitude zonal wind during the NH early winter. It ther efore seems that planetary wave variability in the lower stratosphere is the direct cause of much of the IAV in the monthly mean stratospher ic zonal wind during the NH winter (especially in December). During la te winter agreement between model and observations is improved by incr easing the damping of planetary waves in the Tropics. Since the variab ility in the NH high-latitude winter zonal wind is so well modeled it does not seem likely that the high-latitude stratosphere is strongly c haotic, at least in early winter and on the timescale of a month. It a lso implies that if the stratospheric equatorial quasibiennial oscilla tion (QBO) affects the extratropics in December (as implied by observa tions) then it must do so by somehow influencing the planetary waves a t 150 mb. It is shown that this influence is disrupted by strong tropo spheric activity, as in the early winter of 1987. The model also produ ces a QBO signal in the Southern Hemisphere Tropics during NH winter, due to the variability in the wave-induced cross-equatorial flow.