Interannual Variability of Stratospheric Zonal Wind Forced by the Northern Lower-Stratospheric Large-Scale Waves

Abstract

An interactive stratospheric model was run for 13 years while being forced with the observed daily varying Northern Hemisphere (NH) waves (numbers 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 variability (IAV) in the high-latitude monthly mean stratospheric zonal winds during NH winter, especially in December. In the model, wavenumber 1 played the major role in producing the high-latitude IAV. In addition, observations (from 1974 to 1995) revealed some strong anticorrelations between the large-scale 150-mb stationary wave-1 amplitude and the 10-mb high-latitude zonal wind during the NH early winter. It therefore seems that planetary wave variability in the lower stratosphere is the direct cause of much of the IAV in the monthly mean stratospheric zonal wind during the NH winter (especially in December). During late winter agreement between model and observations is improved by increasing the damping of planetary waves in the Tropics. Since the variability in the NH high-latitude winter zonal wind is so well modeled it does not seem likely that the high-latitude stratosphere is strongly chaotic, at least in early winter and on the timescale of a month. It also implies that if the stratospheric equatorial quasi-biennial oscillation (QBO) affects the extratropics in December (as implied by observations) then it must do so by somehow influencing the planetary waves at 150 mb. It is shown that this influence is disrupted by strong tropospheric activity, as in the early winter of 1987. The model also produces a QBO signal in the Southern Hemisphere Tropics during NH winter, due to the variability in the wave-induced cross-equatorial flow.