The effect of reflecting surfaces on the vertical structure and variability of stratospheric planetary waves

The effects of an upper-stratospheric reflecting surface on the vertical st ructure of stratospheric planetary waves are considered. A diagnostic of th e basic-state wave propagation characteristics, which is particularly usefu l for determining the existence and location of turning surfaces for meridi onal and vertical propagation, is developed. The diagnostic used is a more accurate indicator of wave propagation regions than the index of refraction because it diagnoses meridional and vertical propagation separately. The diagnostic is tested on a series of simple models, both steady state an d time dependent. It is found that the stratospheric waveguide sets the mer idional wavenumber of the waves, regardless of the characteristics of their tropospheric forcing, making it easier to understand the effects of dampin g and turning surfaces on the vertical structure of the waves. The diagnost ic is then applied to observations of the Southern Hemisphere winter of 199 6. It is shown that the differences in vertical wave structure between midd le and late winter can be explained as a linear response to the seasonal ev olution of the basic state, which involves a formation of a reflecting surf ace in late winter. It is also shown that on daily timescales wave-mean flo w interactions cause significant changes in the basic-state propagation cha racteristics for periods of a few days. These changes, along with the time variations in the forcing of the waves, are responsible for the observed da ily timescale variations in wave structure. The fact that the observed evol ution of the waves and the basic state are consistent with linear or quasi- linear wave theory (depending on the timescale looked at) supports the appl icability of the theory, as well as the validity of the observations.