The Effect of Reflecting Surfaces on the Vertical Structure and Variability of Stratospheric Planetary Waves

Abstract

The effects of an upper-stratospheric reflecting surface on the vertical structure of stratospheric planetary waves are considered. A diagnostic of the basic-state wave propagation characteristics, which is particularly useful for determining the existence and location of turning surfaces for meridional 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 and time dependent. It is found that the stratospheric waveguide sets the meridional wavenumber of the waves, regardless of the characteristics of their tropospheric forcing, making it easier to understand the effects of damping and turning surfaces on the vertical structure of the waves. The diagnostic is then applied to observations of the Southern Hemisphere winter of 1996. It is shown that the differences in vertical wave structure between middle and late winter can be explained as a linear response to the seasonal evolution of the basic state, which involves a formation of a reflecting surface in late winter. It is also shown that on daily timescales wave–mean flow interactions cause significant changes in the basic-state propagation characteristics for periods of a few days. These changes, along with the time variations in the forcing of the waves, are responsible for the observed daily timescale variations in wave structure. The fact that the observed evolution of the waves and the basic state are consistent with linear or quasi-linear wave theory (depending on the timescale looked at) supports the applicability of the theory, as well as the validity of the observations.