Vertical Dependence of the Scale and Structure of Stratospheric Equatorial Waves

The three-dimensional structure of free stratospheric equatorial waves and their vertical dependence are studied in various reanalysis datasets. The structure and temporal variability of the waves are isolated through space-time spectra and EOF analyses of space-time filtered equatorial wind, temperature and geopotential fields. The Principal Components (PCs) associated with each mode can be used to establish its statistical structure by projecting global multilevel dynamical fields from reanalysis onto the PCs in the time domain at lag using linear regression. Here we study the change in the activity, structure, and scale of the waves from the lower to the upper stratosphere. The spectral signals of Kelvin, n=0 mixed-Rossby/eastward inertia-gravity (MRG/EIG), and n=1 westward inertia-gravity (WIG) waves of Matsuno’s (1966) theory can all be readily detected from the tropical tropopause layer (TTL) at 100 hPa all the way to the upper stratosphere at 1 hPa in the reanalysis data. The activity of the equatorially-trapped modes is related to changes in basic state circulation including the seasonal cycle and the stratospheric Quasi-Biennial Oscillation (QBO). At the tropopause, these waves scale to around a 50m equivalent depth, and the corresponding equivalent depths increase monotonically with height, reaching values of around 300 m at 1 hPa. These shifts are shown to be due to wave damping and filtering of lower frequency (slower) waves by the zonal wind. Correspondingly, the waves become faster and they become less progressively trapped about the equator with height, as expected from linear theory. Additionally, the well-documented variability of many of the modes associated with the stratospheric QBO is also evident.