Vertical Variations of Stratospheric Equatorial Wave activity in Various Reanalysis Datasets

The temporal variability and vertical dependence of free stratospheric equatorial waves are studied in various reanalysis datasets, including JRA-2 and ERA-Interim. 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 activity of the equatorially-trapped Matsuno are also related to metrics of convective activity derived from satellite brightness temperature, and to basic state circulation changes including the QBO. The Principal Components (PCs) associated with each mode can then be used to establish its statistical structure by projecting global multilevel dynamical fields from reanalysis and satellite brightness temperature or precipitation onto the PCs in the time domain at lag. The square root of the squared sum of PC pairs also provides a convenient “activity index” for each of the modes. Here we study the change in the activity, structure, and scale of the waves from the lower to the upper stratosphere, and the ability of various reanalysis products to detect these changes. 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. At the TTL, these waves scale to around an 50m equivalent depth, and the corresponding equivalent depths increase monotonically with height, reaching values of around 300 m at 1 hPa. This shift is assumed to be due to wave damping and filtering of lower frequency (slower) waves by the zonal wind as hypothesized by Garcia and Salby (1987). Correspondingly, the waves become faster and they become less progressively trapped about the equator with height, as expected from linear theory. It appears that the activity indices of many of the modes have strong peaks in the range of the Madden-Julian Oscillation (MJO), particularly during northern winter. The potential significance of tropospheric forcing of the stratospheric wave activity will be assessed using wave-activity flux diagnostics. Additionally, the well-documented variability of many of the modes associated with the stratospheric QBO is also evident, as well large variability at interannual and seasonal time scales. Both the JRA55 and ERA Interim reanalyses yield impressively similar bulk statistics of the modes, with other reanalysis products such as MERRA and NCEP agreeing less on the wave properties and variability.