Spectrum Analysis of Symmetric and Antisymmetric Equatorial Waves

Abstract

An attempt is made to separate large-scale equatorial waves into the ODD-mode and the EVEN-mode waves using time series data of four Pacific stations which are located nearly symmetrically on both sides of the equator. At least eight distinct waves are isolated by the spectrum analysis of symmetric and antisymmetric components of winds with respect to the equator. Most of the ODD-mode waves propagate eastward and exhibit only zonal wind oscillations indicatingthat they are Kelvin waves. There are two ODD-mode waves that may be identified with Rossby waves, because they show westward propagation relative to the air. All the EVEN-mode waves (seemingly the mixed Rossby-gravity waves) including the 4- to 5-day waves found by Yanai and Maruyama (1966) move westward. The vertical amplitude distribution and the vertical phase propagation are examined for each of the isolated waves. All the waves show maximum spectral density at about 12-14 km levels and the inclination of phase lines indicates upward propagation of wave energy above the level of maximum spectral density and downward energy propagation below the level. Some waves have very large inclination of phase lines, i. e., very short vertical wavelengths. The observed horizontal wavelength by use of two pairs of stations is compared with the calculation using the frequency equation of Matsuno (1966) and Lindzen (1967) based on the observed period and vertical wavelength.