Equatorially Trapped Waves at the 200 mb Level and Their Association with Meridional Convergence of Wave Energy Flux

Equatorially trapped planetary-scale waves at the 200 mb level are isolated and their characteristics are examined using space-time spectral analyses of wind data in the latitude belt from 20°S to 40°N during the two contrasting northern summers of 1967 and 1972. With the aid of the theoretically known properties of equatorial waves such as the dispersion relation and the symmetry or antisymmetry of wind components with respect to the equator, Kelvin waves with s (zonal wavenumber) = 1 and 2, a mixed Rossby–gravity wave with s = 4, and an equatorially trapped Rossby wave with s = 2 can be clearly identified among the major spectral peaks observed in 1967. However, in 1972 the mixed Rossby–gravity wave is absent and only weak signals of the Kelvin waves with s = 1 and 2 and the equatorially trapped Rossby wave with s = 2 are found. In addition, westward moving waves with s = 1, which are identified as the gravest symmetric free Rossby waves, are observed in both years. All the westward-moving equatorially trapped waves are accompanied by meridional convergence of wave energy flux, suggesting their possible excitation by lateral forcing due to midlatitude disturbances.