Upper Tropospheric Waves in the Tropics. Part I: Dynamical Analysis in the Wavenumber-Frequency Domain

Space and time spectra of large-scale wave disturbances at the 200 mb level in the latitude belt 20°S to 40°N are studied based on wind data for the period June–August 1967. The kinetic energy of the transient waves shows a minimum near 10°N where convective activity attains a maximum. The transient eddies in the zonal wind component have dominant scales of zonal wavenumbers s = 3–6 in most latitudes, while those in the meridional wind component have dominant scales of s = 6–8. By decomposing the wind data into symmetric and antisymmetric components with respect to the equator, three prototype equatorial wave modes are detected: 1) Kelvin waves of zonal wavenumber s = 1 and 2 and a period of 7 days, and s = 1 and periods longer than 20 days; 2) mixed Rossby-gravity waves of s = 4 and a period of 5 days; and 3) Rossby waves (of the lowest meridional nodal number) of s = 2 and a period near 12 days. Westward moving short waves (s = 7–15) gain kinetic energy from the mean easterly flow. Eastward moving waves in the middle latitudes do not propagate into the tropics because of the absorption at critical latitudes. Westward moving long waves of s ≈ 4 and periods near 5 days accompany a distinct peak in the equatorward wave energy flux, suggesting the origin of the observed mixed Rossby-gravity waves.