Power Spectra of Large-Scale Disturbances over the Tropical Pacific

Abstract

The power spectra of the meridional component of the wind from the ground to the 30-kmlevel at 17 stations in the tropical and sub-tropical Pacific are studied based on special upper wind observations taken during the period April through July 1962. In equatorial latitudes, the power spectra show a peak at a period close to 4 days corresponding to the passage of "equatorial waves" in the lower tropospheric easterlies. A very large spectral density is found at the periods 4 to 5 days throughout the upper troposphere and the lower stratosphere where the mean wind is from the west. The peak spectral density reaches a maximum at about the 17-km level near the tropopause. The spectral density gradually decreases with height in the stratosphere where the mean wind is from the east. In sub-tropical latitudes, large disturbance energy is associated with the upper tropospheric westerlies and a large portion of the spectral density is contained in the wave period longer than 5days. A sharp suppression of the spectral density takes place at the lower boundary of the stratospheric easterlies. From the computation of the coherence and the phase difference of the meridional component of the wind, the vertical and horizontal structure of the disturbances is studied. The large horizontal extent and the westward inclination of phase lines of the disturbances in the upper troposphere and in the lower stratosphere are revealed. The horizontal coherence of the lower tropospheric disturbances in the east-west direction is very low. The phase lines of the lower tropospheric disturbances are inclined eastward with height. The average wavelength of the disturbances at various levels is estimated from the relation between the phase difference and the longitudinal difference of the stations.