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-km level 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 5 days. 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.

Structure of Wave Disturbances over the Marshall Islands During the Years of 1956 and 1958

“…We cannot say conclusively about the disturbance in 1958, but we notice for the 1956 data that this wave looks like the type of n =1, is e., the center of the wave lies on latitudes about 10°N. Yanai (1963) has reported the n =1 type disturbance propagating westward with the 6-day period at 200 mb for the 1958 data which are the same data in this study, but we cannot found this type of disturbance in this study. We consider that the disturbance reported by Yanai (1963) is the upper part of the lower tropospheric wave with the wavelength of 3,000 km mentioned above.…”

“…Yanai (1963) has reported the n =1 type disturbance propagating westward with the 6-day period at 200 mb for the 1958 data which are the same data in this study, but we cannot found this type of disturbance in this study. We consider that the disturbance reported by Yanai (1963) is the upper part of the lower tropospheric wave with the wavelength of 3,000 km mentioned above.…”

“…Synoptic-scale disturbances in the tropical trades are known as "easterly waves" and were studied by Riehl (1945Riehl ( , 1948Riehl ( a, b, 1954 Palmer (1951Palmer ( , 1952, Yanai (1961Yanai ( , 1963 and others. Yanai and Maruyama (1966) discovered stratospheric wave disturbances propagating over the equatorial Pacific which correspond to mixed Rossby-gravity waves in Matsuno's theory (1966).…”

“…Yanai and Maruyama (1966) discovered stratospheric wave disturbances propagating over the equatorial Pacific which correspond to mixed Rossby-gravity waves in Matsuno's theory (1966). Wallace and Kousky (1968) found another mode of large-scale waves which were identified with Kelvin waves in Matsuno's theory (1966).…”

“…They also pointed out that the phase lines of wave incline westward with height in the upper troposphere and the lower stratosphere and incline eastward with height in the middle and the lower troposphere. The upper-level waves were identified as the mixed Rossby-gravity waves found by Yanai and Maruyama (1966) (we denote this wave by Y-M wave hereafter). Nitta (1970) extended the studies of Yanai et al (1968) and examined the structure of the tropospheric wave disturbances with period of 4 to 5 days more closely.…”