Large-Scale Disturbances in the Equatorial Lower Stratosphere

Abstract

Westward-moving wave disturbances in the westerlies of the equatorial lower stratosphere recently found by Yanai and Maruyama (1966) are studied in detail by using the data of the special observations over the Marshall Islands area during spring and summer in 1958. Vertical time-sections of upper winds show that the wind direction at about 18 to 24 km height oscillates between southwest and northwest with a period of four to five days. Synoptic analysis shows the existence of the large-scale wave disturbances moving westward against the westerly flow at a speed of about 2,000 km/day. The wave-length may be as long as 10,000 km. The phase lines tilt westward with increasing height. Power spectral analysis of meridional wind component indicates that the oscillation is prevailing in the lower stratosphere confined close to the equator. By removing the basic flow through a simple procedure, the disturbances are found to take the form of eddies centered on the equator. Pronounced eddies observed in April 1958 are investigated in particular to determine threedimensional structure of the disturbances. The west ward propagation of the disturbances is apparently owing to the 13-effect. The phase velocity and the flow-height pattern of the disturbances are compared with those of the theoretical barotropic-divergent equatorial waves discussed by Rosenthal (1965) and Matsuno (1966).