Double Quasi-Biennial Cycles in Observed Winds in the Tropical Stratosphere

Abstract

Height-time sections of mean monthly observed upper-air zonal winds at stations within 20 degrees of the equator show not only an almost biennial oscillation of the wind in the middle or upper stratosphere but that there is a relative wave, 180 degrees out of phase to the first, which occurs in the lower stratosphere. The two waves are merged near the equator; they are best distinguished from each other about 9N to 15N and, although still noticeable as separate occurrences of westerlies from 15N to 20N, the intensity of the westerlies is much reduced. Both waves occur at progressively higher levels from one biennium to the next. This observed double cycle is caused by the combination of the fundamental annual and biennial waves, and the upward progression appears due to the difference in phase lags with height of these two component waves.