Observational Evidence of Kelvin Waves in the Tropical Stratosphere

This study of synoptic-scale wave motions in the equatorial stratosphere is based on the analysis of six months of radiosonde data from three tropical stations. Fluctuations in the zonal wind with an average period of 15 days, and amplitudes in the order of 8–12 m sec−1 are noted. Corresponding fluctuations are found in the temperature field with amplitudes of 3–5C and a phase lead of ¼ cycle with respect to the zonal wind. These wave motions which propagate phase downward do not appear to involve the meridional wind component. The observed fluctuations resemble Kelvin waves, which represent one of the solutions of the wave equation on an equatorial beta plane. One of the notable features of this type of wave is that it produces an upward flux of westerly momentum. The observations indicate that this flux is large enough to account for the westerly accelerations associated with the quasi-biennial oscillation.