Kelvin Wave Activity and the Quasi-Biennial Oscillation in the Equatorial Lower Stratosphere

Abstract

This note presents observational results and their mechanistic understanding of the long-term variation of Kelvin wave activity in the equatorial lower stratosphere. Analyses are made for 15 years (1974-1988), using the rawinsonde station data at Singapore (1°N, 104°E). Occasionally, clear fluctuations with periods near two weeks can be observed in the zonal wind and temperature fields, suggesting the prominent existence of Kelvin waves. The variability of Kelvin wave activity (measured by the power spectral density integrated over periods in the range around two weeks) is clearly related to the quasi-biennial oscillation (QBO) of the zonal wind in the lower stratosphere. Around the 30 mb level, the Kelvin wave activity is vigorous when the wind regime of the QBO rapidly changes from easterlies to westerlies. In order to understand the observational results, we perform a simplified mechanistic calculation following an idea that the momentum flux due to the Kelvin wave is locally constant and suffers damping which is inversely proportional to the vertical group velocity. In this calculation the observed monthly-mean zonal wind data are used for estimating Kelvin wave amplitudes on a monthly basis. Results of the calculation represent well the long-term variation of Kelvin wave activity associated with the QBO.