SHORT-PERIOD DISTURBANCES IN THE EQUATORIAL LOWER STRATOSPHERE

Temperature and horizontal wind fluctuations with periods shorter than 3 days in the equatorial lower stratosphere are examined by using data from routine rawinsonde observations at Singapore (1N, 104E) during 1978-1993. Internal wave-like structures having a period of about 2 days and a short vertical wavelength of 5 km are occasionally observed both in temperature and wind fluctuations. The result of power spectral analysis indicates that short-period fluctuations have significant energy, separated from Kelvin waves and mixed Rossby-gravity waves. Long-term variations of the spectral characteristics are investigated in relation to the quasi-biennial oscillation (QBO). Zonal wind and temperature variances due to the short-period fluctuations are generally large and particularly enhanced in the QBO phase when the direction of mean wind changes from easterly to westerly. Co-spectra of temperature and zonal wind fluctuations at short periods are significantly larger than the quadrature spectra, and the sign of co-spectra changes according to the phases of the QBO. These results indicate strong connection of the short-period disturbances with the QBO.