Annual and QBO-Synchronized Variations of Lower-Stratospheric Equatorial Wave Activity over Singapore during 1961-1989

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Abstract
Annual and QBO-synchronized variations (variations synchronized with the QBO cycle of the zonal wind) of lower-stratospheric equatorial wave activity were investigated by spectrally analyzing the upper-air data at Singapore (1.4N, 104.OE) for the period January 1961 to February 1990. To measure the equatorial wave activity, the quadrature spectra between the zonal wind component and temperature integrated over the period range 7.4-32 days were used for Kelvin waves, and the power spectra of the meridional wind component integrated over the period range 3.3-5.1 days for mixed Rossby-gravity waves. Annual variations in the mixed Rossby-gravity wave activity were found at the 30, 50 and 70hPa levels with a maximum around March. The amplitude was largest at the 70hPa level. For the Kelvin wave, a weak annual variation was found at 70hPa. QBO-synchronized variations were found in the Kelvin wave activity at the 30, 50 and 70hPa levels. A QBO-synchronized variation of mixed Rossbygravity wave activity was found at 30hPa, but was unclear at the 50 and 70hPa levels. Annual variations of mixed Rossby-gravity wave activity in the 50-70hPa layer appear to be related to a rapid descent of the easterly wind regime during March-June and a slow descent in July-February during the layer 30-50hPa layer.