RADIATIVE DAMPING OF EQUATORIAL WAVES IN THE MIDDLE ATMOSPHERE

Theoretical estimates are made of the radiative damping of various Kel vin and Rossby-gravity (RG) wave modes in the 20-70 km region, for a p eriod of 24 months (1979-80), using the observed monthly mean zonal-wa ve profiles at 8.5-degrees-N latitude and realistic values of scale-de pendent cooling coefficients. The net changes in wave amplitudes at va rious height regions are calculated by combining the attenuation due t o radiative cooling and the growth due to density decrease. The calcul ated height variations of wave amplitudes explain quite well several f eatures of the observed height structures of Kelvin waves. They also e xplain the observed rapid decay of RG waves below about 35 km when the ir period exceeds 3 days. The calculated momentum flux convergences of various waves are used to estimate the mean-flow accelerations genera ted by the damped waves. The Kelvin waves are found to be capable of g enerating the accelerations needed to drive the westerly phase of the quasi-biennial oscillation (QBO) and the semi-annual oscillation (SAO) in the zonal wind. However, RG waves seem to be capable of driving th e easterly phase of the QBO but not the SAO. The remarkable consistenc y of our theoretical results and the observed patterns of wave amplitu des and mean-flow accelerations demonstrate that radiative damping is the dominant dissipative process for the equatorial waves in the 20-70 km region and our theoretical approach is capable of characterizing t he equatorial wave behaviour and the associated flow accelerations wit h a good quantitative accuracy.