Mean winds, tides and gravity waves during the westward phase of the mesopause semiannual oscillation (MSAO)

Abstract

MF radar observations of mean winds, tides, and gravity waves (in the period range 30–120 min) over Tirunelveli (8.7°N, 77.8°E) during April 1993, when the mesopause semiannual oscillation (MSAO) was in the westward phase, are presented in this work. The focus herein is on the observed relationship between these dynamical parameters, which is explained in the context of the current understanding of the behavior of MSAO winds, tidal dissipation, and gravity wave-tide interactions. The observed diurnal tide amplitude and phase variations with height indicate (1,1) wave damping and subsequent westward momentum deposition. The gravity wave variances are shown to be modulated by the diurnal tide. Though the analysis does not lead to an estimation of gravity wave contributions to the time-mean zonal flow and its shears, the observations do suggest that the short-term tidal variability contributes, either directly, or indirectly through the gravity wave-tidal interactions, to the variation of the strength of the time-mean westward flow associated with the MSAO.