Summary
The amplitude and phase of the semidiurnal tide in the mesosphere and lower thermosphere above Davis varies through the summer months. In particular, the phase can oscillate around a fixed value or, at some heights, shift by a whole cycle of local time. Recent studies of the semi-diurnal tide at the South Pole have suggested that an s=1 mode is common at high southern latitudes. At mid-latitudes, the s=2 mode is thought to dominate suggesting that a region where these modes overlap is a possibility. Although it is not possible to discern which of these modes are present using single station data, the concept that more than one 12-hour wave is present allows a new interpretation of the Davis semi-diurnal amplitudes and phases. In this study, wind data obtained at Davis, Antarctica, during the summer of 1996/97 are used to show that the complex variations in phase and amplitude can be explained by a sum of two waves. A first order model that combines an invariant and a varying 12-hour wave yields characteristics for each component that are less complex than the original observation. There is also potential for changes to the interpretation of phase vs height profiles.