Observations of the diurnal tide in the mesosphere and lower thermosphere over Christmas Island

Atmospheric tidal variations in the mesosphere wind field have been studied extensively using radars. Most of the climatological studies of the tide have been done at middle and high latitudes. Previous tropical observations of the diurnal oscillation derived from campaign studies have shown the dominance of the migrating diurnal tide in the mesosphere and lower thermosphere. By using wind measurements obtained with a meteor radar system located at Christmas Island; the diurnal tidal behavior in the upper atmosphere of the equatorial Pacific has been characterized over several years. The data collected from August 1988 to December 1993 are presented in this paper and compared to various model results. Our observations show the meridional wind component of the diurnal tide maximizes in April near 94 km. The observed meridional vertical wavelengths are large and indicate upward propagating energy. Even though the vertical wavelengths for the zonal diurnal component are also large, the phase progression is upward corresponding to downward propagating energy. There is considerable interannual and intraseasonal variation in the diurnal amplitudes throughout the year.