Seasonal variations of the terdiurnal tide in the mesosphere and lower thermosphere: a model study

Observations of the terdiurnal (8-hour) tidal wind oscillations in the mesosphere and lower thermosphere (MLT) reveal strong seasonal and latitudinal variations. The tidal amplitudes reach a maximum of about 15-20 m s(-1) at midlatitudes in winter. The vertical phase progression corresponds to very long vertical wavelengths. During summer the amplitudes are typically weaker and the phases are less regular. First simulations of the terdiurnal tide with the Spectral Mesosphere/Lower Thermosphere Model are presented. The model explicitly accounts for the solar thermal excitation in the middle and upper atmosphere and reproduces the salient features of the terdiurnal wind oscillations well. The preferential solar excitation of antisymmetric terdiurnal modes capable of propagating into the MLT combined with an asymmetric atmospheric response appears to be primarily responsible for the observed seasonal variations. Nonlinear interactions of the diurnal and semidiurnal tides are demonstrated to make a noticeable in situ contribution to the excitation of the terdiurnal tide at 95-100 km, especially during equinox.