The dependence of the nonmigrating diurnal tide in the mesosphere and lower thermosphere on stationary planetary waves

The diurnal tide in the mesosphere/lower thermosphere has been studied in a new linear model which facilitates the analysis of the impact of stationary planetary waves on all tidal components. From the ground to the mesopause this model uses a longitude dependent background atmosphere essentially extracted from monthly means of a general circulation model including the middle atmosphere (MA-GCM), and as forcing the monthly mean daily cycle in the heating rates due to solar infrared and ultraviolet radiation, convection, and latent heat release, diagnosed from the same model. A comparison between the solar diurnal tide in the GCM and in the linear model shows good agreement. The planetary-wave impact has been analyzed both for the migrating and the nonmigrating component under solstice conditions. It is found that there is no direct effect on the migrating tide, although such might be mediated by planetary-wave associated changes in the zonal-mean wind. The nonmigrating tide, however, appears to be considerably influenced by the interaction between stationary waves and the migrating part of the diurnal tidal heat source. This stationary-wave affected part is an essential component which is indispensable for a correct interpretation of the total nonmigrating tide. In July the interaction produces, due to the impact of the planetary-wave component at zonal wave number 1 a nonmigrating component dominated by the wave numbers 0 and 2. in January we find a dominant contribution at wave number 3, originating from the impact of the stationary wave at wave number 2. Experiments with increased planetary-wave activity at wave number 1 (oriented at available observations) indicate that under most realistic conditions also the January part of the nonmigrating tide affected by the stationary planetary waves might be dominated by the same wave numbers as in July. (C) 2004 Elsevier Ltd. All rights reserved.