The linear interpretation of semi-annual oscillations of migrating and nonmigrating diurnal tides in the mesosphere and lower thermosphere, revealed from different general circulation models

Three general circulation models (GCM) have been used to analyse the diurnal tides for the migrating (spatial wave number (s) one) and the most important nonmigrating components (s: 2, 0, -1,-2,-3) in the mesosphere and lower thermosphere (MLT), where positive wave numbers describe westwards travelling, and negative ones describe eastwards travelling components. Each model covers the whole atmosphere up to the thermosphere. We analysed model data of a 20-year run of HAMMONIA (Max-Planck Institute for Meteorology, Hamburg, Germany), data of a 2-year run of the CMAM (Canadian Middle Atmosphere Model, Department of Physics, University of Toronto, Canada) and the one-year output of WACCM-1 (Whole Atmosphere Community Circulation Model, National Center of Atmospheric Research, Boulder, USA).

The modelled diurnal tides are comparable with observations based on satellite measurements or ground-based meteor radar soundings.

The migrating component and the nonmigrating one with s=2 show a strong semi-annual oscillation signature with amplitudes of 12 m/s and 5 m/s in the meridional wind tides, respectively. For the nonmigrating components with s=0, -1 the amplitudes are lower than 3-4 m/s.

With the newly developed linear tidal propagation model LINKMCM, taking into account the horizontal and vertical dissipation as well, the linear results show that the quasi semi-annual oscillations (s=2, 1) are mainly controlled by the propagation terms. From the linear results it seems to be evident that mostly the seasonal dependence of background wind fields in the stratosphere and lower mesosphere (about up to 70 km height) are responsible for the tidal structure in the MLT. Only for the nonmigrating components (s=0, -1) the annual dependences of the forcings show remarkable influences onto the annual structure.

Comparing the linear results with and without planetary waves in the background fields there is a clear evidence for the planetary wave influence onto the tidal MLT structure. The strongest influence can be found for the s=2 (up to 60 %) and s=-2, -3 components (about 30 %).