The semiannual oscillation of diurnal tides in the mesosphere and lower thermosphere and their dependence on forcing and wind coupling processes

The vertical structure of diurnal tides (DT) is strongly determined as well by forcing processes as propagation processes controlled by the three dimensional background wind fields. Both processes are important by forming the semiannual oscillation (SAO) of DT in the mesosphere and lower thermosphere (MLT) observed in satellite soundings. We have used as basic data, the tidal responses of a 20 year run of the atmospheric general circulation model (GCM) HAMMONIA, which includes atmospheric chemical interactions, too. The analysed SAO show good agreement with observations by McLandress and Forbes who have investigated both, migrating and nonmigrating components of DT. The newly developed three dimensional linear model LIN-KMCM based on the simple GCM KMCM has been used to study the influence of the background state on the tidal propagation processes as well as the different forcings. It could be shown that the most important process in forming the SAO seems to be the ‘wind' variations through out the year for the migrating component of DT. On the contrary the seasonal dependence of forcings is rather important for the SAO of nonmigrating components (westward moving two (w2) and eastwards moving three (e3)). The influence of stationary planetary waves on the tides in the MLT could be determined to be important for nonmigrating components, only. Nevertheless, their influence on the SAO in the MLT seems to be of secondary importance. Besides the linear processes (tidal propagation in zonally symmetric state or in a longitude dependent state, including planetary waves, and the dependence of the DT on different forcings) the quasi linear behaviour of tides with respect to the background winds will also be discussed. Using the simple GCM KMCM, (which is fully nonlinear, realising the radiation forcings by linear relaxation to a given equilibrium temperature) we study the influence of the wind field on tidal propagation either with or without tidal interactions with the background. The differences are shown and their influence on the formation of the SAO are discussed.