Tidal perturbations (at integral subharmonics of a solar day) are mainly due to vertically propagating waves excited in the stratosphere and troposphere triggered by the absorption of sunlight by water vapor and ozone. Tidal components are typically classified according to Hough modes (a mathematically concise set of functions for the description of motions in a spherical atmosphere) which describe their latitude structure and period. The most important tides are the diurnal (1,1) and semidiurnal (2,2), (2,3), (2,4), (2,5) and (2,6) (Hough-mode) components which are westward-propagating or migrating (i.e., they migrate with the apparent motion of the sun). Planetary waves are longer period waves that can either be stationary (with respect to the Earth's surface) or can also propagate. Tides and planetary waves and their effects have been recently reviewed by Forbes [1994].
Forbes et al., [1993] have discussed the dynamical
effects of the (1,1) diurnal tide, showing how the tide can
accelerate the zonal flow in the lower thermosphere due to
molecular and eddy dissipation. As indicated in Figure 2, the
diurnal tide's growth is limited by ion drag and molecular
diffusion at altitudes greater than about 100km. Forbes et
al. also predicted a multi-cellular structure in the meridional
wind near the equator (<30
latitude) due to the dissipation
of the diurnal tide. The recent UARS measurements of the
climatology of the (1,1) diurnal tide [ Hays et al., 1994]
have revealed significant semi-annual and well as other shorter
term variations in the tidal amplitude. Encouraging agreement
between the UARS observations and the earlier Forbes and
Gillette [1982] theory was obtained, though much more comparative
work with more recent models remains to be done.
Characterization of the global semi-diurnal tides has been the subject of much recent work [see review by Fuller-Rowell, 1994, and references therein]. The magnitudes of the observed amplitudes are highly variable, presumably due to non-linear interactions between the tides and the mean flow, planetary waves and/or gravity waves [ Walterscheid, 1994; Fritts, 1994]. Vial [1993] reviews proposed mechanisms for generating tidal variability. A sequence of papers [ Forbes and Vial, 1991; Fesen et al., 1991a, b; 1993a] has led to the development of an improved specification of tides in terms of phases and amplitudes for specified geophysical conditions. These specifications have been used extensively in earlier versions of the TIMEGCM, though the most recent version of the model allows for self-consistent calculation of the semi-diurnal tides. It is clear, however, that the large degree of tidal variability, and the limited available experimental coverage even with the UARS data, makes the establishment of a complete tidal climatology very difficult to achieve. A climatology in which quantitative confidence can be placed for a range of geophysical conditions will probably require closely coordinated future satellite and ground-based monitoring of mesospheric and lower thermospheric winds and temperatures. The first such attempts are being made through national and international coordinated programs (e.g., NSF's CEDAR program, NASA's TIMED program, and the international Solar-Terrestrial Energy Program, STEP).