Inter-annual variability of tides in the mesosphere and lower thermosphere

The inter-annual variation in diurnal and semi-diurnal atmospheric tides between 85 and 95 km has been studied for various years between 1978 and 1988. Observations comprised wind measurements from the medium frequency SA mode wind radars at Adelaide (35°S), Christchurch (44°S) and Saskatoon (52°N) and the meteor wind radar at Durham (43°N). Although the observations include the interval between solar maximum and solar minimum, there is in general no correlation between tidal amplitudes and solar activity. In contrast with earlier studies there does appear to be a positive correlation between solar activity and the amplitude of the semi-diurnal tide, but only during the southern summer and simultaneous northern winter.     

Dynamic coupling between the lower and upper atmosphere by tides and gravity waves

Results of a General Circulation Model simulation of the dynamics of the middle atmosphere are shown focusing our attention to the tidal wave mean flow interaction and propagation of migrating diurnal and semidiurnal tides in the model. It is shown that migrating tidal waves are well simulated and the amplitude growth with height is effectively suppressed by the convective adjustment in the model. It is also shown that the dissipating solar diurnal tide plays an important role in inducing mean zonal winds in the low latitude region of the lower thermosphere. The behavior of non-migrating diurnal tides is also analyzed to show that non-migrating diurnal tides have significant amplitudes in the lower thermosphere. It is suggested that the non-migrating diurnal tide, which propagates against background mean zonal winds, has the possibility to propagate into the middle to high latitude region due to the Doppler effect.     

Recent progress in tidal modelling

Recent progress in identifying the physical mechanisms governing the propagation of atmospheric tides and in performing numerical simulations of the phenomenon, are reviewed. Specific attention is given to gravity-wave/tidal interactions, non-linear interactions between tides, numerical simulations of the diurnal tide, and month by month simulations of the semidiurnal tide.     

On tidal variability and the existence of planetary wave-like oscillations in the upper thermosphere—I. Observations of tidal variability

The evidence for the existence of tidal variability as observed in the meridional thermospheric wind (approx. 300 km height) is presented for a set of eight ionosonde locations (three in the northern hemisphere and five in the southern hemisphere). The data set corresponds to a full year (1984) of hourly values. The detected variability can be seen in the tidal components of the meridional wind. The diurnal and semidiurnal components are spectrally analysed. The quarterly spectra show that the tidal amplitudes oscillate with periods between 2 and 60 days. The more important oscillations have periods from 15 to 3 days. No direct link between solar and magnetic activity indices was detected. Possible reasons for the observed tidal variability are discussed in the light of the current theory developed for the mesosphere and lower thermosphere.     

Meteor radar observation at Kyoto in two C.T.O.P. periods (July 20–August 7, 1978, and March 13–29, 1979)

Meteor radar data taken in 1978–1979 at Kyoto station (35°N, 136°E) are processed statistically by Groves' algorithm (1959). Analysis is done especially for data in two C.T.O.P. campaigns carried out in summer and in spring to study a seasonal variation of tides. The reliability of the algorithm is clarified by a simulation with a realistic model. Vertical wavelengths of the diurnal tide and the semidiurnal tide are estimated from height profiles. The diurnal tide becomes evanescent in summer, and the semidiurnal tide appears to be the S_2,2 mode in summer and the S_2,4 mode in spring. This suggestion regarding the semidiurnal tidal modes agrees with the latitudinal variation deduced from many other meteor radar data. Tidal wave energy in the meteor region is estimated. A sudden enhancement of energy is observed in both the diurnal and the semidiurnal tides. There is a clear correlation between an energy decrease of the diurnal tide and an energy increase of the semidiurnal tide.     

Tides in the middle atmosphere

Some recent progress in the study of tides in the middle atmosphere are reviewed, with special emphasis placed on radar observations at high latitudes, as well as data analysis methods used in the calculation of tidal structures. Observations carried out outside the meteor zone with MST radars and satellites are also presented. Theoretical and numerical advances on the diurnal tide are extensively discussed. Finally, some outstanding problems, which we hope will be solved in the near future are raised: the existence of hemispheric asymmetries in tidal structure; the role played by non-migrating modes at meteor heights and short time scales variations of tides.     

GPS satellite measurements: ionospheric slab thickness and total electron content

A preliminary analysis was made of ionospheric slab thickness, τ, and total electron content, TEC, for southern Australia using GPS satellite measurements. It was found that at mid-latitudes τ has similar overall diurnal, seasonal and latitudinal variations in the southern hemisphere as in the northern hemisphere. However, there are appreciable differences between τ in the two hemispheres which would justify appropriate modifications to ionospheric models based on northern hemisphere data before being applied confidently to the southern hemisphere. The usefulness of GPS satellites together with ionosondes over a spread of latitudes was demonstrated in determining long-term variations of TEC and τ over a large area. It was concluded that as few as four GPS receivers could provide TEC for the whole of Australia in real-time, though approximately six receivers in convenient locations would be required in practice.     

Asymmetries in mesospheric tidal structure

Radar wind measurements made at Adelaide (35°S, 138°E) and Kyoto (35°N, 136°E) are used to construct climatologies of solar tidal wind motions in the 80–185 km region. The climatologies, in the form of contour plots of amplitude and phase of the diurnal (24 h) and semidiurnal (12 h) tides, show that there are significant asymmetries between Adelaide and Kyoto. The amplitude of the diurnal tide is significantly larger at Adelaide than at Kyoto. At both stations the phase changes in a systematic way with lime such that the phases of the zonal wind components tend to be in anti-phase at the solstices. At Adelaide, there is more evidence of the propagating (1,1) diurnal mode. At both stations, the semidiurnal tide is strongest and has the longest vertical wavelengths (>100 km) in late summer; short vertical wavelength (~ 50–80 km) oscillations are most in evidence in winter. In order to place the Adelaide and Kyoto observations in context they are compared with observations made at other latitudes and with recent numerical simulations. There is encouraging agreement between the observations and models, especially for the semidiurnal tide.     

A tidal wind model for 85–120 km based on CIRA-86: a feasibility study

This paper examines the feasibility of deriving a climatology of the diurnal variations of the wind in the 85–120 km region from the tidal components of temperature, density, and composition contained in the new COSPAR International Reference Atmosphere, CIRA-1986, Part I: Thermosphere Models [(1988), Adv. Space Res.8, 9]. To derive the wind field, we used the zonal and meridional momentum equations which have been modified from the characteristic scales of the tidal components observed in the 85–120 km region. The CIRA temperature and density model was used to derive the eastward (westerly) and northward (southerly) pressure gradient forces which serve as the forcing functions in the coupled momentum equations. Ground-based wind data from the Mesosphere-Lower Thermosphere (MLT) radar network is used as an independent data set to check the accuracy of the derived tidal wind model. At midlatitudes, the model reproduces some of the general features observed in the radar tidal data, such as the dominant semidiurnal tide with increasing amplitude with height and clockwise (counterclockwise) rotation of the velocity vector observed in the northern (southern) hemisphere. The model overestimates the semidiurnal amplitudes observed by radar by 50–75% during most seasons with the best agreement found during the equinoctial months. The model exhibits little phase variation with height or season, whereas the radar data exhibit a downward phase progression during most seasons (other than summer) characteristic of upward propagating tidal waves, and large seasonal phase variations associated with seasonal changes in vertical wavelengths. The diurnal tidal amplitudes, which are generally 5–20 m s⁻¹ at mid-latitude radar stations and are dominant over the semidiurnal amplitudes at lower latitudes, are less than 5 m s⁻¹ at all latitudes in the model.     

A nonlinear simulation of the thermal diurnal tide

A simplified middle atmosphere general circulation model is used to investigate the nonlinear behavior of the thermal diurnal tidal waves. In the model, only a westward moving diurnal tide generated by heating with zonal wavenumber 1 is considered. The tidal wave propagation is simulated by a full nonlinear calculation with a convective adjustment scheme and a Richardson number dependent vertical eddy diffusion.The numerical results show that the growth of the diurnal tide due to the density stratification is effectively suppressed and a relatively constant amplitude distribution with height is realized by the convective adjustment in the lower thermosphere. It is also shown that mean zonal winds and mean meridional circulations are induced by the diurnal tidal waves in the region where the tidal waves are breaking by convective instability, in accordance with the wave-mean flow interaction theorem.     

The effects of non-linearity on thermal tides in a 3-D numerical model

Absorption of solar radiation is taken as the cause of atmospheric tides, which are simulated by a global, 3-D primitive equation model for the altitude region 0 km–120 km. To investigate non-linear effects, two model versions are used, one with the complete non-linear equation set, the other with the linear equation set. Tide simulations are then performed under the same conditions (background atmosphere and radiation for solstice) with both model versions and directly compared. The diurnal tide can be regarded as a linear phenomenon, whereas the semi-diurnal tide is modified in lower latitudes above 65 km by non-linear effects. Due to the interaction of the tidal components with the background wind, a strong westward zonal flow is generated in the non-linear model above 70 km.     

Effects of solar tides on the zonal mean circulation in the lower thermosphere: solstice condition

Effects of momentum deposition due to solar diurnal and semi-diurnal tidal waves on the zonal mean circulation in the mesosphere and lower thermosphere for a solstice condition are discussed. In the present model, the system of zonally averaged equations and the system of perturbation equations are integrated simultaneously, so that the propagation of tidal waves is affected not only by the basic mean fields but also by the induced zonal mean fields due to the momentum deposition. Results for two different vertical eddy diffusion profiles are presented. It is shown that the solar tides make a significant contribution to the generation of the mean zonal winds in the upper mesosphere and the lower thermosphere. Below 120 km the main contribution is due to propagating diurnal tides, while above 120 km it is due to semidiurnal tides.     

Winds oscillations (∼ 6h–6days) in the upper middle atmosphere at Monpazier (France, 45°N, 1°E) and Saskatoon (Canada, 52°N, 107°W) in 1979–1980

The medium frequency radar (∼ 2.2 MHz) at Saskatoon has been run continuously since 1978 and the Meteor Radar at Monpazier ran continuously for ∼ 10 day intervals in most months of 1979/1980. The radars are separated by ∼ 8000 km. Because of the excellent quality of the data, spectral and harmonic analyses have been completed from ∼ 70 to 100km and oscillations with periods of ∼ 6h–6days studied.There are substantial similarities in the mean zonal winds, both with regard to magnitudes and times of seasonal reversals; also in annual and semi-annual oscillations. In general, the semi-diurnal tide has similar amplitudes, phases and vertical wavelengths : there are consistent summer (long λ) and winter (short λ) characteristics, with rapid transitions between them. Differences between the timing of these transitions and in some of the mid-season tides are discussed. The diurnal tide is less regular and of smaller amplitude at both locations, often being too small to reliably characterize at Monpazier. However, seasonal variations between summer and winter months may be discerned.In addition to the 24 and 12 h tidal oscillations, which traditionally are studied in most detail, there is clear evidence for additional osculations near 6,8, ∼ 10 and ∼ 16 h and longer periods of ∼ 2 and ∼ 5 days. The amplitudes of these are often comparable or larger than the ‘dominant’ 24 and 12 h tides. The monthly and seasonal variations of these additional oscillations are studied, as a function of height, at the two locations. There is evidence for large scale (global) and small scale (local) disturbances in the wind field.     

Tides in the 80–100 km height region

The Institute of Experimental Meteorology (U.S.S.R.) has carried out long-term continuous wind velocity measurements by the meteor radar method for the 80–100 km height region. From this experimental data the seasonal and latitudinal variations of atmospheric tides, as well as the spatial and temporal scales of tidal variability, have been determined.Atmospheric variations with a period of about half-a-day are investigated on the basis of a numerical model. A dependence between the mean wind structure and the semi-diurnal oscillation in the lower thermosphere is established. The influence of stratospheric warmings on semi-diurnal oscillations is also discussed. Numerical experiments show that the mean wind variations cannot explain the observed seasonal variations of a semi-diurnal tide.     

Tidal winds from the MLT global radar network during the first LTCS campaign—September 1987

Winds and tides were measured by a number of MLT (Mesosphere, Lower Thermosphere) radars with locations varying from 43–70°N, 35–68°S, during the first LTCS (Lower Thermosphere Coupling Study) Campaign, 21–25 September 1987. The mean winds were globally westerly, consistent with early winter-like (NH) and late winter (SH) circulations.The semi-diurnal tide had vertical wavelengths near or less than 100 km at most locations, with some latitudinal variation (longer/shorter at lower latitudes in the NH/SH)—amplitudes decreased at high latitudes. The global tide was closer to anti-symmetric, with northward components being in phase at 90 km. Numerical model calculations [Forbes and Vial (1989), J. atmos. lerr. Phys. 51, 649] for September have rather similar wavelengths and amplitudes; however, the global tide was closer to symmetric, and detailed latitudinal trends differed from observed.The diurnal tide had similar wavelengths in each hemisphere, with short values (~30 km) at 35°, long (evanescence) at 68–70°, and irregular phase structures at mid-latitudes. The tide was neither symmetric nor anti-symmetric. Model calculations for the equinox [Forbes. S and Hagan (1988), Planet. Space Sci. 36, 579] were by nature symmetric, and showed the short wavelengths extending to mid-latitudes (43–52°). Southern hemisphere phases were significantly (6–8 h) different from observations. Amplitudes decreased at high latitudes in model and observation profiles.     

Monthly simulations of the lunar semi-diurnal tide

Although having a much smaller amplitude than solar tides, lunar tides are also present in the atmosphere. Lunar tides are attractive for theoretical and observational studies because their frequencies and forcing are better determined than for any other atmospheric waves. Lunar tides are generated by the lunar tidal potential which itself is very well determined. However, this potential also affects the Earth and oceans, modifying their mass distributions and elevations, and creating secondary tidal potentials as well as periodic movements of their interfaces with the atmosphere. The periodic load of ocean over the Earth crust due to the tides also induces a secondary modification of tidal potential as well as movement of the atmosphere interface. In our present work we seek to provide a comprehensive model of atmospheric M2 lunar tidal oscillations from the surface to the lower thermosphere (c. 105 km), taking account of the above-mentioned effects i.e. Earth, ocean and load tides. This study is motivated by two facts. First, we now have good determinations of ocean tides using satellite altimetry whereas previous studies of lunar tides were based only on numerical simulations of these ocean tides. Second, some reliable analyses of lunar tides in the lower thermosphere are now available as radars are now operating over a sufficiently long period. Their number should increase in the near future leading to a need for a theoretical study of lunar tides. In this paper, we develop a numerical model and analyse the effects of the different primary and secondary forcings on the lunar tide. Monthly simulations are discussed, and some comparisons with available data (at ground level as well as in the lower thermosphere) are presented.     

Influence of latitudinal and longitudinal variations of ozone and water vapour on the solar semidiurnal tide

A new tidal source model, based on climatological global ozone and water vapour distributions, has been obtained for January, April, July and October. The source model is used for modelling the solar semidiurnal tide in the lower thermosphere within the framework of the classical tidal theory. The observed phase quasibimodality of the semidiurnal tide at middle latitudes is possibly formed, to a great extent, by two types of hemispheric asymmetry (changing sharply near the equinox) of the ozone distribution. Near 95 km at middle latitudes, the ozone and water vapour distribution nonzonality yields typical values ~2 m/s for maximum longitudinal variations of the zonal and meridional wind amplitudes, and the range ~0.2–0.5 h for maximum longitudinal phase variations in the Northern hemisphere, while they can reach ~10 m/s and ~1.5 h in the Southern hemisphere. The hemispheric asymmetry is mainly caused by the effect of the water vapour tidal source.     

Diurnal and semi-diurnal tides in the equatorial middle atmosphere

As part of the DYANA Programme, six rocket launchings (ship-borne) were conducted on three days in the equatorial region (Indian Ocean/Arabian Sea region). Using the temperature and wind data from these launchings, the diurnal and semi-diurnal tidal components in wind and temperature in the middle atmosphere are obtained and are compared with theoretical predictions. It is found that significant departures occur between the observed and theoretical values. The results are discussed in the light of current theoretical understanding of the tides.     

Non-linear interaction between the diurnal and semidiurnal tides: terdiurnal and diurnal secondary waves

Many years of measurements obtained using French meteor radars at Garchy (Lat. 47°N) and Montpazier (Lat. 44°N) are used to show the existence of an 8 h oscillation. Some examples of the structure of this wave are displayed and compared with measurements performed at Saskatoon (Lat. 52°N) and Budrio (Lat. 45°N). This wave can be interpreted as the solar driven terdiurnal tide, or as the result of the non-linear interaction between the diurnal and semidiurnal tides. Both hypotheses are tested with numerical models. Incidentally, the possible existence of a 24 h wave resulting from this interaction is also studied.     

Some results on mean tidal structure and day-to-day variability over Arecibo

A long series of monthly 36 h observations, plus a series of observations at the same local time each day over two months at Arecibo, have been analyzed for tidal structure and variability. Some of the results are as follows. (1) A diurnal tide with vertical structure similar to that of the S_1,1 mode dominates the wind field up to heights of the order of 110 km over Arecibo. A semidiurnal oscillation dominates above that height. For non-winter conditions the semidiurnal oscillation in the 80–200 km region closely resembles the S_2,2 mode, though there is the possibility of contributions from higher order modes in the 100–120 km region. (2) Larger semidiurnal amplitudes are observed in the lower thermosphere for winter conditions. The data appears roughly consistent with Bernard's (1979) hypothesis that S_2,2, S_2,4 and S_2,5 modes are thermally excited, with the S_2,4 and S_2,5 modes out of phase in the meteor region in summer and in phase in winter. (3) The day-to-day variability of the tides is at least half the amplitude of the mean oscillations. The maximum wind variability was observed to occur in the 100–110 km region where the diurnal tide is strongly dissipated. (4) The day-to-day deviations in the wind and temperature oscillations from a long term mean at one local time tend to be wave-like structures which are generally correlated from day to day. The structures tend to move upwards, i.e., appear at a later local time, from day to day.