Short-time variability of the atmospheric tides is frequently observed in the meteor region but is not yet fully explained in terms of production mechanisms. This is probably due to the existence of several such mechanisms acting together or separately. In this paper we show that many observations can be explained by nonlinear interactions between tides and planetary waves having periods corresponding to those of the observed tidal amplitude modulations. These nonlinear interactions generate two secondary waves whose frequencies are the sum and difference of frequencies of the primary waves. These two waves beat with the tide, modulating its amplitude with the planetary wave period. A numerical model is used to demonstrate that with primary waves of reasonable amplitudes the nonlinear interactions can be quite large. This is because the importance of nonlinearity depends essentially on the amplitude of the induced fluid velocity in the direction of wave propagation compared to the wave propagation velocity. When two waves propagate simultaneously, the fluid velocity can have a large component in the direction of propagation of one of the waves, and advective (nonlinear) terms can be large. This point is further illustrated in the case of two gravity waves interacting together. Finally, some observational campaigns carried out above Garchy (45°N) are analyzed using a nonparametric method. The results indicate that nonlinear interactions between tides and planetary waves really take place in the upper mesosphere and lower thermosphere.

On tidal variability induced by nonlinear interaction with planetary waves

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

Radars have been used successfully for many years to measure atmospheric diurnal and semidiurnal tides, but studies of the atmospheric terdiurnal tide have only rarely been reported to date in the literature. In this paper we investigate the amplitude, phase, and vertical wavelength for the terdiurnal tide using the University of Western Ontario (UWO) 2 MHz radar near London, Canada. Emphasis is placed on short-term, monthly, seasonal, and annual variations of the terdiurnal tide. The results of the present study bring out certain new aspects of the terdiurnal tide at midlatitudes in the northern hemisphere. We show that terdiurnal tides have significant amplitudes in the mesosphere and lower thermosphere; indeed, the terdiurnal amplitudes are often comparable to the diurnal ones. We also draw attention to the annual variability and especially highlight the occurrence of the terdiurnal tide in non winter months. For example, the occurrence of the terdiurnal tide in spring months is not uncommon. Thus it seems that the 8-hour component observed over London, Canada, cannot be explained by the terdiurnal tide being due to solar heating alone. Particular cases of the terdiurnal tide are examined, and the observed amplitude and phase profiles of the terdiurnal tide at certain times of the year are discussed in terms of nonlinear interaction between the diurnal and semidiurnal tides. We suspect that this effect may also explain the fact that diurnal and semidiurnal tides at our station often seem suppressed relative to other sites [Thayaparan et al., 1995a]. On the basis of our previous and present observations, it is also possible that large terdiurnal amplitudes observed at certain times of the year may be partly explained in terms of tidal/gravity wave interactions. However, this possibility needs to be further investigated in the future.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/97JD01839

The terdiurnal tide in the mesosphere and lower thermosphere over London, Canada (43°N, 81°W)

The mesopause oxygen rotational temperature imager, MORTI, was operated at Bear Lake Observatory (41.9°N, 111.4°W) during the period November, 1991 to May, 1993. Fluctuations of long period in both emission rate and temperature of the O2 Atmospheric (0–1) nightglow layer are evident that seem related to the diurnal and semidiurnal tides. The data show a dominant semidiurnal tidal mode in January, but a dominant diurnal component at the spring equinox. Other long-period fluctuations also appear for which a linear tidal explanation does not seem applicable.

Tides in emission rate and temperature from the O2 nightglow over Bear Lake Observatory

Tidal winds in the radio-meteor region over Trivandrum (8.5°N, 77°E)

Recent progress in the study of tides in the middle atmosphere ((10÷120) km) includes: i) development of more realistic thermal excitation and numerical simulation models; ii) observational efforts which delineate average seasonal, latitudinal and vertical structures of tides and shorter term variations of these. A survey is presented here of the variation with time and height of atmospheric tides observed at Budrio (45° N, 12° E) in the wind field between 75 and 115 km of altitude during the period 1978–82. Thermal excitation of the tides is mainly due to absorption of solar incident flux by ozone in the stratosphere and water vapour in the troposphere, release of latent heat in clouds, and heat conduction at the ground. The spectral analysis of wind data, obtained at Budrio in a few days of each month of 1978, shows that the semi-diurnal tide is the most easily observable by its regularity and amplitude: consistent amplitudes (around 20 m/s) are recorded in Summer and Winter months at 95 km. The vertical profiles of amplitudes of the diurnal, semi-diurnal and terdiurnal tide observed in zonal winds in some periods of 1979 (January 11–26, March 14–28, July 26–August 4, and September 11–23) demonstrate the great variability of tides not only over a large time scale (seasonal variations) but also on a reduced scale (variations in a few days).

G. Cevolani

Papers

Tidal activity in the middle atmosphere