Quadratic nonlinear interactions between atmospheric tides in the mid-latitude winter lower thermosphere

Abstract

Data obtained from the Wuhan/China MF radar in 2001 are used to study the quadratic nonlinear interactions between tides in lower thermosphere. It is observed that diurnal, semidiurnal and terdiurnal tides are the prominent perturbations in the meridional wind component near mid-latitude winter mesopause region, and the quarterdiurnal tide is clear. By bicoherence spectrum analysis, it is revealed most prominent bicoherence peaks stand for phase correlation between tidal harmonics or self-coherence of a single tidal wave. By examining the vertical wavelength time variations, a significant correlation is found between the vertical wavelength of the observed terdiurnal tide and that of the supposed nonlinearly generated one. In the interval of 94.0–98.0km, there exist not only a certain phase correlation and vertical wavenumber correlation but also a strong amplitude correlation of the oscillatory amplitudes equivalent and oscillatory phases synchronous or reversed between the prominent tides, indicating a wave–wave quadratic interaction has occurred. The time and height variations of tidal amplitudes just combine to show that the nonlinear interactions between tides make their respective energy redistributed through the accumulation of interactions and thus change the power spectral structure. Below 94.0km, the various correlations between the tides grow weaker and weaker with descending height and hence the tidal quadratic interaction is more likely a local and temporary phenomenon. Overall, the spectral amplitudes of all concerned tidal harmonics gradually increase, reach their 80.0–98.0km interval maxima, and then decay in turn from lower frequency components to higher frequency components with increasing height. Besides probably associating with the vertical distribution variations of the local generating sources and various complex dissipative effects, this “pattern” closely correlates with the quadratic nonlinear interactions between tides and gravity waves and planetary waves.