Increased attention is currently paid to studying the so-called "secondary" acoustic-gravity waves (AGWs), which appear due to instabilities and nonlinear interactions of "primary" wave modes generated by atmospheric sources. This report is devoted to the study of horizontal spatial spectra of primary and secondary AGWs at fixed altitude levels in the middle and upper atmosphere using a high-resolution three-dimensional nonlinear model AtmoSym. It is found that in a short time after turning on the source of plane waves at the lower boundary of the model, the spectrum contains mainly a peak related to the primary AGW. Later, spectral peaks corresponding to secondary AGWs appear at horizontal wave numbers that are multiples of the wave numbers of the primary wave. This study allows estimating relative contributions of secondary AGWs at different heights, different times, and for different atmospheric conditions.
The evolution of various nonlinear processes associated with the propagation of stationary planetary waves (SPW) during sudden stratospheric warmings (SSW) is studied from the troposphere up to the lower mesosphere levels. Based on data of the UK Met Office (UKMO), the spatiotemporal structure of the processes of planetary waves interaction with each other and with the mean flow, as well as wave activity, potential enstrophy flux divergence and advection, was analyzed. Such an analysis was performed for the 2008-2009 winter, when an SSW, accompanied by the splitting of the stratospheric polar vortex, was observed and for the 2018-2019 winter, when an SSW with the displacement of the stratospheric polar vortex was observed. The results show that these SSWs are succeeded by significant differences in the nonlinear processes under consideration. It is also demonstrated that during the SSW with the stratospheric polar vortex displacement the contribution of various processes to the balance of the disturbed potential enstrophy for SPWs with different zonal wavenumbers is comparable. In addition, during the SSW with the stratospheric polar vortex splitting, the exchange of waves with the mean flow makes the greatest contribution to the perturbed potential enstrophy balance.
In the stratosphere, there is a correlation between changes in the amplitudes of stationary planetary waves. This correlation is most clearly manifested during sudden stratospheric warmings (SSWs). An analysis of wave-wave and wave-mean flow interactions during winter 2008-2009 and 2018-2019 sudden stratospheric warmings was made using the equation of perturbed potential enstrophy. It is shown that wave-wave interactions make the least contribution to the wave activity variation during the 2008-2009 SSW, the contribution of all interactions is comparable during the 2018-2019 SSW.
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