Abstract
This paper studies inter-annual variations of 6.5-Day Waves (6.5DWs) observed at altitudes 20−110 km between 52°S−52°N latitudes during March 2002−January 2021, and how these variations were related to the equatorial stratospheric Quasi-Biennial Oscillation (QBO). Temperature amplitudes of the 6.5DWs are calculated based on SABER/TIMED observations. QBO zonal winds are obtained from an ERA5 reanalysis dataset. QBO phases are derived using an Empirical Orthogonal Functions (EOF) method. Wavelet analysis of the observed 6.5DW variations demonstrates obvious spectral maximums around 28−38 months at 32°N−52°N, and around 26−30 months at 32°S−52°S. In the Northern Hemisphere, peak periods lengthened poleward; in the Southern Hemisphere, however, they were unchanged with latitude. Residual 6.5DWs amplitudes have been determined by removing composite amplitudes from 6.5DWs amplitudes. Comparisons between QBO and monthly maximum residual 6.5DWs amplitudes (<inline-formula><tex-math id="M1">\begin{document}$ {A}_{\mathrm{M}\mathrm{m}\mathrm{a}\mathrm{x}} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="RA276-huangyingying-F_M1.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="RA276-huangyingying-F_M1.png"/></alternatives></inline-formula>) show clear correlations between the QBO and 6.5DWs in both hemispheres, but the observed relationship is stronger in the NH. When <inline-formula><tex-math id="M2">\begin{document}$ {A}_{\mathrm{M}\mathrm{m}\mathrm{a}\mathrm{x}} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="RA276-huangyingying-F_M2.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="RA276-huangyingying-F_M2.png"/></alternatives></inline-formula> were large in the NH, the mean QBO profile was easterly at all levels from 70 to 5 hPa; when the <inline-formula><tex-math id="M3">\begin{document}$ {A}_{\mathrm{M}\mathrm{m}\mathrm{a}\mathrm{x}} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="RA276-huangyingying-F_M3.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="RA276-huangyingying-F_M3.png"/></alternatives></inline-formula> were weak, the mean QBO wind was weak westerly below 30 hPa. Linear Pearson correlation coefficients between QBO phases and <inline-formula><tex-math id="M4">\begin{document}$ {A}_{\mathrm{M}\mathrm{m}\mathrm{a}\mathrm{x}} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="RA276-huangyingying-F_M4.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="RA276-huangyingying-F_M4.png"/></alternatives></inline-formula> show large positive values at 60−110 km between 20°N−52°N in April and around 64 km at 24°S in February, and large negative values from 80 to 110 km between 20°N−50°N in August and at 96−106 km between 20°S−44°S in February. These results indicate quantitative correlations between QBO and 6.5DWs and provide credible evidences for further studies of QBO modulations on long-term variations of 6.5DWs.
Highlights
6.5DWs studied here are reasonable to be recognized as 5-day waves (5DWs), 6-day waves (6DWs) or 5−7-day waves mentioned in those studies. 6.5DWs’ amplitudes in temperature and horizontal wind are found dominant at three separate vertical layers around stratosphere (40−50 km), mesosphere (80−90 km) and the lower thermosphere (100−110 km) (Talaat et al, 2001; Liu et al, 2004; Jiang et al, 2008a; Huang, et al, 2017), respectively
Quasi-Biennial Oscillation (QBO)-like modulations are found from inter-annual variations of different wave parameters in mesosphere and the lower thermosphere (MLT) regions, e.g., vertical flux of both zonal and meridional momentum of gravity waves (GWs), amplitudes of semi-diurnal migrating tides (Laskar et al, 2016) and planetary waves (PWs) (Miyoshi and Hirooka, 2003; Kishore et al, 2004; Lima et al, 2005; Li et al, 2008; Pancheva et al, 2010; Merzlyakov et al, 2013), etc
Data and processes utilized in this paper are introduced in Section 2; inter-annual variation characteristics of 6.5DWs are shown in Section 3; relations between inter-annual variations of 6.5DWs and QBO are shown in Section 4; discussions and conclusions are demonstrated in section 5 and 6, respectively
Summary
6.5-Day Waves (6.5DWs) are westward propagating planetary scaled waves in earth’s atmosphere from stratosphere to the lower thermosphere with period 6−7 days and zonal wavenumber 1 (Wu et al, 1994; Meyer and Forbes, 1997; Talaat et al, 2001; Kishore et al, 2004; Liu et al, 2004; Lima et al, 2005; Jiang et al, 2008a, 2008b). For most studies on correlations between QBO and atmospheric dynamics, QBO phases are determined by zonal wind directions at a single level over the equatorial region, usually presentative 30 hPa or 50 hPa (de Wit et al, 2016; Laskar et al, 2016; Merzlyakov et al, 2013; 2015) It brings out ambiguity (Anstey and Shepherd, 2014; Baldwin and Dunkerton, 1989; Solomon et al, 2014) as that, on one hand, altitudes used to define QBO phase is a posteriori in order to maximize the correlation with extratropical variables (Anstey and Shepherd, 2014; Baldwin and Dunkerton, 1989), and on the other hand, the correlation between QBO phases and wave intensities obtained in these studies should be opposite, because zonal winds of QBO vary or even reverse with altitude. Data and processes utilized in this paper are introduced in Section 2; inter-annual variation characteristics of 6.5DWs are shown in Section 3; relations between inter-annual variations of 6.5DWs and QBO are shown in Section 4; discussions and conclusions are demonstrated in section 5 and 6, respectively
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