## 2 Observations and Method for Data Analysis
### Climatology, Seasonal and Interannual Variability of the Nonmigrating Diurnal Tides
A careful inspection of the decomposition results indicated that the 4- and 3-month components are quite weak in comparison with the AC and SAC ones, so they will no longer be considered. Figure 3a shows the latitude-time structures of the AM (upper plot), AC (middle plot), and SAC (bottom plot) separated from the monthly mean DE3 amplitudes at an altitude of 110 km, while Figure 3b,c presents the same as Figure 3a, but for DE2 and DW2, respectively. Figure 3a reveals quite large AM and AC of DE3 amplitudes reaching \(\sim\)13.5 K and \(\sim\)9.5 K. Both seasonal components are centered over the equator having large amplitudes between latitudes of \(\pm\)10\({}^{\circ}\). The quasi-2-year variability of these seasonal components can be well distinguished. The SAC of DE3 is significantly weaker than that of AC with the largest amplitude of \(\sim\)4.3 K. It is seen that this component is distributed between latitudes of \(\pm\)30\({}^{\circ}\); it demonstrates some quasi-2-year variability, but there is also another, longer-term one. Considering the main seasonal component of DE2 (Figure 3b), it is right away evident that the SAC (4.3 K) is larger than the AC (3.8 K) one, i.e., a feature that was noticed from Figure 2 (middle plot) and mentioned above. The AM (7.1 K) of DE2 has maxima located between 15\({}^{\circ}\) N and 10\({}^{\circ}\) S depending on which solstice (June or December) maximum has the largest amplitude at the given month. During most of the time, the AC of DE2 is distributed between 10\({}^{\circ}\) N and 20\({}^{\circ}\) S, while the SAC is between \(\pm\)20\({}^{\circ}\). The quasi-2-year variability is best seen in AM, but can also be noticed in the other two seasonal components of DE2. Figure 3c presents the main seasonal components of DW2. According to the climatology of this tide (Figure 1) the seasonal components are also located over the equator  
and around \(\pm 50^{\circ}\). Considering the AM (5.3 k) of DW2, there is a better synchrony between the enhancements above the equator and those around \(50^{\circ}\) N than with those in the SH. The use of the full 21 years of observations to investigate some nonmigrating tides at lower thermosphere provides an opportunity to examine their interannual variations. It is known that the 11-year solar cycle (SC) and the quasi-biennial oscillation (QBO) has long been observed to be globally prevalent, as the QBO impact, particularly at low and low-middle latitudes, is a predominant one [80; 81]. The QBO is the dominant signal in the stratosphere, and although it is a tropical phenomenon, it modulates the stratospheric flow from pole to pole [82]. The solar radio flux index F10.7 is used as a proxy of solar activity, and the monthly mean data were downloaded from the website: [[https://omniweb.gsfc.nasa.gov/ow.html](https://omniweb.gsfc.nasa.gov/ow.html)]([https://omniweb.gsfc.nasa.gov/ow.html](https://omniweb.gsfc.nasa.gov/ow.html)) (accessed on 9 April 2024). The QBO index at 30 hPa is obtained from [[https://www.cpc.ncep.noaa.gov/data/indices/](https://www.cpc.ncep.noaa.gov/data/indices/)]([https://www.cpc.ncep.noaa.gov/data/indices/](https://www.cpc.ncep.noaa.gov/data/indices/)) (accessed on 10 April 2024). The SC and QBO impact on the seasonal components of the considered DE3, DE2, and DW2 tides will be investigated using across-correlation analysis between the monthly mean F10.7 index and QBO data and the respective monthly mean seasonal components presented in Figure 3.  
Figure 2: Latitude-time structures of the monthly mean DE3 (**upper** plot), DE2 (**middle** plot), and DW2 (**bottom** plot) tidal amplitudes at an altitude of 110 km. Figure 4a (left column of plots) presents the cross correlation functions between the QBO at 30 hPa and the AM (upper plot), AC (middle plot) and SAC (bottom plot) of the DE3, while Figure 4b (middle column of plots) and 4c (right column of plots) show the same as Figure 4a, but for DE2 and DW2.