## 3 Results and Analyses
### Time-Height Distributions of the Monthly and Zonal-Mean GW Ep  
To investigate the temporal variations of the global GW activities, the time-height distributions of the monthly and global/zonal-mean GW Ep from 2007 to 2020 at 20-35 km height are drawn and are presented using logarithmic scale in Figures 6 and 7, respectively. In these heat maps, the horizontal axis is the time series with the step length of one month, and the vertical axis is the altitude. As we can see in Figure 6, in general, the magnitudes of the monthly global-mean GW Ep in 20-30 km are greater than those in 30-35 km. The maximum values of the global mean GW Ep appear in the summer solstice and the winter solstice every year, which is the most distinct in 25-30 km. This is consistent with [PERSON] et al. [9], although the altitude range of the GW activity they were concerned about was 30-100 km. Moreover, Figure 6 presents that the magnitudes of GW Ep values before 2016 were significantly larger than those after 2016, which may be related to the sharp weakening in the solar activity during 2015-2017 (shown in Figure 3c), and will be discussed later. Figure 7 presents the time-height distributions of the monthly zonal-mean GW Ep at different latitudes of the NH (left column) and the SH (right column). The QBO wind time series presented in Figure 3b are overlapped on the Ep distribution at the latitude bands of \(10^{\circ}\) N/S and the equator. It can be seen that at the latitudes higher than \(30^{\circ}\) N/S the monthly zonal-mean GW Ep exhibits distinct annual variations, with GW Ep peaks appearing in the winter months of the corresponding hemisphere, which is consistent with the findings of [PERSON] et al. [62], by using lidar observations in southern France (44\({}^{\circ}\) N), and those of [PERSON] et al. [9], by using 14 years of SABER data. The monthly zonal-mean GW Ep exhibits semiannual variations at \(20^{\circ}\) N/S and \(30^{\circ}\) N/S, with GW Ep peaks appearing in both the winter and summer months, just similar to the global-mean GW Ep shown in Figure 6.  
The QBO signals can be identified in the time variations of GW Ep at \(10^{\circ}\) N/S and over the equator. The monthly zonal-mean GW Ep at 20-30 km over the tropics peaks when the QBO eastward wind phase reaches its peak too, which is consistent with some previous studies [57; 61; 63; 64]. What needs to be mentioned is that during January 2007 to September 2013, the variation of Ep over the equator shown in Figure 7 is similar to Figure 6 of [PERSON] et al. [57]. By comparing the temporal variations in the GW Ep with those of the zonal wind field, [PERSON] et al. [57] pointed out that, due to the selective filtering effect of stratospheric wind systems on GWs, maximum GW Ep values generally appear where the  
Figure 6: Time–altitude distribution of the monthly and global-mean GW Ep at 20–35 km during 2007 to 2020. Logarithmic scale is used. direction of the wind changes from westward to eastward [63]. The detailed relationship between Ep and zonal wind field over the tropics will be further discussed in Section 4. At 10\({}^{\circ}\) N/S and 20\({}^{\circ}\) N/S, the GW Ep has larger values in the summer months than in the winter months, which is consistent with the results of [PERSON] et al. [64] and [PERSON] et al. [9]. Moreover, at 20\({}^{\circ}\) N/S, 30\({}^{\circ}\) N/S, and 40\({}^{\circ}\) N/S, the zonal-mean GW Ep values at the zonal bands of the NH are larger than those of the corresponding zonal bands of the SH. Using eight-year SABER/TIMED data, [PERSON] et al. [64] also found that the GW Ep at 10\({}^{\circ}\) N-30\({}^{\circ}\) N was larger than at 10\({}^{\circ}\) S-30\({}^{\circ}\) S in 21-26 km height. However, in the latitudes at and higher than 60\({}^{\circ}\) N/S, GW activities over the zonal bands of the SH are stronger than those over the corresponding zonal bands of the NH, which has already been described in Section 3.2.