## 4 Discussion  
The comparison between Figure 9a,b demonstrates that the height-latitude distribution pattern of the trends in the GW Ep and that of the response of GW Ep to the solar activity show distinct correlations, i.e., the overall negative trends in GWs correspond to the overall positive responses of the GW Ep to the solar activity. Considering that the solar activity weakened significantly after the year 2015, as shown in Figure 3a, it can be deduced that the sharp decline in the solar activity after the year 2015, together with the positive response of the GW Ep to the solar activity, might have contributed significantly to the overall attenuation of the gravity wave activity during the 14 years from 2007 to 2020. What needs to be mentioned is that [PERSON] et al. (2016) and [PERSON] et al. (2019) both observed negative responses in GW Ep to F10.7 indexes at the altitude range of 30-35 km above low and middle latitudes by using the SABER temperature data, which are different from our results, while the time periods researched by [PERSON] et al. (2016) and by [PERSON] et al. (2019) are 2002-2011 and 2002-2015, respectively, and the F10.7 data after the year 2015 were not used in their studies. Moreover, [PERSON] et al. (2019) pointed out that differences in GW sources, in conditions of wave propagations, and in measurement methods might all have impacts on the analyses concerning the responses of GW to solar activities. We suggest that a possible reason why our results are inconsistent with those of [PERSON] et al. (2016) and [PERSON] et al. (2019) is that the solar activity weakened sharply during 2015 to 2017 (as shown in Figure 3a), which partially contributed to the general weakening of GW activities after 2016. The specific dynamic mechanisms which directly lead to the negative trend in global GW Ep are still unclear. Considering that the significant negative trend is particularly notable in the polar regions, these background dynamic mechanisms might be related to the enhancement/weakening of polar circulations or the expansion and break in polar vortexes. On the other hand, as shown in Figures 5 and 7, the GW Ep at high latitudes has been declining extremely rapidly since 2016, accompanied by a decrease in solar activity. Taking into consideration the strong positive correlation between GW Ep and solar activity revealed in the present study, it is speculated that the dynamic mechanisms which lead to the negative trend in global GW Ep should also be strongly affected by the solar activity. More work will be done on this issue in our future work. The comparisons of the QBO time series, the time-height distributions of the monthly and zonal-mean GW Ep over the equator, and the latitudes of 10\({}^{\circ}\) N and 10\({}^{\circ}\) S, which are presented in Figure 7, reveal that during the time periods when peak values appear in the QBO eastward wind phase time series, the monthly zonal-mean GW Ep values in 20-30 km also reach peak values over the equator and over 10\({}^{\circ}\) N/S, where the value of ln(Ep) is larger than 0.6 ln(J/kg) and is shown with red color in Figure 7. Figure 10 further presents the time-height cross section of Ep and zonal wind field at 20-35 km over the equator during 2007 and 2020. As mentioned in Section 3.3, due to the selective filtering effect of stratospheric wind systems on GWs, the GW Ep value above the 0 m s\({}^{-1}\) wind level line is usually significantly attenuated compared with the value below it. We also can see that the Ep value at 20-30 km is generally large (>1.8 J/kg) in the eastward wind time period and small (<1.8 J/kg) in the westward wind time period. This is a manifestation of the significant positive correlation between the GW Ep in the altitude range of 20-30 km and the QBO over the tropics, and is consistent with the significant positive responses of GW Ep below 30 km over 15\({}^{\circ}\) S-15\({}^{\circ}\) N, as shown in Figure 9c.