## 3 Results and Analyses
### Global Trends in GWsDuring 2007 to 2020  
Following the procedure introduced in Section 2.4, by analyzing the time series of the monthly zonal-mean GW Ep values from 2007 to 2020 with the MLR method, the linear trend (\(\alpha\) in Equation (4)) in the GW Ep values for each 2.5\({}^{\circ}\times\) 0.1 km (latitude\(\times\) height) grid cell is obtained for each month. For a certain month, if the value of the coefficient \(\alpha\) of a grid cell is greater than 0, then the GW Ep of the corresponding cell has a positive trend. Otherwise, the GW Ep of the corresponding cell has a negative trend. The final latitude-height distributions of the GW linear trends for each month are presented in Figure 8. In each subfigure of Figure 8, the regions covering the cells with the trends passing the student \(t\)-test with a confidence level of 90% are shown as protuberance. It can be seen from Figure 8 that in the middle and high latitudes, GW Ep values generally show significant negative trends (\(<\)\(-\)0.02 J/kg per year) in almost all months, especially in the altitude range of 25-35 km, which is consistent with the phenomena, shown in Figures 5 and 7, that the GW Ep decreases significantly in middle and high latitudes after 2016. Over the tropics, positive trends appear from March to August, while these trends are mostly not significant. Besides, relatively large positive trends appear in the latitudes of 25\({}^{\circ}\) N-45\({}^{\circ}\) N in November and December, and these trends are not significant either, which is consistent with [PERSON] et al. [9]. The low confidence level of the trends over these latitudes might be partly due to the strong planetary wave activity and the impacts of SSWs in the northern high latitudes. It should be noted that due to the strong extension and subsequent division of the polar vortex, almost the whole NH to the north of about 30\({}^{\circ}\) N will be influenced by the enhanced GW activities induced by SSWs [65]. The positive trends occurring in 40\({}^{\circ}\) N-70\({}^{\circ}\) N during March and April are also consistent with the findings of [PERSON] et al. [9]. In addition, the significant negative trends over 5\({}^{\circ}\) N-30\({}^{\circ}\) N in January and the negative trends over 25\({}^{\circ}\) N-40\({}^{\circ}\) N in February in the range of 30-35 km, which were presented in Figure 5 of [PERSON] et al. [9], are also shown here in Figure 8.  
Considering that a half-year shift exists in the atmospheric conditions between the two hemispheres, some distinct characteristics of GW trends can be determined, which are almost symmetrical between the SH and the NH. For example, in the altitude range of 30-35 km, both the high latitudes of the SH during June, July, and August and the high latitudes of the NH during December, January, and February have significant negative trends, which are generally lower than -0.03 J/kg per year. In the NH from June to September and in the SH from December to March, the GW Ep values at 25-35 km over the mid-high latitudes generally show significantly negative trends, which are of the magnitudes of -0.01 to -0.03 J/kg per year. Whether for the more active winter months or for the relatively less active summer months of the gravity waves, the GW Ep trends generally show a consistent symmetry over the two hemispheres. Furthermore, the GW activities in 20-35 km are generally declining from 2007 to 2020. The latitude-height distributions of the deseasonalized trends in the monthly zonal-mean GW Ep values at 20-35 km during 2007 to 2020 and the responses of GW Ep to solar flux, QBO, and ENSO are presented in Figure 9. Similar to Figure 8, in each subfigure of Figure 9, the regions where the results are statistically significant at the confidence level of 90% are shown as protuberance. The deseasonalized trends shown in Figure 9a are derived by analyzing the deseasonalized monthly zonal-mean GW Ep time series using the MLR method [46; 66].