## 5 Conclusions  
By using multi-mission GNSS RO data from 2007 to 2020, the present study studies the variations in the gravity waves at the lower stratosphere of 20-35 km over the globe. The dry temperature profiles (atmPrf files) from six different RO missions provided by CDAAC, after a two-step quality check, are used to retrieve the GW Ep profiles, based on which the temporal variations of the GW activities are derived and the linear trends of the GWs and the responses of GW activities to solar activity, QBO, and ENSO are further analyzed using the MLR method. To verify the validity of our procedure for extracting the GW Ep profiles with the horizontal detrending method and to understand the consistency of the GWs derived from different RO missions, firstly, the global distributions of the seasonal means of GW Ep values at the altitude range of 20-35 km, derived respectively from three RO datasets, i.e., data of all the missions, COSMIC data only, and METOP-A/B data only, during the period from February 2013 to January 2016, are compared. It is found that the distribution patterns of the seasonal means of GW Ep values derived from the three datasets are all consistent with previous works, and there are no significant differences among the magnitudes of the Ep  
Figure 10: Time–height distributions of GW Ep and zonal wind at 20–35 km over the equator during the 14 years from 2007 to 2020. The eastward and westward winds, with the unit of m s\({}^{-1}\), are represented by the black solid lines and the black dashed lines, respectively. The 0 m s\({}^{-1}\) wind level is represented by the red solid lines. Zonal wind data from ERA5 are used. values derived from different RO datasets, which confirms the feasibility of the method for extracting the GW Ep and suggests that it is statistically reasonable to use the data from multi-GNSS RO missions to study the trend in the GW activities. By analyzing the time-latitude and the time-height distributions of the GW Ep values during the 14 years from 2007 to 2020, we found that GW activities in 20-35 km show distinct annual variations in mid and high latitude regions with latitudes higher than 40\({}^{\circ}\) in both of the two hemispheres, reaching the peaks during winter months of the corresponding hemisphere. The stronger and longer-lasting eastward wind during the winter seasons of the SH compared with the winter seasons of the NH should partly contribute to the GW Ep distribution pattern that GW activities are stronger in winter months at high latitudes of the SH than that of the NH. At lower latitudes of 20\({}^{\circ}\) N/S and 30\({}^{\circ}\) N/S, GW activities at the altitudes of 20-35 km exhibit semiannual variations, with Ep values peaking in both winter and summer months. At the equator and the latitudes of 10\({}^{\circ}\) N/S, QBO signals of GW Ep are observed, which should be related to the selective filtration of gravity waves by the stratospheric wind system. Moreover, the zonal-mean GW activities over the latitudes of 20\({}^{\circ}\) N, 30\({}^{\circ}\) N, and 40\({}^{\circ}\) N are generally larger than those of the same latitude bands of the SH, which is contrary to the comparison between the high latitudes of the two hemispheres. In addition, in both the time-latitude and the time-altitude distributions of the GW Ep values, it is clearly observed that in high latitude regions, there has been a significant decrease in the strength of the GW activities since 2016. From the latitude-height distributions of the trends in the GW Ep values during each month, it is found that positive, while not significant, trends appear from March to August over the tropics. Being consistent with [PERSON] et al. (2018), relatively large positive trends which are not significant appear in the latitudes of 25\({}^{\circ}\) N-45\({}^{\circ}\) N in November and December, and positive trends also occur in the latitudes of 40\({}^{\circ}\) N-70\({}^{\circ}\) N during March and April.