## 4 Discussion
### Quantification of the Impacts of Selected Factors on LS GW Activities in the Indian Region
over both of the two grid points and the maximum wind speeds appear at around 16 km, reaching higher than 20 m/s, which is due to the TEJ being prevalent over the Indian region in JJA [30; 39]. At the same time, the easterly wind shears are generated within a height range of about 16-20 km above the two grid points (see Figure 5f,g), which becomes one of the important sources of the GWs in the LS of the Indian region, and, influenced by the strong westward winds, the GWs generated propagate to higher than 20 km. The altitude-time distributions of GW Ep over the two grid points, which are shown in Figure 5b,c, both present peak GW Ep during JJA. This is attributed to the contributions of both the TEJ and the convection activity. As we can see in Figure 3a,i, at grid point B, the magnitudes of the RIC of the mean zonal wind speed between 15 and 17 km (0.19) and that of the OLR (0.22) are close, which indicates that the contributions of the two factors to the high GW Ep over this grid point are similar. However, at grid point A, the RIC of the mean zonal wind speed between 15 and 17 km and that of OLR are 0.09 and 0.03, respectively, which indicates that between the two factors, the contribution of TEJ on the GW Ep is greater than that of convective activity and that at this grid point, the impacts of both of the two GW source factors on the LS GWs are very limited (RIC \(<\) 0.1). Figure 5b indicates that at grid point A, during JJA of both years, due to the contributions of the two GW source factors, the mean zonal wind speed between 15 and 17 km and the convection activity, high GW energy is generated at the heights lower than 20 km. Figure 3a,i reveal that for the LS GW activity, i.e., GWs between 20 and 30 km, the RICs of the two source factors are both very low, and this should be mainly due to the significant influence of the background LS zonal wind shear on the upward propagation of the GW energy. During JJA of 2020 at grid point A, the high GW energy propagates up to the whole height range of 20-30 km, while during JJA of 2021, the high GW energy propagates upward only to the height of about 22-23 km and then dissipates, which should be mainly attributed to the large westward wind shears in the 22-28 km height range above this grid point, considering that the GW energy can be attenuated by large background wind shears [57; 58; 34; 59]. Figure 5d shows that during JJA of 2021, the zonal wind speeds of 14-18 km and those above 25 km over grid point A are very close, both of which are westward winds with speeds higher than 30 m/s, but the westward wind speeds at 18-23 km are generally less than 15 m/s. The large differences between the zonal wind speeds in the 18-23 km height range and those of the upper and lower altitude regions result in large zonal wind shears, especially between 22 and 28 km during JJA of 2021, as shown in Figure 5f, which are beneficial for the filtering of GWs. In comparison, during JJA of 2020, at this grid point, the zonal wind shears are generally very small at altitudes higher than 20 km, and so the GWs can propagate up to 30 km and higher, as presented in Figure 5b. Figure 6 further presents the time series of the mean LS GW Ep and the mean zonal wind shear between 20 and 30 km at grid point A. They both reach peak values in JJA of 2020, and the CC between them is 0.78, indicating a strong positive correlation between the mean LS GW Ep and the mean zonal wind shear in the 20-30 km height range at this grid point. Figure 6 also shows that at grid point A, the peak value of the mean zonal wind shear between 20 and 30 km obtained in JJA of 2020 is around 0 m/s, which provides stable atmospheric conditions favorable for the upward propagation of the high GW energy generated mainly by TEJ at around 16 km in the stratosphere.