## 4 Discussion
### Quantification of the Impacts of Selected Factors on LS GW Activities in the Indian Region
During the other time periods, the mean zonal wind shears between 20 and 30 km over grid point A are generally westward, and the filtering effect of the background wind should bring the dissipation of the high GW energy, which explains the distinct reduction of GW Ep at the altitudes higher than around 23 km at this grid point in JJA of 2021. Figure 5b,d,f and Figure 6 together should explain the high RICs of the mean zonal wind shear between 20 and 30 km around grid point A, as presented in Figure 3.  
In comparison, at grid point B, GW energy propagates upward to the height of around 30 km during JJA of both 2020 and 2021, as shown in Figure 5c. As revealed by the comparison of Figure 5c,e, at grid point B, during JJA of both years, the peak GW Ep at 20-30 km is generally accompanied by strong westward winds in the whole altitude range of 10-30 km. Note that, besides the core of TEJ between 15 and 17 km, strong westward winds also prevail at higher altitudes between 17 and 30 km, which facilitate the upward propagation of GWs. [PERSON] et al. [60] pointed out that strong background winds should contribute to the upward propagation of GWs, and the propagation of the GW energy generated in the troposphere to the stratosphere under strong background winds has been observed in previous studies [2; 18; 61]. Moreover, as presented in Figure 5g, the zonal wind shears formed in the whole altitude range of 20-30 km are generally very small, which is also beneficial for the high GW energy to propagate to higher than 30 km. At grid point B, during JJA of both years, besides the small zonal wind shear between 20 and 30 km, the strong background wind field in the UTLS provides one of the important GW sources (TEJ) and the favorable conditions for the upward propagation of GWs, which should partly explain the comparatively high RICs of the mean zonal winds between 15 and 17 km and 20 and 30 km at this grid point, as presented in Figure 3.  
Note that the propagation and the filtering effect of the background wind field on GW should be identified more clearly by the correlation between the background wind direction and the propagation direction of GW. For example, it is very clear that over grid point A, westward winds are very strong beyond 20 km in JJA 2021, which could have attenuated the upward propagating GWs. As proposed by [62], westward waves propagate freely through eastward wind, and if there is a reversal phenomenon in the background wind field, the westward propagating GWs are easily filtered by the westward wind, and vice versa. This probably indicates that GWs are mostly propagating westward at grid point A over 20 km during JJA 2021. Figure 7 further shows the time-longitude distribution of the mean GW Ep value between 20 and 21 km at 10\({}^{\circ}\)N in the Indian region, and at grid point A, this height range is just below the strong westward wind shear. A fact that may support the westward propagation of GWs at 10\({}^{\circ}\)N during JJA 2021 is that GW Ep reaches peak values higher than 2.8 J/kg in the latitude range of 80\({}^{\circ}\)E-100\({}^{\circ}\)E while decreasing significantly within the latitude range of 60\({}^{\circ}\)E-80\({}^{\circ}\)E during this period. It can be reasonably inferred that the GWs propagate from east to west and gradually dissipate due to the strong shear caused by strong westward winds above 20 km at 10\({}^{\circ}\)N during JJA in 2021. Furthermore, it may indicate that during JJA 2021, over grid point A, which is at 10\({}^{\circ}\)N, the westward propagating GWs only propagate upwards to around 22-23 km and are filtered out by strong westward winds in the higher altitude. Figure 6: The time series of the mean zonal wind shear at 20–30 km and mean LS GW Ep at grid point A during December 2019 and November 2021. However, since Ep is scalar, inferring the propagation direction of GW through the peak distribution of Ep is only conjecture, and the true propagation direction of gravity waves can only be obtained through spectral analysis of GWs.