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Deep convection from monsoons has been shown to be a major tropospheric source of gravity waves (GWs) in the summer hemisphere. These GWs can propagate up to the upper mesosphere, either vertically (over the same latitude) or obliquely (latitudinal propagation away from their source), where they dissipate and release their momentum. These waves play an important role in the global dynamical structure of the middle atmosphere. Understanding their hemispheric and seasonal variations could improve the GW parameterization schemes in present global models. To this end, this paper reports on a GW ray‐tracing analysis using the GROGRAT model to simulate the propagation of GWs from the monsoon regions in the northern and the southern hemispheres during both the summer and the winter seasons. The 20 simulations show the southern hemisphere to be more conducive to both the vertical and the oblique propagation of mesospheric GWs compared to the northern hemisphere, regardless of season. This is partially due to a stronger GW filtering in the northern hemisphere near the tropopause where a third of the waves have been vertically reflected. We also show that an increase in the horizontal wavelength increases not only the latitudinal component but also the longitudinal component of the oblique propagation of GWs. The broad spectrum of waves with different horizontal wavelengths and horizontal phase speeds used in this study highlights the existence of an upper limit in the horizontal wavelength of GWs that can reach the upper mesosphere. Plain Language Summary: Atmospheric gravity waves (GWs) propagating through the middle atmosphere play an important role in its global dynamical structure. As the waves propagate upward, the density of the atmospheric background exponentially decreases, resulting in an exponential increase in the wave amplitude and thus the wave energy. GWs from the tropospheric monsoon convection can propagate up to the upper mesosphere where they can break and transfer a significant amount of energy to the background flow. Previous studies have shown the atmospheric coupling between the low‐latitude troposphere and the high‐latitude mesosphere through the oblique propagation of GWs. Using the ray‐tracing model GROGRAT, we first perform a hemispheric comparison of the monsoon‐generated GW propagation between the summer northern hemisphere in July 2016 and the summer southern hemisphere in January 2017. Second, GWs were launched from the same regions but during the winter seasons to study the seasonal differences of this atmospheric phenomenon. The 20 simulations lead to a discussion on the spectral dependencies of the GW propagation and dissipation in the middle atmosphere, and on the nature of the mesospheric GWs in the two hemispheres and the two seasons. Key Points: The vertical reflection of gravity waves at the low‐latitude tropopause is significantly stronger in the northern hemisphereThe southern hemisphere appears more conducive to the gravity‐wave propagation from the troposphere to the mesosphere in summer and winterThere is an upper limit in the horizontal wavelength relative to the phase speed for gravity waves to reach the upper mesosphere [ABSTRACT FROM AUTHOR]

Oblique propagation of gravity waves (GWs) refers to the latitudinal propagation (or vertical propagation away from their source) from the low‐latitude troposphere to the polar mesosphere. This propagation is not included in current gravity wave parameterization schemes, but may be an important component of the global dynamical structure. Previous studies have revealed a high correlation between observations of GW pseudomomentum flux (GWMF) from monsoon convection and Polar Mesospheric Clouds (PMCs) in the northern hemisphere. In this work, we report on data and model analysis of the effects of stratospheric sudden warmings (SSWs) in the northern hemisphere, on the oblique propagation of GWs from the southern hemisphere tropics, which in turn influence PMCs in the southern summer mesosphere. In response to SSWs, the propagation of GWs at the midlatitude winter hemisphere is enhanced. This enhancement appears to be slanted toward the equator with increasing altitude and follows the stratospheric eastward jet. The oblique propagation of GWs from the southern monsoon regions tends to start at higher altitudes with a sharper poleward slanted structure toward the summer mesosphere. The correlation between PMCs in the summer southern hemisphere and the zonal GWMF from 50°N to 50°S exhibits a pattern of high‐correlation coefficients that connects the winter stratosphere with the summer mesosphere, indicating the influence of Interhemispheric Coupling mechanism. Temperature and wind anomalies suggest that the dynamics in the winter hemisphere can influence the equatorial region, which in turn, can influence the oblique propagation of monsoon GWs. Plain Language Summary: Propagation of waves throughout the Earth's atmosphere is a key phenomenon to understanding atmospheric dynamics, as it changes temperature, pressure, density, and composition. Due to the exponentially decreasing density, the amplitude and energy carried by these waves increase exponentially as they propagate vertically. When the waves break, their energy is released and transferred to the background flow. Gravity waves (GWs) can propagate up to the middle atmosphere but are too small to be resolved by most global‐scale atmospheric models. The deep convection from monsoon regions is known to be a major source of mesospheric GWs and previous studies on summer northern hemisphere have shown that monsoon GWs tend to propagate obliquely from the low‐latitude stratopause up to the high‐latitude mesopause. We focus this study on the summer southern hemisphere and the Interhemispheric Coupling (IHC) between the summer mesopause, where Polar Mesospheric Clouds (PMCs) form, and the winter stratosphere where sudden warmings occur. PMCs are excellent indicators of atmospheric changes. Their correlations with wind, temperature, and GW pseudomomentum flux highlight the consequences of anomalies in the winter stratosphere, such as warmings, on the oblique propagation of GWs that influence the PMC formation in the summer southern hemisphere. Key Points: Polar Mesospheric Clouds in the southern hemisphere can be influenced by obliquely propagating monsoon gravity wavesStratospheric sudden warmings enhance the GW propagation at the midlatitude winter hemisphere which appear to be slanted equatorwardStratospheric sudden warmings appear to alter the GW momentum flux distribution along the oblique GW propagation path in summer hemispheres [ABSTRACT FROM AUTHOR]