Journal of Geophysical Research, Atmospheres: In the lower thermosphere, there is a large circulation from the winter polar region to the summer polar region at an altitude around 120 km. This study analyzed the role of gravity waves contributing to this circulation using output from a high-resolution simulation. In the winter middle atmosphere, gravity waves with eastward phase speeds are generated around the polar vortex and propagate into the lower thermosphere. Gravity waves are typically assumed to originate in the troposphere and are thought to be unable to propagate above the strong winds in the mesosphere. This study highlights the importance of gravity wave generation in the middle atmosphere and suggests a method for improving parameterized gravity waves. Additionally, the vertical structure of the zonal mean zonal wind is important through the selective filtering of gravity waves. In the summer hemisphere, semidiurnal tidal forcing largely contributes to the lower thermospheric circulation, as do gravity waves. Thus, the lower thermospheric circulation is driven by gravity waves generated in the mesosphere and filtered in the upper mesosphere, as well as by the semidiurnal tide.
Time-latitude sections of (a) the meridional component of the residual mean circulation and (b) the vertical momentum flux divergence for small scale waves based on high-resolution (0.25 degree) WACCM-X simulations. The results illustrate the role of small scale wave forcing on driving the interhemisphere circulation in the lower thermosphere, including its seasonal variation.