Contribution of Gravity Waves to the Lower Thermospheric Winter‐To‐Summer Meridional Circulation in High‐Resolution WACCM‐X

Abstract The mechanism driving the lower thermospheric meridional circulation is analyzed using the high‐resolution Whole Atmosphere Community Climate Model with thermosphere/ionosphere extension (WACCM‐X). In the winter lower thermosphere, eastward forcing is dominant around z =120 km, consistent with equatorward circulation. Following previous studies, the vertical flux of the zonal momentum is examined for small‐scale waves with zonal scales smaller than 2,000 km through the depth of the middle atmosphere. Wave decomposition analysis reveals that eastward waves with a wide range of phase speeds are generated around the polar vortex height. This result differs from the conventional understanding in gravity wave parameterizations, which states that eastward waves slower than the strong mesospheric wind cannot propagate to higher altitudes. Quasi‐stationary and westward waves are also active in the mesosphere, some of which originate in the troposphere. However, these waves more often dissipate in the upper mesosphere at critical levels and easterly shear, which results in the dominance of eastward waves in the lower thermosphere. The horizontal and temporal variations of gravity wave activity in the lower thermosphere are associated with the structure of the mesospheric jet. These results suggest a possible method for diagnosing wave generation that could improve gravity wave parameterizations. In the summer hemisphere, westward forcing is expressed by the semidiurnal tide, as well as by filtered gravity waves. Thus, the driving mechanisms of the lower thermospheric circulation are the contribution of gravity waves, mainly generated in the mesosphere and selectively filtered in the upper mesosphere, and the semidiurnal tide.