Sources, Sinks, and Propagation Characteristics of the Quasi 6‐Day Wave and Its Impact on the Residual Mean Circulation

Abstract This study employs a troposphere to lower thermosphere assimilation model data set generated by the Whole Atmosphere Community Climate Model with data assimilation provided by the Data Assimilation Research Test Bed (WACCM + DART) to explore the sources, sinks, and propagation characteristics of the quasi 6‐day wave (Q6DW) in the year 2007. WACCM + DART reproduces the burst‐like Q6DW and compares well with Sounding of the Atmosphere using Broadband Emission Radiometry and Thermosphere, Ionosphere, Mesosphere Energetics and Dynamics Doppler Interferometer observations. The most prominent Q6DW took place in later February and mid‐October, while the Q6DW was absent during solstice conditions in 2007. The occurrence of a large Q6DW in the equinoctial mesosphere and lower thermosphere is highly dependent on wave amplification and overreflection processes associated with barotropic/baroclinic instabilities and wave critical layers defined by the zonal mean zonal winds. During solstices, the winter hemisphere waveguide is negative and prevents the vertical wave propagation from the source region into the mesosphere and lower thermosphere. Meanwhile, the critical layer for the Q6DW encloses the unstable region in the summer hemisphere and thus blocks the energy conversion from the mean flow to the wave. The resulting circulation pattern due to the Q6DW momentum deposition is upward and poleward in both hemispheres and thus weakens the residual mean circulation in the summer hemisphere but strengthens it in the winter hemisphere. Also, the Q6DW impact on the residual mean circulation points to broader implications for the mean state of the upper atmosphere, for example, the thermospheric O/N 2 ratio due to upward constituent transport and related changes in the ionospheric plasma.