The quasi 2 day wave (QTDW) is a traveling planetary wave that can be enhanced rapidly to large amplitudes in the mesosphere and lower thermosphere (MLT) region during the northern winter postsolstice period. In this study, we present five case studies of QTDW events during January and February 2005, 2006 and 2008–2010 by using the Thermosphere‐Ionosphere‐Mesosphere Electrodynamics‐General Circulation Model (TIME‐GCM) nudged with the Navy Operational Global Atmospheric Prediction System‐Advanced Level Physics High Altitude (NOGAPS‐ALPHA) Weather Forecast Model. With NOGAPS‐ALPHA introducing more realistic lower atmospheric forcing in TIME‐GCM, the QTDW events have successfully been reproduced in the TIME‐GCM. The nudged TIME‐GCM simulations show good agreement in zonal mean state with the NOGAPS‐ALPHA 6 h reanalysis data and the horizontal wind model below the mesopause; however, it has large discrepancies in the tropics above the mesopause. The zonal mean zonal wind in the mesosphere has sharp vertical gradients in the nudged TIME‐GCM. The results suggest that the parameterized gravity wave forcing may need to be retuned in the assimilative TIME‐GCM.

Sudden stratospheric warmings (SSWs) refer to rapid warming events in the polar stratosphere, which usually occur during mid-winter (Andrews et al., 1987). These large-scale phenomena are triggered by enhancement of vertically propagating quasi-stationary planetary waves (QSPWs) (Labitzke, 1981) generated in the troposphere by land-sea thermal differences and/or large-scale topography. The breaking of QSPWs in the polar stratosphere can disrupt the wintertime polar vortex and affect the mean circulation (Matsuno, 1971). The polar vortex can either be displaced and/or split due to the forcing from the breaking of QSP-Ws. To this extent, in the polar stratosphere the background eastward wind would be decelerated and the temperature can increase tens of K very rapidly. This aspect of stratospheric dynamics is the key driver of the SSWs and can impact the dynamics of the upper atmosphere (

In this study, the mechanism driving the narrow lower‐thermospheric winter‐to‐summer meridional circulation is thoroughly investigated for the first time using the Specified Dynamics configuration runs of the Whole Atmosphere Community Climate Model eXtended (SD‐WACCMX) simulations and the TIMED Doppler Interferometer (TIDI) observations. The mean meridional circulation in the SD‐WACCMX is qualitatively consistent with the TIDI measurements, though the magnitude in the SD‐WACCMX is about 50% weaker. The lower‐thermospheric winter‐to‐summer circulation is mainly driven by the resolved wave forcing, including the tides and internally generated inertia gravity waves (GWs). The momentum forcing from the parameterized sub‐grid scale GWs is not as significant as the resolved wave forcing in driving the lower‐thermospheric meridional circulation. The GW parameterization scheme in the SD‐WACCMX only includes GWs with phase velocities in the range of ±45 m/s, which might result in most of the parameterized sub‐grid GWs dissipating and breaking in the mesosphere and hardly impacting the lower thermosphere. Only including slow GWs in the SD‐WACCMX parameterization could potentially lead to the underestimation of the meridional wind in the model. Analysis also indicates the lower‐thermospheric meridional circulation is stronger in the summer hemisphere, which is attributed to the hemispheric asymmetry in the resolved wave momentum forcing. This study underlines the importance of the whole atmosphere coupling through wave propagation and dissipation. This understanding can guide the model development with an accurate representation of underlying physical processes in the mesosphere and lower thermosphere which drives the lower‐thermospheric circulation as well as the overall dynamics of this region.