Tidal Variations in the Mesosphere and Lower Thermosphere Before, During, and After the 2009 Sudden Stratospheric Warming

This work presents a comprehensive evaluation of tides in the mesosphere and lower thermosphere (MLT) of the Whole Atmosphere Community Climate Model with thermosphere-ionosphere eXtension (WACCMX) and data assimilation provided by the Data Assimilation Research Testbed (DART) ensemble Kalman filter. A total of 26 diurnal and semidiurnal tidal components are calculated using the latest hourly data assimilation cycling during January 12–March 15, 2009 time period. When averaged over this entire yaw cycle, most tidal components display a latitudinal and vertical structures similar to that observed by the Sounding of the Atmosphere using Broadband Emission Radiometry (SABER). However, WACCMX + DART amplitudes of some diurnal tidal components are a factor of ∼1.2–2 times smaller and the peak altitudes and latitudes are shifted compared to SABER. The amplitudes of semidiurnal tides are likewise underestimated in the model, as well as being shifted in altitude and latitude, and some wavenumbers are not captured. Daily tidal variations are calculated in the model and compared with SABER and Super Dual Auroral Radar Network (SuperDARN) radar observations. Both the model and the radars show an amplification of the migrating semidiurnal tide during the week following the peak SSW. Enhancements in nonmigrating diurnal tides are also shown around the SSW peak, which could be produced by the nonlinear interaction between migrating tides and planetary waves. This study finds that, despite using data assimilation to constrain model dynamics, more work is needed in order to accurately simulate the full tidal spectrum and responses to SSWs in the MLT region.