On the Westward Quasi-8-Day Planetary Waves in the Middle Atmosphere During Arctic Sudden Stratospheric Warmings

Using the geopotential height measurements from the Aura Microwave Limb Sounder and the Sounding of the Atmosphere using Broadband Emission Radiometry instrument on the Thermosphere-Ionosphere-Mesosphere Energetics and Dynamics satellite, we found eight westward quasi-8-day wave (Q8DW) events with zonal wavenumber 1 (W1) during the Arctic sudden stratospheric warmings (SSWs) in 2005-2020. The W1 Q8DW perturbations are mainly confined to the middle and high latitudes in the Northern Hemisphere, and peak at similar to 50-80 km. Specified Dynamics Whole Atmosphere Community Climate Model eXtended version simulations are utilized to reproduce the Q8DW activities. The diagnostic results indicate that the W1 Q8DWs originate from the high latitudes in the Northern Hemisphere stratosphere. Their excitation, propagation, and amplification are primarily influenced by the critical layers and baroclinic/barotropic instability, which are dependent on the background conditions during SSWs. Our statistical results show that the W1 planetary waves with periods shorter than 12 days during the Arctic SSWs are most frequently observed at the period of similar to 8 days, which is significantly different from the climatological periods of the Rossby (1, 1) and (1, 2) normal modes (6.14 and 9.81 days, respectively). These results indicate that the W1 Q8DW may be a new wave mode. Finally, we found that the reversal (or deceleration) and the recovery of the zonal mean flow during SSWs can result in a zonally symmetric Q8DW, and the W1 Q8DW is likely the child wave generated by the nonlinear interaction between the stationary planetary wave with zonal wavenumber 1 and zonally symmetric Q8DW.