The westward quasi‐6‐day wave (Q6DW) with zonal wavenumber 1 is a prominent and recurrent phenomenon in the mesosphere and lower thermosphere (MLT) and has a significant impact on day‐to‐day ionospheric variability. Geopotential height measurements from Aura Microwave Limb Sounder and Specified Dynamics Whole Atmosphere Community Climate Model eXtended Version simulations during 2005–2019 are utilized to study the climatological variations of Q6DW. The spectral analysis clearly indicates that four typical Q6DW events occur in the MLT region before and after the two equinoxes in 1 year. The wave amplitudes in the summer hemisphere are considerably larger than the amplitudes in the winter hemisphere. The Eliassen‐Palm flux diagnostics show that the wave source of the post‐September equinox event is located in both hemispheres, while the sources of the other three Q6DW events are in the winter hemisphere. The diagnostic analysis results show that the climatological features of Q6DW are primarily due to the seasonal variations of the mean flow, which can determine the Q6DW critical layers, baroclinic/barotropic instability, and waveguides. Specifically, the Q6DW can be amplified in the summer hemisphere mesosphere at high latitudes during pre‐ and postequinox periods when the critical layers penetrate the unstable region. At the two equinoxes, the Q6DW can also propagate into the MLT region with similar amplitudes in both hemispheres due to the weak zonal mean flow but without additional amplification. At the two solstices, the Q6DW is suppressed because the critical layers envelop the whole unstable region, which prevents its amplification through wave‐mean flow interaction.

Using the Modern‐Era Retrospective analysis for Research and Applications version 2 reanalysis and Aura Microwave Limb Sounder geopotential height and temperature observations, two unusual planetary waves (PWs) with westward zonal wavenumber 1 (W1) and periods of ∼6.0 and ∼9.6 days are identified during a rare Antarctic sudden stratospheric warming (SSW) event in September 2019, which follow a strong W1 PW with period of ∼7.4 days. It is found that the W1 ∼7.4‐day wave is actually the representation of the climatological W1 quasi‐6‐day wave before the September equinox, which peaks on September 10, 2019. Meanwhile, a strong zonally symmetric wave with period of ∼32.2 days occurs in the Southern Hemisphere with a maximum zonal wind amplitude of ∼27 m/s at 65°S and 48 km. Interestingly, the frequencies and zonal wavenumbers of the W1 ∼7.4‐day, ∼6.0‐day, ∼9.6‐day and zonally symmetric ∼32.2‐day waves exactly satisfy the nonlinear interaction theory between PWs. We thus proposed for the first time the occurrence of nonlinear interaction between a W1 ∼7.4‐day wave and a zonally symmetric ∼32.2‐day wave, which generates two W1 PWs with periods of ∼6.0 and ∼9.6 days. The Eliassen‐Palm flux diagnostics suggest that the zonally symmetric ∼32.2‐day wave also contributes to the wind deceleration during this SSW. Moreover, the baroclinic/barotropic instabilities related to the vertical shear in the zonal wind during the SSW considerably enhance the PW activities in the Antarctic stratosphere. The climatological variations of the instabilities and critical layers in the mesosphere can also influence the propagation of these three W1 PWs.

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 ∼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 ∼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.

The westward quasi‐6‐day planetary wave (Q6DW) with zonal wave number 1 is a prominent oscillation in the mesosphere and lower thermosphere region, which causes significant variabilities in the ionosphere through E region wind dynamo. In this paper, the neutral wind observations from the Thermosphere Ionosphere and Mesosphere Electric Dynamics Doppler Imager experiment, the kinetic temperature observations from the Sounding of the Atmosphere using Broadband Emission Radiometry instrument, and the total electron content (TEC) maps from globally distributed Global Positioning System (GPS) receivers during 2003–2017 are utilized to statistically study the Q6DW in both the neutral atmosphere and the ionosphere. Our statistical results show that (1) the Q6DW is most likely to occur during May and September, exhibiting a quasi‐semiannual oscillation. (2) The periods of the Q6DW show two statistical crests at ~6 and ~7 days separately. (3) The ionospheric oscillations due to the Q6DW are positively related to both the 10.7‐cm solar flux index and the neutral wave amplitudes in the mesosphere and lower thermosphere region. The stronger oscillations in TEC under higher solar activity conditions are due to the larger ionization rate and thus larger background TEC. A stronger Q6DW results in stronger wind dynamos in the E region, which leads to stronger F region responses indirectly. (4) The interhemispheric asymmetries of the ionospheric responses to Q6DW show weak negative correlations with both solar activities and neutral wave perturbations, the mechanism of which needs our further investigation.

In this study, the neutral winds from meteor radars and the total electron content (TEC) from Beidou Geostationary (GEO) satellites were utilized to study the responses of the mesosphere and lower thermosphere and the ionosphere in the Asian‐Australian sector to the recent Sudden Stratospheric Warming (SSW) occurred in the Antarctic polar region during September 2019. It was demonstrated that the ionospheric TEC increased significantly, especially around the equatorial ionization anomaly (EIA) region during this SSW. A pronounced semidiurnal signature in the deviation TEC was observed near the EIA crest region, which was more significant in the southern hemisphere. Meanwhile, the enhancement period of the amplitudes of solar and lunar semidiurnal tides in winds in the mesosphere was coincided with that in TEC. The solar semidiurnal amplitudes increased from about 18 to 35 m/s in the zonal wind and from 15 to 25 m/s in the meridional wind at Mengcheng (33.4°N, 116.5°E, MLAT 27.4°N), and from about 15–38 m/s in the zonal wind at Kunming (25.6°N, 103.6°E, MLAT 19.3°N). The lunar semidiurnal amplitudes increased from 2 to 6 m/s in the zonal wind and from about 3–9 m/s in the meridional wind at Mengcheng, and from about 3–7 m/s in the meridional wind at Kunming, respectively. These indicated that the semidiurnal signature in TEC during the 2019 southern hemisphere SSW could be associated with the SSW induced by the atmospheric semidiurnal tide variations in the mesosphere.