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.

The sudden stratospheric warming (SSW) is one of the dramatic dynamical phenomena occurring in the polar region of the winter hemisphere, which is accompanied by changes in the wind and temperature fields (Andrews et al., 1987; Butler et al., 2015; Chandran & Collins, 2014). The interaction between the upward propagation of quasi-stationary planetary waves from the troposphere and the stratospheric mean flow is believed to be the cause of SSW events (Liu & Roble, 2002; Matsuno & Sciences, 1971). Long-term atmospheric data set indicates that SSW events occur rarely in the southern hemisphere (SH), which is most likely due to the weaker planetary wave (PW) forcing and smaller topographical and land-sea differences in the SH. It has been found that the PWs may exhibit anomalous behaviors during SSW periods. For example, Gu, Dou, et al. (2018) and Ma et al. (2017) observed the enhancements of quasi-two-day wave activities during several SSWs. As for the quasi-6-day wave (Q6DW), it also presents an enhancement during some SSWs with either zonal wavenumbers 1 or 2 (Gong et al., 2018; Pancheva et al., 2018). Similarly, the amplification of the quasi-10-day wave may also occur after the collapse of the polar vortex in the stratosphere during some SSW events (

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. QIN ET AL.

The ionospheric disturbance is a wave-like plasma activity that impacts remote sensing systems, navigation and positioning, and other long-range communication works. To maintain the safety of human space activities and reduce related economic losses, the study of ionospheric disturbances is necessary and has been a popular topic in space physics.Internal atmospheric dynamics have been identified as one of the main causes of ionospheric disturbances, especially convective activity. The study of the relationship between convective activity and the ionosphere can provide a unique perspective on ionosphere-atmosphere coupling (

The quasi‐two‐day waves (QTDWs) with westward‐propagating wavenumbers 2 (W2), 3 (W3), and 4 (W4) during the boreal summers were statistically examined using TIMED/SABER temperature observation datasets during 2002–2019 with a 6‐day two‐dimensional least‐squares fitting window. The W4 (W3) and W2 QTDWs were observed at ∼67–73 km and ∼30–40°N, and ∼89–95 km and ∼20–30°N, respectively. The W4 mode occurred 57 times over the past 18 years compared to 43 (52) times for the W3 (W2) QTDWs. The W4 QTDW reached maximum amplitudes of ∼9–10 K during 2006, 2009, and 2017, whereas W3 and W2 QTDWs attained maximum amplitudes of ∼8 K during 2017 and 2012. In addition, W4, W3, and W2 occurred more frequently with periods of 41–44 hr, 47–53 hr, and 44–50 hr, respectively. QTDW events with longer periods took place later than those events with shorter periods. Statistically, the W4 (W2) QTDW events were significantly more frequent during days 195–210 (165–180). In contrast, the W3 QTDW tended to occur during days 180–195 but was only slightly less frequent during days 195–210 and 210–225. Diagnostic analysis of the modern‐era retrospective analysis for research and applications ‐2 reanalysis dataset indicates that the attribution variations of the QTDWs, including their amplitudes and periods, were intimately related to the corresponding variabilities of the background zonal wind.