Abstract. According to Modern-Era Retrospective Research Analysis for Research and Applications (MERRA-2) temperature and wind datasets in 2019, this study presents the global variations in the eastward-propagating wavenumber 1 (E1), 2 (E2), 3 (E3) and 4 (E4) planetary waves (PWs) and their diagnostic results in the polar middle atmosphere. We clearly demonstrate the eastward wave modes exist during winter periods with westward background wind in both hemispheres. The maximum wave amplitudes in the Southern Hemisphere (SH) are slightly larger and lie lower than those in the Northern Hemisphere (NH). Moreover, the wave perturbations peak at lower latitudes with smaller amplitudes as the wavenumber increases. The period of the E1 mode varies between 3–5 d in both hemispheres, while the period of the E2 mode is slightly longer in the NH (∼ 48 h) than in the SH (∼ 40 h). The periods of the E3 are ∼ 30 h in both the SH and the NH, and the period of E4 is ∼ 24 h. Despite the shortening of wave periods with the increase in wavenumber, their mean phase speeds are relatively stable, ∼ 53, ∼ 58, ∼ 55 and ∼ 52 m/s at 70∘ latitudes for E1, E2, E3 and E4, respectively. The eastward PWs occur earlier with increasing zonal wavenumber, which agrees well with the seasonal variations in the critical layers generated by the background wind. Our diagnostic analysis also indicates that the mean flow instability in the upper stratosphere and upper mesosphere might contribute to the amplification of the eastward PWs.

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.

Atmospheric tides are global-scale atmospheric oscillations. The periods of atmospheric tides are subharmonic and corresponding to solar or lunar days, and the maximum atmospheric tide amplitude in the neutral atmosphere mainly occurs at altitudes ranging between 80 and 150 km (Forbes, 1995). Atmospheric tides can propagate westward or eastward, and several westward propagating atmospheric tides that are synchronized with the motion of the sun or moon are called migrating solar or lunar tides. Since migrating tides are a large component of all atmospheric tides, many studies have focused on their occurrence (Chang et al., 2013;Hagan et al., 1995;Lin et al., 2019). Meanwhile, due to the presence of large vertical wavelengths, atmospheric tides can propagate from the lower atmosphere to the upper atmosphere and easily cause variabilities in the atmosphere. More systematic and detailed descriptions of atmospheric tides have been provided by Forbes (1995), Forbes and Garrett (1979), Lindzen and Chapman (1969), and the references therein.During the annual Northern Hemisphere (NH) winter, a dramatic large-scale meteorological phenomenon referred to as sudden stratospheric warming (SSW) occurs in the winter polar stratospheric region. Current theories mainly suggest that upward propagating quasi-stationary planetary waves (SPWs) play a crucial role in the dynamics of SSWs and that the interaction between quasi-SPWs and mean flow patterns in the stratosphere leads to SSW (Andrews et al., 1987;Butler et al., 2015;Matsuno, 1971). All SSW events are accompanied by substantial changes in both the temperature and horizontal wind components, and the temperature and winds have different changes at different altitudes. SSW can also occur in the Southern Hemisphere (SH) during winter but much less frequently than that during the NH winter due to the smaller SPW amplitudes (

The temporal variations in the sudden stratospheric warming (SSW) events in the winter stratosphere always coincide with the quasi-2-day wave (Q2DW) in the summer mesosphere, and the impact of SSW on Q2DW is interesting but still a mystery. Major SSWs occurred in both 2006 and 2009, while the Q2DW activity was quite different. The Second Modern Era-Retrospective Analysis for Research and Applications (MERRA-2) reanalysis dataset was used to comparatively analyze these two major SSW events and elucidate the reasons for the different Q2DW behaviors. We noticed that the summer easterly jet shows a large interannual variability. We conclude that the summer mesospheric Q2DWs are modulated by the winter SSW, whereas the modulation process is also affected by the interannual variability of the summer easterly flow itself. The effects of the SSW on the Q2DWs may differ from year to year due to the variability of the summer easterly flow itself, resulting in different anomalous Q2DW behavior. This conclusion may also be true for the interannual variability of other phenomena during the SSW period.

We report observations of the wave number transition between two types of westward-propagating quasi-2 day wave (QTDW) with zonal wave number s = 3 and s = 4 in the mesosphere. The Aura/MLS temperature datasets from January and February 2011 are utilized in the analysis. A strong W4 was observed in the Southern Hemisphere in early February, following the fading away of a strong W3 in late January. The W3 and W4 were maximized on days 23 and 36 with maximum amplitudes of ~15 K and ~8 K, respectively. Both the amplitudes and periods were nearly equal on day 30, which coincided with the peak of the Sudden Stratospheric Warming (SSW) on day 31 in the polar region of the Northern Hemisphere. Our analysis shows that the equatorial barotropic/baroclinic and inertial instabilities, which are related with the winter planetary wave activities that lead to the SSW, may have triggered this wavenumber transition event and contributed significantly to the growth of the QTDW. Meanwhile, the relatively strong summer easterly jet at middle and high latitude regions in early February also provided favorable conditions for the amplification of W4 during 2011, and thus facilitated the wavenumber transition.