Abstract
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.
Highlights
The dominance of large-amplitude planetary waves in the stratosphere, mesosphere and lower-thermosphere regions and their interactions with zonal mean winds are the primary driving forces of atmospheric dynamics
In the Southern Hemisphere (SH), the strongest eastward-propagating wavenumber 1 (E1) and E2 events occur on days 209–218 and 167–172, while E3 and E4 events occur on days 151–154 and 139–142
While the results demonstrate the decline in the peak amplitude with increasing zonal wavenumber in the Northern Hemisphere (NH), the occurrence date is irregular
Summary
The dominance of large-amplitude planetary waves in the stratosphere, mesosphere and lower-thermosphere regions and their interactions with zonal mean winds are the primary driving forces of atmospheric dynamics. Some recent studies have discovered significant eastward planetary waves in the polar stratosphere and mesosphere regions, with periods of nearly 2 and 4 d (Gu et al, 2017; Sandford et al, 2008; Merzlyakov and Pancheva, 2007; Coy et al, 2003; Manney and Randel, 1993). Sandford et al (2008) reported on significant fluctuations in E2 Q2DWs in the polar mesosphere They indicated the influence of changes in mean zonal winds during a major SSW on the propagation of polar E2.
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