Planetary Wave Spectrum in the Stratosphere–Mesosphere during Sudden Stratospheric Warming 2018

Abstract

The planetary wave activity in the stratosphere–mesosphere during the Arctic major Sudden Stratospheric Warming (SSW) in February 2018 is discussed on the basis of microwave radiometer (MWR) measurements of carbon monoxide (CO) above Kharkiv, Ukraine (50.0° N, 36.3° E) and the Aura Microwave Limb Sounder (MLS) measurements of CO, temperature and geopotential heights. From the MLS data, eastward and westward migrations of wave 1/wave 2 spectral components were differentiated, to which less attention was paid in previous studies. Abrupt changes in zonal wave spectra occurred with the zonal wind reversal near 10 February 2018. Eastward wave 1 and wave 2 were observed before the SSW onset and disappeared during the SSW event, when westward wave 1 became dominant. Wavelet power spectra of mesospheric CO variations showed statistically significant periods of 20–30 days using both MWR and MLS data. Although westward wave 1 in the mesosphere dominated with the onset of the SSW 2018, it developed independently of stratospheric dynamics. Since the propagation of upward planetary waves was limited in the easterly zonal flow in the stratosphere during SSW, forced planetary waves in the mid-latitude mesosphere may exist due to the instability of the zonal flow.

Mid-Latitude Mesospheric Zonal Wave 1 and Wave 2 in Winter 2020–2021

“…It was shown that the altitude dependence is not only in the amplitudes of wave 1 and wave 2 [23], but also in the direction of wave migration. Wave 1 migrates westward (eastward) in the stratosphere (mesosphere) in the pre-warming period and the dominant westward wave 1 exists in the post-warming mesosphere [5]. In the events SSW 2018 and SSW 2019, a similar equatorward shift of the maximum wave amplitude at 10 hPa was found: 60-80°N (wave 1), 50-70°N (wave 2), and 50-60°N (wave 3) [24].…”

“…The variations in Z1 and Z2 are analyzed during the two winter months, December 2020-January 2021, when a SSW event occurred, and are compared with the three preceding winters of 2018-2020. A zonal wave analysis was carried out along the 50°N latitude circle, passing through the northern Ukraine and northern China, in order to continue the recent studies of mid-latitude SSW manifestations by Wang et al [5,23,34] and Shi et al [24].…”

“…Particular attention is paid to the problem of sudden stratospheric warmings (SSWs), which are associated with large disturbances of the zonal flow of the polar regions [1][2][3]. The observed zonal circulation anomalies in the winter troposphere-stratosphere-mesosphere indicate that they are formed mainly due to variations in the amplitude of planetary waves with zonal wave numbers m = 1 (wave 1) and m = 2 (wave 2) [1, 4,5]. Propagating upward from the troposphere, wave 1 and wave 2 can decelerate the strong westerly flow in the stratospheric polar vortex region.…”

“…5], cannot propagate upward through easterly stratospheric flow [3,15]. However quasi-stationary zonal wave 1 and wave 2 have been observed in the mesosphere after the reversal to westerly flow during a SSW [4,5,16]. From their absence below 50 km, it can be inferred that an independent source of quasistationary waves lies in the mesosphere [16][17][18][19].…”

“…The mid-latitude zonal planetary waves around 50°N have been recently analyzed by Wang et al [5,23] and Shi et al [24]. It was shown that the altitude dependence is not only in the amplitudes of wave 1 and wave 2 [23], but also in the direction of wave migration.…”