Mid-latitude mesospheric zonal waves in the 2018 and 2021 major sudden stratospheric warming events

The planetary wave activity in the stratospheremesosphere during the major Sudden Stratospheric Warmings (SSW) at mid-latitudes is discussed. Because some indications exist of their independent evolution in the stratosphere and mesosphere, the study of the relationships between zonal wave 1 and wave 2 is the subject of interest. Aura Microwave Limb Sounder (MLS) measurements of carbon monoxide (CO), temperature and geopotential height were used to estimate wave amplitudes during the 2018 and 2021 winter major sudden stratospheric warmings (SSWs) with a focus on the mid-latitude mesosphere at 50 N latitude. Eastward and westward migrations of wave 1/wave 2 spectral components were distinguished. Sharp changes in zonal wave spectra occur with the zonal wind reversal in the SSW 2018. Wavelet power spectra of mesospheric CO variations show statistically significant periods of 2030 days. Although westward wave 1 in the mesosphere dominates with the onset of the SSW 2018, it develops independently of stratospheric dynamics. In the case of SSW 2021, wavelet analysis shows a clear 8-day period in waves 1 and 2 in the mesosphere at 0.01 hPa (80 km). Unlike this, the period in the stratospherelower mesosphere is twice as long as 16 days and is statistically significant only in wave 2. Since the propagation of upward planetary waves is limited in the easterly zonal flow in the stratosphere during SSW, forced planetary waves in the mid-latitude mesosphere may exist. As shown earlier, mesospheric planetary waves can be generated in situ by breaking or dissipation of gravity waves, instabilities in the zonal mean zonal wind, or a mixture of gravity wave drag and instabilities. Possible sources of mid-latitude mesospheric planetary waves in the SSWs in 2018 and 2021 are discussed.