Planetary Wave Activity During 2019 Sudden Stratospheric Warming Event Revealed by ERA5 Reanalysis Data

The minor sudden stratospheric warming (SSW) event and the relevant planetary waves are investigated by analyzing ERA5 reanalysis data from July to December 2019. Frequency-wavenumber spectral analysis shows that the quasi-10-day and quasi-16-day waves dominate the stratosphere over the Southern Hemispheric polar region with the eastward-propagating wavenumber 1 during the SSW event. The corresponding amplitudes and phases of each wave mode have been fitted using the two-dimensional harmonic fitting method. The result suggests that quasi-16-day and quasi-10-day waves prior to the SSW event had an important effect on the occurrence of the SSW event. Furthermore, the Eliassen–Palm flux diagnosis shows that the quasi-16-day wave and quasi-10-day wave had poleward and equatorward-propagating components. The poleward-propagating component may have come from the tropical tropospheric convective activity. The equatorward component may have been excited by the atmospheric barotropic/baroclinic instability.