A New Mechanism for the Generation of Quasi‐6‐Day and Quasi‐10‐Day Waves During the 2019 Antarctic Sudden Stratospheric Warming

Using the Modern‐Era Retrospective analysis for Research and Applications version 2 reanalysis and Aura Microwave Limb Sounder geopotential height and temperature observations, two unusual planetary waves (PWs) with westward zonal wavenumber 1 (W1) and periods of ∼6.0 and ∼9.6 days are identified during a rare Antarctic sudden stratospheric warming (SSW) event in September 2019, which follow a strong W1 PW with period of ∼7.4 days. It is found that the W1 ∼7.4‐day wave is actually the representation of the climatological W1 quasi‐6‐day wave before the September equinox, which peaks on September 10, 2019. Meanwhile, a strong zonally symmetric wave with period of ∼32.2 days occurs in the Southern Hemisphere with a maximum zonal wind amplitude of ∼27 m/s at 65°S and 48 km. Interestingly, the frequencies and zonal wavenumbers of the W1 ∼7.4‐day, ∼6.0‐day, ∼9.6‐day and zonally symmetric ∼32.2‐day waves exactly satisfy the nonlinear interaction theory between PWs. We thus proposed for the first time the occurrence of nonlinear interaction between a W1 ∼7.4‐day wave and a zonally symmetric ∼32.2‐day wave, which generates two W1 PWs with periods of ∼6.0 and ∼9.6 days. The Eliassen‐Palm flux diagnostics suggest that the zonally symmetric ∼32.2‐day wave also contributes to the wind deceleration during this SSW. Moreover, the baroclinic/barotropic instabilities related to the vertical shear in the zonal wind during the SSW considerably enhance the PW activities in the Antarctic stratosphere. The climatological variations of the instabilities and critical layers in the mesosphere can also influence the propagation of these three W1 PWs. Plain Language Summary A very rare Antarctic sudden stratospheric warming (SSW) event occurred in September 2019, which led to an unexpectedly strong quasi‐6‐day wave (T≈ $T\approx $ 6.0 days) and an unusual quasi‐10‐day wave (T≈ $T\approx $ 9.6 days) with westward zonal wavenumber 1 (W1) in the Southern Hemisphere mesopause. In this article, the Modern‐Era Retrospective analysis for Research and Applications version 2 reanalysis and Aura satellite Microwave Limb Sounder observations are analyzed jointly to study the excitation mechanism for the W1 ∼6.0‐day and ∼9.6‐day waves during this SSW. Before the occurrence of the W1 ∼6.0‐day wave, a climatological W1 quasi‐6‐day wave (T≈ $T\approx $ 7.4 days) is also captured in the mesopause. The nonlinear interaction between the climatological W1 ∼7.4‐day wave and a zonally symmetric ∼32.2‐da