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During February 2023, the quasi‐4‐day wave (Q4DW) with westward zonal wavenumber 2 (W2) reached its largest amplitude of ∼400 m in the Southern Hemisphere (SH) geopotential height observations since 2004, which occurred simultaneously with an Arctic major sudden stratospheric warming (SSW) with an elevated stratopause (ES). However, the Q4DW‐W2 perturbations in the Northern Hemisphere (NH) were unexpectedly suppressed despite the unstable Arctic stratosphere and mesosphere during the 2023 ES‐SSW. Diagnostic analysis shows that the westward winds at ∼54°N–70°N in the upper stratosphere of ∼‐79 m/s during the 2023 ES‐SSW were the strongest during boreal winters over the past two decades, which benefited from the onset of a preceding minor SSW at the end of January. The strongest westward wind generated a wave geometry configuration of full reflection for Q4DW‐W2 in the NH, while the Q4DW‐W2 enhancement in the SH was induced by the in‐situ amplification of the surviving seeding perturbations. Plain Language Summary: Among the planetary waves with a period of about 4 days, the eastward quasi‐4‐day wave (Q4DW) excited by the double‐jet structure in the Southern Hemisphere is the most famous one, while another Q4DW with westward zonal wavenumber 2 (W2) belonging to the normal modes of the Earth's atmosphere has attracted little attention. During the February 2023 sudden stratospheric warming (SSW) with an elevated stratopause (ES), the exceptional enhancement and suppression of Q4DW‐W2 were captured in the Southern Hemisphere and Northern Hemisphere, respectively. Such global structure is first observed and abnormal among traveling planetary waves during SSW since they usually have a peak region in the unstable winter hemisphere. We found that the rare sequence of stratospheric disturbances in the 44‐year historical record that a minor SSW followed by an ES‐SSW led to the excessively strong westward wind during the 2023 ES‐SSW, which resulted in a wave geometry configuration of full reflection for Q4DW‐W2 in the Northern Hemisphere. Our current results well establish the relationship between a new global structure of traveling planetary waves during SSW and the rare sequence of stratospheric disturbances. This will deepen the understanding of the less studied Q4DW‐W2 and the Earth's whole atmospheric coupling during SSW. Key Points: The anomalous burst and suppression of quasi‐4‐day wave (Q4DW) with westward zonal wavenumber 2 (W2) were observed in the Southern Hemisphere and Northern Hemisphere (NH) respectively during the February 2023 major sudden stratospheric warming (SSW) with elevated stratopause (ES)The rare sequence of SSWs in January and February 2023 led to the strongest westward wind during boreal winters over the past two decadesThe strongest westward wind during the 2023 ES‐SSW created a wave geometry configuration of full reflection weakening the Q4DW‐W2 in the NH [ABSTRACT FROM AUTHOR]

We report observations of the wave number transition between two types of westward-propagating quasi-2 day wave (QTDW) with zonal wave number s = 3 and s = 4 in the mesosphere. The Aura/MLS temperature datasets from January and February 2011 are utilized in the analysis. A strong W4 was observed in the Southern Hemisphere in early February, following the fading away of a strong W3 in late January. The W3 and W4 were maximized on days 23 and 36 with maximum amplitudes of ~15 K and ~8 K, respectively. Both the amplitudes and periods were nearly equal on day 30, which coincided with the peak of the Sudden Stratospheric Warming (SSW) on day 31 in the polar region of the Northern Hemisphere. Our analysis shows that the equatorial barotropic/baroclinic and inertial instabilities, which are related with the winter planetary wave activities that lead to the SSW, may have triggered this wavenumber transition event and contributed significantly to the growth of the QTDW. Meanwhile, the relatively strong summer easterly jet at middle and high latitude regions in early February also provided favorable conditions for the amplification of W4 during 2011, and thus facilitated the wavenumber transition. [ABSTRACT FROM AUTHOR]