First Observation of Dominant Quasi‐Two‐Day Wave With Westward Zonal Wavenumber 3 at the December Solstice During Austral Summer: Links to Persistent Winter Stratopause Warming

Abstract During the 2023/2024 austral summer, the quasi‐two‐day (QTDW) with westward zonal wavenumber 3 (W3) abnormally reached its maximum amplitude at the December solstice (22 December 2023) for the first time in 20 years of Aura Microwave Limb Sounder observations, while the strongest event during austral summer usually occurs ∼2–6 weeks after the December solstice (on average January 21). Diagnostic analysis reveals that the westward winds in the Southern (summer) Hemisphere were anomalously strong (maximum of ∼90 m/s) during December 2023, which significantly shortened the e‐folding time of QTDW‐W3, and additionally generated the QTDW‐W3 critical layers at the tropical summer stratopause from December 7. These two factors contributed to the earliest amplification of QTDW‐W3. In essence, the cold equatorial stratosphere triggered the exceptionally strong westward winds in the Southern Hemisphere via thermal wind balance, which was related to the enhanced upward middle‐atmosphere Hadley circulation during a prolonged Arctic stratopause warming event.