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Abstract:
The wintertime circulation of the extratropical stratosphere is characterized by a strong band of circumpolar eastward winds called the polar vortex. Several external factors, including tropical stratospheric Quasi Biennial Oscillation (QBO), El-Nino Southern Oscillation (ENSO), Madden-Julian Oscillation (MJO), the 11-year solar cycle, and arctic snow have been found to influence the polar vortex. Recent studies have revealed that the tropical upper stratospheric winds also have a robust influence on the polar vortex. In this study, we use reanalysis datasets to study the coupling between the northern hemispheric polar vortex and the tropical upper stratosphere where the Semiannual Oscillation (SAO) is dominant. We identify a remarkable association between the temporal evolution of the polar vortex and SAO zonal winds. The results are contrasted for winters with and without major Sudden Stratospheric Warming (SSW) events. Eliassen-Palm flux diagnostics are used to explain the physical mechanism underlying this connection. Finally, long-term trends in the strength of the SAO zonal winds are presented, and potential implications for polar vortex dynamics are discussed.
