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Vortex Preconditioning Due to Planetary and Gravity Waves Prior to Sudden Stratospheric Warmings
Using the composite analysis as a guide, we conduct a case study of the 2009 SSW in order to evaluate the roles of planetary and gravity waves for preconditioning the polar vortex in terms of two SSW triggering scenarios: anomalous planetary wave forcing from the troposphere, and resonance due to either internal or external Rossby waves. The results support the view that split SSWs are caused by resonance rather than anomalously large wave forcing. Given these findings, we suggest that vortex preconditioning – which is traditionally defined in terms of vortex geometries that increase poleward wave focusing – may be better described by wave events that `tune' the geometry of the vortex towards its resonant excitation points.