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ESRL/PSD Seminar Series

Relative Roles of Gravity and Planetary Waves in Vortex Preconditioning Prior to Sudden Stratospheric Warmings

John Albers
National Science Foundation Fellow, NOAA/ESRL PSD


Recent observational research has suggested that large disruptions of the stratospheric polar vortex - such as those that occur during a major sudden stratospheric warming (SSW) - may modulate deep convection by injecting intrusions of high potential vorticity air into the deep tropical upper troposphere. In particular, because SSWs occur over very particular geographic regions, they provide an interesting opportunity to use knowledge of the state of the stratosphere to help predict where and when convection may be triggered in the deep tropics. However, taking advantage of such a predictive capability will require forecast skill in predicting the occurrence of SSWs themselves. Unfortunately, predicting when a SSW will occur is not yet possible given our current knowledge on how warmings are actually triggered. We make progress on this front as follows.

We conduct a composite analysis using reanalysis data to evaluate the evolution of polar vortex geometry prior to the onset of two distinct categories of SSWs: vortex splitting events versus vortex displacements events. 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.

Wedndaday, Feb 21
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