Dynamically Motivating the Definition of Sudden Stratospheric Warmings

The polar stratospheric vortex impacts stratospheric ozone concentrations and couples with surface weather and climate. The preconditions and effects of its most profound dynamics -- sudden stratospheric warmings (SSWs) -- are still not fully understood. In analyzing SSW dynamics, many researchers simplify or alter the thresholds in the World Meteorological Organization (WMO) definition of SSWs. The WMO defines major SSWs based on a reversed temperature gradient and reversed circulation at 60N, 10 hPa. Modern Era Retrospective Analysis for Research and Applications (MERRA) and an idealized polar disc model are used to (1) examine whether altering the definition impacts the number and type of events identified, and (2) dynamically motivate the WMO latitude and circulation thresholds. Common alterations to the latitude and circulation criteria are found to change the number of major SSWs identified by up to 40%, and to select types of warmings with differing latitudinal extent, coherence, and intensity. Dynamical motivations that justify the 60N threshold emerge because 60N (a) marks the edge of the polar vortex, (b) exhibits maximal sensitivity to SSW dynamics, and (c) marks the transition between the coherent vortex zone and the surf zone. Dynamical motivations for the circulation threshold emerge because (a) 0 m/s marks the inflection point of the cumulative frequency wind distribution, and (b) 0 m/s possibly maximizes the average number of events identified across all latitudes.