AbstractAbstractOn the subseasonal-to-seasonal (S2S) time scale, roughly corresponding to the period between 2 weeks and 2 months, high-latitude stratospheric variability impacts tropospheric weather. During the winter, changes to the stratospheric polar vortex serve as a key driver of tropospheric S2S variability. The state of the stratospheric polar vortex and its extremes, including sudden stratospheric warmings (SSWs), are commonly characterized through metrics defined by the zonal-mean state. Other nonzonal approaches provide a view of stratospheric behavior that is well suited for analyses of stratospheric impacts. Ellipse metrics can define the polar vortex and approximate its shape and position by quantifying vortex area, central latitude and longitude location, elongation, and orientation. Contours of physical variables, such as potential vorticity or trace gases, at levels within the stratosphere were previously used for calculating ellipse metrics. More recent methods have shown that similar results can be obtained using reference contours of geopotential height. This analysis explores variations in the ellipse metric calculation to support the use of a best-fit ellipse methodology on a contour of geopotential height for defining the vortex edge. The proposed method for assessing stratospheric polar vortex variability yields comparable results to previous studies in identifying and classifying SSWs using ellipse metrics, including distinctions between split and displaced vortex events. Additionally, the method yields promising results for identifying conditions associated with stratospheric wave reflection events. Overall, this analysis provides an alternative framework for applying ellipse metrics, supports previously established methodologies, and highlights the potential utility of ellipse metrics in forecasts of stratospheric polar vortex variability.Significance StatementInteractions between the stratosphere and troposphere are important for assessing forecast skill on longer lead times. To better understand these interactions, we must effectively and efficiently measure the behavior of the wintertime (November–March) stratospheric polar vortex. This paper proposes and assesses an adaptive method for calculating geometries for use in forecast analyses that represent the stratospheric polar vortex, referred to as ellipse metrics. The new ellipse metrics build on previous methods of calculating the vortex ellipse and yield comparable results to previous studies in identifying and classifying stratospheric behavior. The ellipse method also shows promise in identifying types of variability that occur in the wintertime stratosphere, such as sudden stratospheric warming and wave reflection events. Both of these phenomena are known to have downward impacts on North American temperatures.
