Elliptical Orbit Representation for the Annual Evolution of the Northern Hemisphere Stratospheric Polar Vortex. Part I: Diagnosing Interannual Variability

Abstract Using a single annual frequency with each year defined from July 1 to June 30 of the following calendar year, this study fits an ellipse to the yearly phase space trajectory of the daily time series of MU (stratospheric mass‐weighted zonally integrated zonal wind at 60°N) and M (total polar stratospheric air mass over 60–90°N) to represent the annual evolution of the Northern Hemisphere stratospheric polar vortex (SPV) in individual years. The utility of this elliptical representation rests on two principal capabilities. First, interannual variability in the SPV's annual trajectory, as well as its sub‐seasonal and seasonal anomalies, can be captured by yearly variations in the six ellipse parameters. Second, the temporal lead of winter maxima in MU with respect to winter minima in M derived from the elliptical representation provides the general lead time information of MU anomalies with respect to M anomalies from sub‐seasonal to seasonal scales. This serves as direct evidence indicating that sub‐seasonal and seasonal anomalies are closely linked to annually varying SPV trajectories. Moreover, the annually varying ellipse parameters and their derived quantities contain statistically significant information on the timing of sudden stratospheric warmings (SSWs). By distilling the SPV's complex seasonal evolution into six ellipse parameters, this framework offers a concise empirical basis for long‐lead forecasts of MU and M throughout the cold season, as well as timing of SSW events, provided that these ellipse parameters can be predicted before the cold season.