Harnessing Subseasonal-to-Seasonal Predictability from Annual Evolution

(a) Climatological (solid circles) and individual-year (open circles) annual phase-space trajectories of the stratospheric polar vortex (SPV). The color bar indicates the progression through the calendar year. (b) Correlation skill of forecasts for monthly-mean SPV anomalies derived from differences between the predicted and climatological annual phase-space trajectories, shown as a function of lead time (left ordinate) and the day of year at which the forecasts are verified (right ordinate). Each predicted SPV annual phase-space trajectory is constructed from statistical predictions of its center, semi-major and semi-minor axes, orientation, and initial phase, using predictors available before October 1 prior to the forecasts of cold-season anomalies. The predictors consist of yearly varying indices representing climate modes, surface forcings, stratospheric ozone, atmospheric circulation state, isentropic mass circulation, and phase-space trajectory precursors. Credit: Secor et al. [2026]