Sudden stratospheric warmings as nonequilibrium transitions: evidence from eigen-microstate entropy

Sudden stratospheric warmings (SSWs) are among the most dramatic regime transitions in the winter stratosphere, yet their onset remains difficult to diagnose and predict. We explore SSWs from a critical-transition perspective using Eigen Microstate Theory (EMT), which provides an entropy-based measure of how the circulation reorganizes during the event life cycle. In reanalysis composites of major SSWs, we identify a robust, non-monotonic entropy evolution: it rises during vortex deceleration, reaches a maximum prior to onset, and then collapses sharply as the vortex breaks. This “order–disorder–order” sequence provides direct empirical evidence that SSWs exhibit signatures of phase transitions and criticality in the real atmosphere. 

To connect this statistical signature to dynamics, we analyze a one-dimensional wave–mean-flow interaction model that captures the nonlinear feedbacks underpinning vortex destabilization. The model reproduces the same entropy peak and collapse when the system is driven toward instability, supporting the interpretation of eigen-microstate entropy as an order parameter for an intrinsically nonequilibrium transition. Across both reanalysis and model experiments, supported by analytical considerations, the entropy shows a pronounced response as the system approaches loss of stability and provides a clearer precursor than conventional single-series early-warning indicators such as lag-1 autocorrelation (AR1). These results suggest a physically interpretable, entropy-based diagnostic of SSW criticality with potential value for subseasonal prediction.