Dynamical linkage between blocking predictability and jet stream quasi-stationary states

This study investigates atmospheric blocking from the perspective of the instantaneous stationarity of the jet stream. The framework of the quasi-stationary state (QS) dynamical theory is applied to characterize the behavior of ensemble prediction members. Using the Japanese Reanalysis for Three Quarters of a Century (JRA-3Q), we classified atmospheric conditions over the Northern Hemisphere into states characterized by small and large temporal variability in jet stream tendency, referred to as QS and Non-QS respectively, and examined the relationship between the former and blocking patterns.

During QS conditions, the westerlies exhibited significant meandering, and blocking occurred regardless of the blocking type (Omega or Dipole). These results are consistent with blocking defined by potential vorticity reversal at the dynamical tropopause and its persistence.

Based on linearized equations, a relationship is identified between QS and the non-stationary minimum point (MP), where at least one of its eigenvalues is zero. Analysis of forecast data from JMA's Global Ensemble Prediction System (GEPS) revealed that ensemble spread tends to increase with forecast time when the initial state is QS. This result is consistent with the proposed dynamics. Conversely, under a Non-QS initial state, initial uncertainty persists throughout forecast evolution.

These findings suggest that atmospheric blocking is a manifestation of the instantaneous stationarity of the jet stream, indicating that this theoretical framework is valuable for examining the predictability of blocking and interpreting ensemble forecasts.