Subseasonal Predictability and Rossby Wave Dynamics of Blocking High during Transitional Seasons: Insights from Three Successive Events in May–June 2023

Abstract. Extended-range prediction during the transitional seasons remains particularly challenging due to the volatile large-scale circulation background. This study investigates the dynamical linkages, Rossby wave characteristics, and subseasonal predictability of three successive atmospheric blocking events in May–June 2023, which contributed to severe Canadian wildfires, anomalous East Asian precipitation, and European heatwaves. The results indicate that the three blocking episodes are not independent but interconnected through downstream propagation of quasi-stationary Rossby wave energy. Spatiotemporal local diagnostics of phase speed, amplitude, and zonal wavenumber further reveal that all episodes are dominated by slowly propagating, large-amplitude planetary-scale Rossby waves in the troposphere, with zonal wavenumbers typically below the climatological zonal wavenumber. Day-to-day evolution shows abrupt transitions during blocking onset from an eastward-propagating synoptic-scale small-amplitude regime to a quasi-stationary or westward-propagating planetary-scale large-amplitude regime, with the reverse during blocking’s decaying stage. ECMWF ensemble forecasts exhibit high predictability of 500 hPa geopotential height at 15–19-day lead times, with the best performance for the Ural blocking, where skillful members capture both the amplification of wave amplitude and spatial scale. In contrast, forecasts for the Canadian and European blocking show limited growth in wave parameters and associated geopotential height. For all episodes, skillful subseasonal predictions depend on capturing upstream quasi-stationary troughs over the North Pacific or North Atlantic, potentially influenced by exceptionally high sea surface temperatures in these basins during May–June 2023. These findings underscore that the growth in both amplitude and scale contributes to forecast errors in blocking circulation, while upstream wave precursors and external boundary forcing provide key sources of subseasonal predictability for persistent blocking circulation.