Subseasonal predictability and Rossby wave dynamics of blocking high during transitional seasons: insights from three successive events in May–June 2023

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 episodes over Canada, the Ural Mountains, and Europe in May–June 2023, which contributed to severe Canadian wildfires, persistent East Asian precipitation, and European heatwaves. The results indicate that these blocking episodes are not independent but interconnected through downstream energy dispersion of Rossby waves. 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, thereby contributing to the exceptional persistence and large spatial extent of the surface weather extremes. 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 subseasonal-to-seasonal (S2S) ensemble forecasts exhibit high predictability of 500 hPa geopotential height anomalies at 15–19 d lead times for these three blocking episodes. However, the S2S forecasts underestimate the amplification of wave amplitude and spatial scale, particularly for the Canadian and European blockings. For all episodes, skillful predictions beyond two weeks depend on the successful prediction of upstream quasi-stationary troughs and the associated downstream energy dispersion of quasi-stationary Rossby waves. In contrast, poorly performing members are characterized by the propagation of synoptic-scale waves along the subtropical or mid-latitude jet. Moreover, the Canadian blocking provides a window of opportunity for the extended predictability of the subsequent Ural and European blockings through their dynamical interconnections. These findings underscore that successful prediction of upstream Rossby wave propagation is crucial for the subseasonal predictability of persistent blocking.