Rossby wave diversion as a source of subseasonal predictability for extreme Meiyu-Baiu events

Meiyu-Baiu, the distinct rainy season in subtropical East Asia, is characterized by a quasi-stationary Meiyu-Baiu Front (MBF) along which excessive rainfall and mega floods often occur. This study shows that the spatial stationarity of MBF, and thus its potential to generate catastrophic flooding is inherently modulated by synoptic-scale disturbances over northeastern East Asia. The activity of these disturbances is coupled with the Northern Annular Mode (NAM) and the subtropical westerly jet across the subseasosnal timescales. A prominent negative phase of the NAM shifts the jet and its associated Rossby waveguide equatorward, channeling wave energy toward subtropical East Asia instead of northeastern East Asia. This “diversion” of Rossby wave leads to suppressed development of synoptic-scale disturbances and a more stable mid-latitude airmass, ultimately stabilizing the MBF and thus persistent and extreme Meiyu-Baiu rainfall. This mechanism clearly operated in the extreme Meiyu-Baiu season in 2020. Subseasonal hindcasts of the event further confirm that their skills are directly proportional to their capability in predicting the NAM transition to a negative phase. This finding underscores the need to improve the representation of the extratropical atmospheric variability in forecast models to better predict and prepare for extreme rainfall and hydrological hazards typical of Meiyu-Baiu seasons.