Synergistic Effect of the 25–60‐day Tropical and Midlatitude Intraseasonal Oscillations on the Persistently Severe Yangtze Floods

Abstract Significant 25–60‐day intraseasonal oscillations (ISOs) in rainfall over the middle‐lower reaches of the Yangtze River (MLYR) are found for eight summers during 1979–2020. These 25–60‐day ISOs of MLYR rainfall account for 66.7% of persistently severe floods (PSFs) during 1979–2020, indicating their major role in inducing PSFs and the potential for extended‐range forecasting of PSFs. The 25–60‐day ISO in MLYR rainfall results from the synergistic effect of tropical and midlatitude ISOs. During the eight identified summers, more barotropic energy is transferred from summer‐mean to 25–60‐day circulation, thus forming a midlatitude ISO wave train, which cooperates with the tropical ISO in triggering twin vertical cells. Phase locking of ascending branch from the vertical cell induced by the tropical ISO with that from the reversed vertical cell triggered by the midlatitude ISO brings about the wet phase of 25–60‐day ISO of MLYR rainfall.