Influence of Boreal Summer Intraseasonal Oscillation on Persistent Heavy Rainfall Events During the Plum Rain Period

Persistent heavy rainfall events (PHREs) over the middle and lower reaches of Yangtze River (MLYR) exert substantial societal impacts during the plum rain season, yet the role of intraseasonal variability in their occurrence remains insufficiently understood. This study investigates how two types of boreal summer intraseasonal oscillation (BSISO) modulate the frequency, intensity and location of PHREs. Statistical analyses show that majority (92%) of PHREs occur during active BSISO phases, highlighting its critical intraseasonal influence. During phases 2–5 (5–8) of BSISO1 (BSISO2), enhanced convection over the northern South China Sea and western North Pacific triggers prevailing anticyclonic circulation anomalies, strengthening moisture transport toward the MLYR and increasing PHREs frequency by 70–90% (over 80%) relative to non-BSISO periods. PHREs associated with BSISO1 exhibit a broader and more northward distribution, whereas those related to BSISO2 are more intense and concentrated over the south-central MLYR. When both BSISO modes are simultaneously active in the convective phases, PHREs intensify and form a pronounced rain belt along the MLYR. Scale-decomposed moisture budget diagnostics further reveal that interactions between BSISO-related flows and the low-frequency background moisture field precondition and sustain PHREs. These results highlight the essential role of such multiscale moisture-circulation interactions in predicting the PHREs over the MLYR.