Analysis of the 2021 Henan extreme rainfall event from the perspective of boreal summer intraseasonal oscillation

In July 2021, a rare and extremely severe rainstorm and flood disaster occurred in Henan Province, China. Using multiple daily observational and reanalysis data, this study investigates the modulation mechanisms of the boreal summer intraseasonal oscillation (BSISO) on the Henan extreme rainfall event. The analysis is conducted within the framework of the dual-mode BSISO indices (BSISO1: 30–60-day oscillation; BSISO2: 10–30-day oscillation), employing wavelet analysis, dry and wet phase analysis, and refined identification of active convection source regions. The results show that the extreme rainfall event was jointly modulated by BSISO1 and BSISO2, with BSISO2 being stronger than BSISO1. The deep convection of BSISO1 mainly originated from the western Indian Ocean, and its wet phase evolved through three stages. The deep convection of BSISO2 mainly originated from the South China Sea and the western North Pacific; convection from the South China Sea persisted throughout the rainstorm period, while convection from the western North Pacific was even stronger. This study not only provides direct evidence from BSISO phase composite evolution confirming that BSISO2 is stronger than BSISO1, but also refines the source region distinctions within the Indian Ocean and between the South China Sea and the western North Pacific. Against the background of increasing extreme events, this study offers a new perspective for understanding the multi-scale mechanisms of extreme precipitation under La Niña conditions, and provides a valuable reference for improving subseasonal-to-seasonal prediction of summer precipitation in Central China.