Frequent Floods in the Yangtze River Basin Linked to a shifted Indian Ocean Wave Regime

Six major summer monsoon floods occurred in the Yangtze Basin between 1992–2024, compared to only one during 1960–1991.This significant rise in hydroclimatic extremes is closely linked to enhanced variability at the quasi-biennial (QB) timescale. In this study, using sea surface height and thermocline depth from the ORAS5 reanalysis and EN4 observational analysis, we demonstrate that the increased QB variability in East Asian summer monsoon (EASM) rainfall over the Yangtze River Basin is strongly coupled with intensified QB-scale wave dynamics in the Indian Ocean. We provide evidence of fundamental changes in the characteristics of baroclinic waves in the tropical Indian Ocean over recent decades. We find that the mean phase speed of westward-propagating tropical Rossby waves has increased by 50–60%, along with their overall variance. These shifts are likely associated with changes in both oceanic circulation and large-scale atmospheric forcing. Our findings highlight that evolving Indian Ocean wave characteristics are a key contributor to the amplification of monsoon variability in QB timescales and associated hydrological extremes over East Asia. Focused scientific efforts are further needed to investigate the specific roles of possible drivers in altering wave properties and to assess their implications for EASM predictability and associated impacts.