Intensified Indian Ocean Rossby Wave Dynamics as a Driver of Increased Quasi-Biennial Summer Monsoon Floods in the Yangtze River Basin

Six major summer monsoon floods occurred in the Yangtze Basin between 1992–2024 affecting millions of people, compared to only one during 1960–1991. This significant rise in hydroclimatic extremes is closely associated with an approximately 50% increase in variability at the quasi-biennial timescale. In this study, using sea surface height and thermocline depth from the ORAS5 reanalysis and EN4 observational analysis, we demonstrate that the increased quasi-biennial variability in East Asian summer monsoon rainfall over the Yangtze River Basin is strongly coupled with intensified quasi-biennial 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 70%, along with their overall variance. These shifts are likely associated with changes in large-scale atmospheric forcing. Our findings highlight that evolving Indian Ocean wave characteristics are a key driver of changes in East Asian summer monsoon variability at quasi-biennial timescales and the associated hydrological extremes over East Asia, with important implications for the predictability of East Asian summer monsoon rainfall at these timescales.