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 during 1992-2024 compared to only one during 1960-1991. This significant increase in hydroclimatic extremes, which affected millions of people, is closely linked to a 50% enhancement in quasi-biennial variability in East Asian summer monsoon rainfall and associated Yangtze River discharge. This quasi-biennial variability is strongly coupled with intensified tropical baroclinic wave activity in the Indian Ocean under enhanced ENSO forcing. During the later period, amplified westward propagating downwelling baroclinic Rossby waves in late boreal spring and summer strongly suppressed vertical mixing and entrainment in the western tropical Indian Ocean, maintaining anomalously warm sea surface temperatures. This anomalously warm ocean enhanced local atmospheric convection and tropospheric heating, which remotely strengthened the western Pacific subtropical high via atmospheric Kelvin wave processes, thereby increasing moisture transport and summer monsoon rainfall over the Yangtze River Basin. A 70% increase in the phase speed of the baroclinic Rossby waves during the later period also favoured the seasonal phase locking of ocean wave-driven tropical Indian Ocean warming with the East Asian summer monsoon.