Rising flood risks in semiarid South Asia driven by changing intraseasonal oscillations under global warming.

Northwestern South Asia, encompassing the historically semiarid regions of Pakistan and northwest India, has seen increased persistent heavy rainfall and catastrophic flooding in recent decades. While most studies emphasize seasonal mean changes, the role of subseasonal dynamics remains unclear. Here, we present observational evidence that the northward-propagating monsoon intraseasonal oscillation (ISO) has strengthened and penetrated farther inland, with its convective anomalies amplifying rainfall extremes. Simultaneously, the southeastward-propagating mid-latitude ISO along the westerly jet has slowed, prolonging anomalous circulations that sustain rainfall episodes. Together, these ISOs account for ~44% of the observed increase in flood frequency, a contribution comparable to that from mean-state changes (~40%). CMIP6 projections suggest that these ISO-driven processes will further intensify flood risks, posing escalating threats to this climate-sensitive region under continued global warming. Our findings reveal a fundamental yet overlooked mechanism linking subseasonal variability to emerging hydroclimatic extremes in a warmer world.