Climate change quadruples flood-causing extreme monsoon rainfall events in Bangladesh and northeast India

Bangladesh and northeast India are the most densely populated regions in the world where severe floods as a result of extreme rainfall events kill hundreds of people and cause socio-economic losses regularly. Owing to local high topography, the moisture-carrying monsoon winds converge near southeast Bangladesh (SEB) and northeast Bangladesh and India (NEBI), which produces significant extreme rainfall events from May to October. Using observed data, we find an increasing trend of 1-day extreme event (>150 mmday(-1)) frequency during 1950-2021. The extreme rainfall events quadrupled over western Meghalaya (affecting NEBI) and coastal SEB during this period. Composite analysis indicates that warm Bay of Bengal sea-surface temperature intensifies the lower tropospheric moisture transport and flux through the low-level jet (LLJ) to inland, where mountain-forced moisture converges and precipitates as rainfall during extreme events. To understand the role of climate change, we use high-resolution downscaled models from Coupled Model Intercomparison Project phase 6 (CMIP6). We find that the monsoon extreme event increase is ongoing and the region of quadrupled events further extends over the NEBI and SEB in the future (2050-2079) compared with historical simulations (1950-1979). A quadrupling of the intense daily moisture transport episodes due to increased LLJ instability, a northward shift of LLJ, and increased moisture contribute to the increased future extreme events. This dynamic process causes moisture to be transported to the NEBI from the southern Bay of Bengal, and the local thermodynamic response to climate change contributes to the increased extreme rainfall events. The CMIP6 projection indicates that more devastating flood-causing extreme rainfall events will become more frequent in the future.