Increasing 2020-Like Boreal Summer Rainfall Extremes Over Northeast Indian Subcontinent Under Greenhouse Warming

Due to windward slope topography and monsoon activities, the populated Northeast Indian subcontinent (NEI) suffers from heavy rainfall and floods almost every year. Extreme persistent downpours lashed NEI in summer 2020, ranked the second heaviest on record since 1901. This event caused about 550 fatalities and economic loss up to hundreds of millions of dollars. It is highly compelling but challenging to understand the weather drivers and future risks of this high-impact event. Here, we suggested this event was likely caused by the anomalous anticyclone (AAC) over the Indo-Northwest Pacific region and La Niña-induced Walker circulation intensification. The overall effect of current human-induced climate change contributed little to the occurrence probability of this event, as most of the warming and wetting effects of greenhouse gases were canceled out by anthropogenic aerosols. Climate models project an increasing risk of 1.77 (1.97), 2.08 (2.59), 2.58 (3.88), and 3.10 (5.52) times of such extreme event in the median-term (long-term) future under SSP1-2.6, SSP2-4.5, SSP3-7.0, and SSP5-8.5, respectively. It is mainly caused by the increases in atmospheric water vapor and 2020-like AAC frequency. Our findings indicate that future flooding risk over NEI will increase robustly if greenhouse warming continues.