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

Extreme persistent rainfall poses serious impacts on human and natural systems, predominately through its related hydrogeological disasters. Due to sustained heavy downpours, the summer of 2020 was the second wettest on record over Northeast Indian subcontinent since 1901. Here, we find that this orographically anchored extreme rainfall event was largely associated with the anomalous anticyclone (AAC) over the Indo‐Northwest Pacific region and La Niña‐induced Walker circulation intensification. The overall effect of anthropogenic forcings contributed little to the occurrence probability of this event, because the warming and wetting effects of greenhouse gases were almost negated by anthropogenic aerosols. Climate models project a prominent increasing trend of such extreme event under future greenhouse‐induced warming due to increase in atmospheric water vapor and 2020‐like AAC frequency. Our findings thus call for scaling up climate change adaptation efforts for increasingly extreme persistent rainfall in highly populated but low‐resilience South Asian developing countries. Plain Language Summary: 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. Key Points: Extreme persistent rainfall in boreal summer 2020 over Northeast Indian subcontinent was the second heaviest on record since 1901This event was likely caused by the Northwest Pacific anomalous anticyclone (AAC) and La Niña‐induced Walker circulation intensificationIncreasing 2020‐like rainfall extremes are projected due to increase in future atmospheric water vapor and AAC frequency [ABSTRACT FROM AUTHOR]