Post-2000 Escalation of Humid Heatwaves and Emerging Cloudburst Risks across the South Asia

Abstract

The IPCC AR6 (IPCC, 2022) identifies South Asia, particularly India and Bangladesh, as one of the regions most affected by climate change, where extreme weather events are intensified by rising sea levels, melting glaciers, and high social and economic vulnerability. Recent attribution studies confirm that anthropogenic warming has doubled the likelihood of pre-monsoon extreme rainfall events in Bangladesh, such as the 2017 floods that inundated more than 220,000 hectares of cropland. Yet the Northeast India–Bangladesh (NEI–BD) region remains critically underexplored in terms of both thermal and hydrological extremes.

This study assesses the dual vulnerability of the NEI–BD monsoon core, focusing on the post-2000 escalation of humid heatwaves and the emergence of cloudburst hazards. Using ERA5 reanalysis, we derived humidex-based heatwave metrics to quantify human-perceived heat stress. Independently, cloudburst occurrences were identified from GPM-IMERG and tipping-bucket rain-gauge observations established under the Kagawa University–JSPS project (Terao et al., 2023), which recorded 39 extreme hourly rainfall events (≥ 100 mm h⁻¹) across eight stations in NEI–BD.

Results show a nonlinear intensification of humid heat stress after 2000, with total heatwave days tripling and multi-week humidex anomalies exceeding 90^th percentile of day of the year, while Northwest India (NWI) shows slower change. Simultaneously, clusters of cloudburst events are concentrated over the Meghalaya Plateau and the Himalayan foothills of Arunachal Pradesh. However, cloudburst documentation across NEI remains severely limited owing to the lack of high-resolution data and restricted accessibility of rainfall records from multiple agencies, leading to large observational gaps.

Although the two extremes are not causally linked, their spatial co-occurrence within the same moist-warm environment highlights a shared climatic driver—a warming, moisture-rich boundary layer that fuels both thermal stress and convective deluges. The NEI–BD corridor is therefore emerging as a dual-exposure hotspot, demanding integrated monitoring and adaptation strategies for compound climate risks.