Post-2000 Nonlinear Escalation of Humid Heat Stress Over South Asia

South Asia has experienced a nonlinear escalation of humid heat extremes since 2000. Using the humidex index to capture the combined effects of temperature and humidity, we show that over Northeast India and Bangladesh (NEI–BLG), total heatwave days have increased by about 10 days per decade, event frequency by 5 per decade, and the longest-duration spells by nearly 3 days per decade. In contrast, Northwest India exhibits a much slower increase of about 2 days per decade, with comparatively smaller changes in intensity. This pronounced east–west asymmetry reflects multiple compounded phenomena, including concurrent increases in daytime and nighttime temperatures within a persistently moisture-laden environment, together with a growing frequency of dry days or monsoon breaks over NEI–BLG. Hourly analysis further indicates extreme exposure exceeding 800 hour season⁻¹ in recent decades, while El Niño years amplify humidex anomalies by 2–2.5°C. Collectively, these findings provide the first long-term evidence that humid heatwaves are intensifying faster than dry heat events, shifting the epicenter of heat stress from the arid northwest to the humid northeast of South Asia.