Nocturnal boundary-layer ventilation failure governs heatwave persistence in South Asia

Persistent heatwaves across South Asia impose severe and growing impacts, yet the atmospheric processes that sustain extreme heat over multiple days remain incompletely understood. This study aims to determine whether heatwave persistence is driven by failures of nocturnal boundary-layer ventilation, rather than by daytime temperature extremes alone. We analyze pre-monsoon (March–May) heatwaves across South Asia (65°E–98°E, 5°N–35°N) from 1981 to 2024 using ERA5 hourly reanalysis, which includes near-surface air temperature, boundary-layer height, near-surface winds, and surface sensible heat flux. Heatwaves are identified using a percentile-based definition of daily maximum temperature, and nighttime conditions are diagnosed consistently using local solar time. Nocturnal ventilation is quantified through a physically interpretable ventilation potential combining nighttime boundary-layer height and near-surface wind speed, complemented by diagnostics of turbulent mixing and nocturnal cooling. We find that heatwave nights are consistently characterized by suppressed nocturnal ventilation, including shallow boundary layers, weak winds, and reduced turbulent exchange, and that reductions in nighttime ventilation are more strongly associated with heatwave duration and nighttime heat accumulation than daytime temperature anomalies. Composite analyses further indicate that ventilation and turbulent mixing weaken before heatwave onset and remain suppressed throughout the persistence phase, with particularly pronounced effects in humid regions such as Bangladesh. Our findings demonstrate that nocturnal boundary-layer ventilation failure is a central physical mechanism controlling heatwave persistence and suggest that incorporating nighttime atmospheric processes into heatwave monitoring and early-warning frameworks is essential for anticipating prolonged and high-impact heat extremes.