Multi-scale drivers of compound day‒night heatwaves in Shanghai, China (1873–2023): The role of asymmetric warming, oceanic modes, and urbanization

A noticeable gap in urban climate science lies in the limited understanding of multiscale drivers (global climate, oceanic variability, urbanization interactions) governing the centennial evolution of compound day‒night heatwaves (CDNHWs) in major megacities. This study addresses it by investigating 150-year (1873–2023) evolution and drivers of CDNHWs in Shanghai, China—a representative megacity for rapidly urbanizing global metropolises. Two distinct CDNHW phases are identified: a stable low-occurrence phase (1870s–1980s) characterized by weak diurnal temperature synergy, persistently low frequency, and weak anthropogenic forcing; and an accelerated high-occurrence phase (1990s–2020s) marked by strong concurrent warming of days and nights, with CDNHW probability rising from 1.2% (mid-1920s–mid-1950s) to 11.1% (post-1990s). Approximately half of the post-1980s CDNHW increase stems from global warming, with nighttime minimum temperature ( T min ) warming (0.22 °C per decade) occurring three times faster than daytime maximum temperature ( T max ) warming (0.06 °C per decade). Multi-scale oceanic and atmospheric processes modulate CDNHW occurrence. At decadal timescale, the synergistic AMO-positive/PDO-negative phase amplifies warm nights via cloud-radiative feedbacks that suppress diurnal temperature range, with warm nights showing strong sensitivity to both AMO and PDO. Interannually, CDNHWs are favored by a seasonal transition from El Niño & warm Indian Ocean basin in winter to La Niña & cold Indian Ocean basin in summer, combined with persistent tropical North Atlantic warming. Sub-seasonally, the Madden–Julian Oscillation (especially Phases 1–2, linked to ∼50% of CDNHW events) and intra-seasonal atmospheric variability further regulate heatwave occurrence, exerting a stronger influence on daytime extremes. Locally, urbanization has shifted from daytime- to nighttime-dominated warming, reinforcing nocturnal heat retention and further promoting CDNHWs contributing approximately 7% of the observed total CDNHW days. These findings advance the understanding of centennial CDNHW drivers in megacities, highlighting megacities’ escalating CDNHW risk from the interplay of global, regional, and local forces, and provide a scientific basis for integrated prediction and urban resilience strategies.