Spatiotemporal dynamics of urban extreme heat under intensifying potential future heatwaves: Evidence from the Tokyo megalopolis

Global warming intensifies heatwaves and associated risks in metropolitan areas, particularly in coastal cities where marine–terrestrial thermal forcing interacts with the urban heat island (UHI). Using the Tokyo Metropolitan Area (TMA) as a case, this study dynamically downscales Coupled Model Intercomparison Project Phase 6 projections with the Weather Research and Forecasting model, incorporating gridded urban canopy parameters to examine future heatwave characteristics across emission scenarios. Nighttime heatwaves are projected to intensify more than daytime ones. Under Shared Socioeconomic Pathway (SSP) 2–4.5, post–mid-century daytime heatwaves show moderate intensification driven mainly by background warming, with the baseline rising by ∼2–3 °C while extreme amplitudes slightly weaken. The downscaled results further indicate the strongest increases in daytime air temperature during heatwaves over coastal transition areas of the TMA, reaching ∼2 °C under SSP2–4.5 and up to ∼3.5 °C under SSP5–8.5 by end-century. At night, spatial responses diverge markedly between scenarios. Under SSP5–8.5, temperature increases exceed 4 °C in western TMA, overlapping with the UHI-intensified areas and thereby expected to pose substantial urban hazards. In contrast, under SSP2–4.5, nighttime intensification is more pronounced in the eastern TMA, driving hotspot migration that is most evident in suburban and rural areas. Attribution analysis further shows that daytime warming is dominated by marine–terrestrial thermal forcing, while local urban form becomes increasingly influential at finer administrative scales. At night, marine–terrestrial forcing weakens as urban-form effects strengthen, and during high-intensity heatwaves their spatial convergence in inland regions may further intensify urban heat hazards. • High-resolution future heatwave evolution is projected across emission scenarios. • A computationally efficient downscaling framework with detailed urban morphology is proposed. • Under SSP2–4.5, heatwave intensification leans toward background warming over peaks. • Suburban and rural areas show stronger nighttime hotspot migration than urban cores. • Marine–terrestrial thermal forcing dominates coastal warming patterns over urban form.