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

Source abstract

Global warming intensifies heatwaves and heightens heat risks in metropolitan areas, particularly in coastal cities where interactions between marine–terrestrial thermal forcing and urban heat island (UHI) effects complicate intra-urban heat risk changes. Using the Tokyo Metropolitan Area (TMA) as a case, this study employs the Weather Research and Forecasting model to dynamically downscale Coupled Model Intercomparison Project Phase 6 projections and examine potential heatwave characteristics through the end of the century under different emission scenarios. We find that nighttime heatwaves intensify more than daytime ones; under Shared Socioeconomic Pathway (SSP) 2–4.5, post–mid-century daytime heatwaves show moderate increases driven by background warming, while the baseline rises but the amplitude of extremes decreases. The downscaled results show that rising sea surface temperature produces the strongest increases in mean daytime heatwave intensity over coastal transition areas of the TMA. At night, however, the spatial trends diverge sharply between scenarios: under SSP5-8.5, heatwaves pose greater urban hazards, with larger increases and stronger overlap with the UHI in western TMA; under SSP2-4.5, stronger nighttime intensification occurs in the eastern TMA, which also drives hotspot migration—more evident in suburban and rural areas where the UHI is weaker. We further find that daytime warming patterns are largely governed by marine–terrestrial thermal forcing, but at finer administrative divisions, local variations induced by urban form are reflected. At night, marine–terrestrial influence weakens, urban-form influence strengthens, and under more severe heatwaves their combined effects become more evident.