Enhanced nighttime heatwaves over African urban clusters

Despite the threat that nighttime heatwave poses to public health and the environment in the developing world, it remains relatively understudied in Africa, especially in its rapidly expanding urban areas with large populations. Using meteorological observations, climate reanalysis, remote sensing datasets, and integrated methodology, we found that during 1981-2020, nighttime heatwaves dramatically increased with early onset dates over Africa. Large scale circulation induced dry conditions over land which explains the major heatwaves across all climate zones. Meanwhile, the increasing trend of nighttime heatwaves in urban areas than rural areas in both tropical and temperate climates is largely due to urbanization, which amplifies heatwaves with urban heat island (UHI) effects. The strongest contribution of urbanization to nighttime heatwaves was observed in temperate climate, leading to a 46% increase in the heatwave duration. In arid climate, urban expansion has a negative impact on nighttime heatwaves, due to the cool island effect of urban greens and weak urbanization. The major contribution of urbanization to the urban-rural contrast of nighttime heatwave trends in temperate climate can be attributed to stronger UHI intensity resulting from lower windspeed and less evapotranspiration. Without appropriate adaptation efforts to reduce heatwave exposure, the risks posed in Africa's urban areas will continue to increase under future warming.

Automated summary

Despite being relatively understudied, nighttime heatwaves in Africa, especially in rapidly expanding urban areas, have dramatically increased during 1981-2020 due to urbanization and large scale circulation induced dry conditions. The strongest contribution of urbanization to nighttime heatwaves was observed in temperate climate, while in arid climate, urban expansion had a negative impact on nighttime heatwaves. Without appropriate adaptation efforts, the risks posed by heatwaves in Africa's urban areas will continue to increase under future warming.