Population exposure to concurrent daytime and nighttime heatwaves in Huai River Basin, China

Abstract Heatwaves are likely to increase over different regions of the world as the climate warms, which poses potential risk to the environment, society, and public health. Concurrent daytime and nighttime heatwaves have a more significant impact on human health than individually occurring heatwaves, especially in regions with high population density. The Huai River Basin (HRB) is taken as a case in this study to explore the characteristics of concurrent daytime and nighttime heatwaves in a high-population-density area, including quantification of the population’s exposure to concurrent heatwaves. Nighttime hot events are found to increase to greater extent than daytime hot events from 1961 to 2017. A single daytime or nighttime hot event can provide 40-60% capacity for the concurrent daytime and nighttime hot events. The concurrent daytime and nighttime heatwaves show an obvious southeast-northwest gradient with high values in the southeast and low values in the northwest of the HRB. Daytime hot events, nighttime hot events, and concurrent daytime and nighttime heatwave events all show significant upward trends from 1987 to 2017. The population exposed to the heat extremes increased significantly over this period, especially in regards to concurrent daytime and nighttime heatwaves. The population exposure in terms of the quantity, duration, and magnitude of concurrent daytime and nighttime heatwaves increased by around 5-, 9-, and 35-fold from 1984 to 2017. Atmospheric circulation analysis of the 2013 concurrent daytime and nighttime heatwaves in the HRB shows a long-lasting anomalous circulation background (e.g., anomalous high pressure system and low cloud water content) leading to severe concurrent daytime and nighttime heatwaves.