Urbanization Amplified Compound Hot Extremes Over the Three Major Urban Agglomerations in China

Understanding how urbanization affects compound hot extremes (CHEs) is important for the sustainable development in urban agglomeration under global warming. In this study, we investigate the changes of CHEs in summer over the Beijing-Tianjin-Hebei region (BTH), Yangtze River Delta (YRD) and Pearl River Delta (PRD) in China, and assess the effects of urbanization on these changes. We find that the frequency and intensity of CHEs in summer show significantly increasing trends in these agglomerations in the recent five decades, particularly in megacities such as Beijing, Shanghai, Guangzhou and Tianjin. The urbanization contributions to CHEs in the YRD and PRD were estimated to 36 similar to 58%, while relatively small in the BTH, ranging 16 similar to 29%. They were bonded to the unban size, expansion speed and local climate. In addition to global warming and urbanization, the strengthening continental high and enhancing Western Pacific Subtropical High and its westward displacement were favor to the increased CHEs. Compound extreme hot events have a great adverse impacts on the natural environment and human society in urban region. Here, we show that frequency and intensity of compound extreme hot events in summer have significantly increased in the three major urban agglomerations during the recent 50 years. These changes have been more intensive in the downtown areas of megacities such as Beijing, Shanghai, Guangzhou and Tianjin. We estimated the urbanization contributions accounting for 36 similar to 58% in Yangtze River Delta and Pearl River Delta, 16 similar to 29% in the Beijing-Tianjin-Hebei region. Furthermore, the strengthening continental high and Western Pacific high were favor to the increase of compound extreme hot events. It is urgent to take effective measures to reduce the risk of compound extreme hot events in urban agglomeration. Compound hot extremes in summer have significantly increased in the three major urban agglomerations of China, particularly in megacitiesUrbanization contribution to the increased compound hot extremes accounts for 16 similar to 58%, which is larger than to the increased hot daysRobustly intensified and westward extended Western Pacific Subtropical High may be bonded to the increased compound hot extremes