Spatiotemporal Variability of Heat Waves in Beijing-Tianjin-Hebei Region and Influencing Factors in Recent 54 Years

It indicates that hot summers will become more frequent in eastern China in the future. The region will face a great risk in the absence of any adaptation measures taken towards reducing its vulnerability to effects of extreme heat. Beijing-Tianjin-Hebei Region is identified as the biggest metropolitan in northern China. Rapid urbanization and the recent frequent occurrence of hot summers in the region raises questions about influencing factors at the regional scale and the spatiotemporal variability of heat waves. Using the newly developed Heatwave Index (HI), a statistical analysis is conducted on the temporal and spatial distribution characteristics of heat waves in Beijing-Tianjin-Hebei Region over a period from 1960 to 2013. More specifically, based on the history of relocations, the heat wave trends between Beijing and Fengning is compared to investigate the influence of urbanization, and also analyse the relationship between atmospheric circulation anomalies and observed heat wave trends. It shows that based on variations in heat wave trends, two distinct phases are identified in Beijing-Tianjin-Hebei Region. Owing to some abrupt changes in the mid-1970s, the frequency of heat waves decrease from 1960 to 1973, and then increase from 1974 to 2013. Heat waves show a decreasing trend in the southern part and an increasing trend in the northern part of Beijing-Tianjin-Hebei Region. A significant increasing trend is found in the northern and western biological conservation area, and decreasing trend in south-eastern plains. At the regional scale, urbanization and relocations affect the occurrence of slight to moderate rather than extreme heat waves. In the period of global warming and rapid urbanization, the frequency of heat waves in Beijing is higher than that of Fengning. In recent global warming hiatus, the frequency of heat waves in Beijing is lower than Fengning. Driving factors behind temporal and spatial patterns are deemed complicated. The inter-decadal variations are significantly and closely related to the offsetting of western Pacific subtropical high (WPSH) ridge and the anomalous anticyclone over the Tibetan Plateau (TPAI) in summer. In other words, there is a positive correlation between the number of heat wave days and WPSH and TPAI. Furthermore, the probability of a summer with a mega-heat wave would increase with the anomalies in WPSH and TPAI.