Classification and occurrence characteristics of regional atmospheric heatwaves in North China and its marginal seas

Under the background of global warming, the frequency of heatwaves is increasing, and pose a serious threat to ecological and environmental security. North China and its marginal seas is a region highly sensitive to climate change. With it as the study area, we identified the regional atmospheric heatwaves during 1980-2024 using a three-dimensional connectivity approach. We applied a hierarchical clustering method to classify these regional heatwaves according to their formation mechanisms, and analyzed the spatiotemporal variations of different event types. The results showed that the 247 identified regional atmospheric heatwaves could be divided into two major categories and four subtypes, with 21, 67, 64, and 95 events being classified into type 1 to 4, respectively. Type 1 and type 2 belonged to the first major category, characterized as cold-season-dominated heatwaves mainly driven by warm advection associated with a weakened Siberian High. In addition, type 1 was strongly influenced by warm water masses transported by ocean currents. The fisrt two types feature relatively shorter durations but larger affected areas and higher mean intensities, with more occurrence days over land. Type 3 and type 4 belonged to the second major category, characterized as warm-season-dominated heatwaves primarily driven by the westward extension of the Western Pacific Subtropical High and the eastward expansion of the South Asia High, with radiative heating as the dominant mechanism. Type 3 events, which occurred mainly in July and August, exhibited the most pronounced and typical characteristics of this category. Compared with the first two types, type 3 and type 4 had longer durations and more heatwave days but smaller affected areas and lower mean intensities. The number of heatwave days of type 3 and type 4 showed highly significant increasing trends of 11.5 and 5.1 d·(10 a)^-1, respectively, and were more frequently observed over the ocean. The results would provide a scientific basis for regional heatwave risk assessment and adaptation planning.