Increasing trend of extreme winter warm spells in China and the intra-seasonal differences

Based on the daily temperature observation data at 1992 stations in China, this study investigates the intra-seasonal variations and trends of extreme warm spells during 1981–2022. The results indicate that the nationwide extreme cold and warm spells have both increased rapidly since the 21st century. However, different from cold extremes which occur evenly in winter months, the nationwide super warm spells have distinct intra-seasonal differences, with more occurrences in February. The number of nationwide extreme warm spells has increased from 8 during 1981–2001 to 14 during 2002–2022, and the intensity has also increased obviously. Due to the spatial differences, the study area is divided into three regions to reveal the detailed features. It is found that the warm spell frequencies in most of China increase from December to February while decrease in northeastern China. Results demonstrate that extreme warm spells are concentrated in late winter in most regions (stations). The significant increasing trends are widespread from northern China to most parts of southern China, and the areas with increasing trend exceeding +0.6 day/decade are concentrated in North China and the middle reaches of the Yangtze River. Linear trends indicate that the significantly increasing trends in late winter contribute a major part to the variation in the whole winter in China.