Reconstruction of July temperature based on tree-rings during 1845-2002 in North China

Qi Liu ^1,2 Shuheng Li ^1,2* Jiachuan Wang ^1,2 Yijie Han ^1,2 Yili Guo ^1,2 Yiqi Zhao ^1,2 Siqin Zhao ^1,2 Maoxin Du ^1,2

• ¹ Northwest University, Xi'an, China
• ² Key Laboratory of Earth Surface System and Environmental Carrying Capacity, College of Urban and Environmental Science, Northwest University, Xi'an, China

The reconstruction of climate factor fields is essential for understanding large-scale climate change. The study utilized climate data from 29 meteorological stations in North China to analyze the spatial distribution of the maximum temperatures in July in the region. It reconstructed the maximum temperature series for July from 1845 to 2002 using 30 tree-ring width chronologies from the area. The findings reveal several key points: (1) July maximum temperatures in North China exhibit four distinct spatial variation types, categorized as Region 1 (West), Region 2 (East), Region 3 (South), and Region 4 (North). (2) Temperature is identified as the primary limiting factor for the radial growth of trees in the study area, with July maximum temperature exerting the most significant influence. (3) A sliding analysis indicates the presence of two relatively cold periods and two relatively warm periods within the reconstructed series, highlighting a prolonged cold phase during the mid to late 20th century. (4) The July maximum temperature series across different regions demonstrates that temperature fluctuations in Region 4 (North) are less pronounced compared to the other three regions; an upward trend in temperatures is observed across all four regions. (5) Morlet wavelet analysis and cross-wavelet analysis of the reconstructed series suggest that the El Niño-Southern Oscillation (ENSO), Sunspot, and the Pacific Decadal Oscillation (PDO) are the primary factors influencing July maximum temperatures in the study area.