Characteristics and potential drivers of extreme high-temperature event frequency in Eurasia

Abstract

This study investigates decadal variations in the frequency and intensity of extreme high temperature events (EHEs) during the summer months of July and August across the Northern Hemisphere from 1979 to 2023. Research results indicate that the frequency and intensity of EHEs on the Eurasian continent have increased more rapidly than in other Northern Hemisphere landmasses over time. By applying Empirical Orthogonal Function analysis, two dominant modes of EHEs were identified: a spatial consistency pattern and a quadrupole anomaly pattern. The spatial consistency pattern shows significant anomalies centered around the Caspian Sea and East Asia, with a notable upward trend in intensity. This pattern is strongly associated with atmospheric warming and increased sea surface temperatures in the tropical North Atlantic, which amplifies the North Atlantic-Eurasian wave train. The eastward propagation of wave activity flux, driven by the shifting positive geopotential height anomaly, further enhances the frequency and intensity of EHEs. The quadrupole anomaly pattern is characterized by four centers located in the mid-latitude region (30°N-50°N, 25°E-150°E), West Asia-South Asia-Southeast Asia, Central Europe-Northern Europe, and East Asia-Eastern West Asia. The EHEs in these regions exhibit anti-phase characteristics, meaning that while one region experiences higher-than-average frequency of EHEs, others simultaneously show lower-than-average frequency of EHEs. The formation of this quadrupole anomaly pattern is closely associated with the negative phase of the North Atlantic Oscillation (NAO). NAO influences regional temperatures by modulating the jet stream and geopotential height, forming anticyclones or cyclones that, in turn, increase or decrease EHEs. Under NAO influence, a double jet state is formed, and a blocking anticyclone emerges in the weak wind zone between the two zonal wind maxima, thus increasing the EHEs in local areas. This study underscores the importance of understanding these distinct patterns and their underlying mechanisms to better predict and manage the regional impacts of extreme heat in a changing climate.