Footprints of Natural Internal Variabilities in the Low-Frequency Variations of Extreme High Temperature in the Northern Hemisphere

The frequency and intensity of extreme high temperature (EHT) in the northern hemisphere exhibit remarkable low-frequency (LF) variations in summer during 1951–2017. Seven hotspots featuring both high occurrence and large LF variations in EHT were identified. The probability density functions show that the more EHT occurrence over these hotspots in recent decades are consistent with the shifted average and increased variances in daily mean temperature. The common features of the LF variation in EHT frequency over all domains are remarkable increasing trends and evident decadal variations (DV). The DV component makes the main contribution to the LF variations. Further analysis shows that the DV over the six hotspots are the footprints of two dominant natural internal signals: the Atlantic Multi-decadal Oscillation (AMO) and the Inter-decadal Pacific Oscillation (IPO). The coherent variation in EHT over western North America–Mexico, eastern Siberia, Europe, the Mongolian Plateau and southeastern China is significantly influenced by the AMO. It contributes to the variations in temperature over the first four hotspots via barotropic circum-global teleconnection, which imposes striking anomalous pressure over these regions. The IPO governs the DV in temperature over the Indo-China Peninsula through anomalous Hadley circulation driven by the sea surface temperatures in the tropical Indian Ocean. The inconsistency between in situ observations and reanalysis datasets causes the uncertainty in the physical linkage between natural internal variabilities and variation in EHT over southeastern China and Indo-China Peninsula. This study implies that natural internal forcing plays an important role in making hotspots more vulnerable to EHT.