Relative Contribution of Trend and Interannually Varying SST Anomalies to the 2018 Heat Waves in the Extratropical Northern Hemisphere

The northern extratropics-including regions in northern Europe, northeast Asia, and North America-experienced extremely prolonged heat waves during May-August 2018. Record-breaking surface temperatures, which caused numerous deaths, were observed in several cities. The 2018 heat waves exhibited a circumglobal characteristic owing to a circumpolar perturbation (CCP) in the middle-upper troposphere of the Northern Hemisphere (NH). The CCP had two parts: a wavelike perturbation and a hemispheric perturbation that was almost zonally symmetric. Singular-value decomposition analysis revealed that the zonally symmetric perturbation was coupled to the SST warming trend, whereas the wavelike perturbation was primarily coupled to the interannually varying SST anomaly (SSTA), particularly in the tropical North Pacific, which reached an extreme in 2018. Numerical experiments confirmed that the zonally symmetric component primarily resulted from the SSTA associated with the warming trend, whereas the interannually varying SSTAs in the NH contributed mostly to the wavelike perturbation. The warming trend component of SSTA, especially that in the tropics, compounded by the unusually large SSTAs in 2018, was hypothesized to have contributed to inducing the circumpolar circulation anomaly that caused the record-breaking heat waves in the extratropical NH in 2018.

Automated summary

From May to August 2018, the northern extratropics experienced prolonged heat waves with record-breaking temperatures, causing numerous deaths in cities. The heat waves in 2018 showed a circumglobal characteristic due to a circumpolar perturbation in the middle-upper troposphere of the Northern Hemisphere, consisting of a wavelike perturbation and an almost zonally symmetric hemispheric perturbation. The warming trend component of sea surface temperature, especially in the tropics, compounded by unusually large SST anomalies in 2018, played a significant role in inducing the circumpolar circulation anomaly that led to the extreme heat waves in the northern extratropics.