This study analyzed the causes of the extreme heatwave in East Asia in 2018, which brought a record-breaking event in South Korea in terms of the number of hot days and the intensity in history. Long-lasting atmospheric patterns were observed during the 2018 heatwave, and the patterns were similar to those known as the Pacific-Japan (PJ), circum-global teleconnection (CGT), and Arctic Oscillation (AO) patterns identified in a previous study. In 2018, all three patterns appeared to exhibit a strong positive phase. In particular, the PJ and AO Indices showed the highest values within the analysis period, having a major impact on the heatwave. According to a quantitative analysis through multi-linear regression (MLR) of how much each of the three patterns affects the heatwave, all three patterns of PJ, CGT, and AO have significant effects on the heatwave in Korea. The statistically reconstructed heatwave days (HWD) time series with the three indices by MLR account for 51% of the total variability at the interannual timescale. Especially in 2018, most of the occurred HWD could be restored using the three indices. Also, in 1994, when the three indices had a high peak, the number of HWDs was the second-longest ever, while in 1993, when all three indices had a low peak, the heatwave was the lowest. This study quantitatively examined whether the large-scale extreme heatwave in 2018 greatly increased because of PJ, CGT, and AO. This study suggests that extreme heatwave can occur again if three apparently independent atmospheric patterns appear at once in the future.
