What Caused the Extraordinarily Hot 2018 Summer in Korea

In 2018, heatwaves (HWs), defined as a period of abnormally hot weather with a daily maximum temperature (T_Max) exceeding its 95th percentile threshold for at least 3 consecutive days, were prevalent from June through August, and temperature records exceeded the reference values in many countries over East Asia (EA), including China (CH), Japan (JP), and the Korean Peninsula (KP). Particularly, extreme HWs from July through August lasted for the longest duration of 21.3 days, with T_Max reaching 36.9 degrees C. The highest T_Max recorded since 1907 was 41 degrees C in Hongcheon, located east of Seoul in the KR Here, we examined the factors that influenced the 2018 HW, and how these relate to the 1994 HW, which was the second longest HW recorded in the KR The results showed that abnormally strong and northwestward extended anticyclone features observed in July 2018 lasted as a persistent North Pacific anticyclone anomaly until August 2018, centered at the northern KR. These anticyclone features subsequently formed a modon-like blocking, with a cyclonic anomaly in the East China Sea. In August 1994, the North Pacific High (NPH) extended to eastern EA, which broke down the meridional dipole structure and HWs did not persist. The NPH, which persisted until August 2018, was accompanied by a sinking motion, suppression of precipitation, anomalous maximum temperature, weakening of the westerly jet stream, and increased insolation due to clear sky. We found that the prolonged and northwestward-shifted NPH, including the KP, drove the extraordinarily hot 2018 summer in Korea. In addition, low precipitation and massive evapotranspiration with persistent insolation in July 2018 influenced the dry condition at the surface. We suggest that the predictions for the location and duration of the NPH associated with the HWs are required to reduce heat-related mortality and the impact on agriculture because of excessive evapotranspiration.