On the 2nd, the temperature in Yangsan, South Gyeongsang, rose to 42.5 C, surpassing 42 C in Korea for the first time since weather observations began, and the center of the extreme heat has shifted to the capital area. On the 4th, Seoul's official daytime high was 36.9 C, a record high for this summer. At the automatic weather stations (AWS) in Seoul, 39.2 C was observed in Gangnam District, 39.0 C in Guro District, and 38.3 C in Gangseo District.
The Korea Meteorological Administration issued an extreme heat emergency warning for southeastern and southwestern Seoul from 11 a.m. on the 4th, then expanded it to all of Seoul from 11 a.m. on the 5th. The extreme heat emergency warning is the highest level in the newly established heat advisory system this year. It is issued when an area with heat warning-level conditions is expected to see an extreme high of a daily maximum heat index of 38 C or higher or a maximum temperature of 39 C or higher.
◇ With a "double high" and even an easterly wind… the center of the heat moves west
Professor Yun Jin-ho of the GIST School of Earth Sciences and Environmental Engineering cited the developed North Pacific high and the Tibetan high that partially overlaps aloft as the main causes of this heat wave. Currently, the hot, dry Tibetan high sits in the upper levels of the atmosphere, and the hot, humid North Pacific high sits in the mid to lower levels. The two highs have stacked a thick layer of hot air over the Korean Peninsula.
Inside a high-pressure system, air descends from above to below. As the descending air compresses, its temperature rises, and clouds struggle to develop. With fewer clouds, strong sunlight reaches the surface and heats the ground.
If the two highs set the basic conditions for nationwide heat, changes in wind shifted where the heat concentrated. Until the end of last month, westerly-component winds crossed the mountains and blew eastward, sharply raising temperatures in Yeongnam and the East Sea coast. Since this month, after lower-level winds turned easterly, high temperatures have stood out in the capital area west of the Taebaek Mountains and in Chungcheong and Honam.
The Korea Meteorological Administration said winds crossing the mountains created temperature differences by region. Air coming in from the East Sea cools as it climbs the eastern slopes of the Taebaek Mountains, and in the process some of the moisture the air holds falls out as clouds or rain. The relatively drier air then descends the western slopes and warms again. This phenomenon, in which air that has crossed the mountains becomes warm and dry on the downwind side, is called the "foehn effect."
Professor Yun said, "The foehn effect likely influenced the capital area's high temperatures this time," adding, "It is the same principle as when westerly winds raised temperatures in Yangsan, South Gyeongsang, through the 2nd, only with the wind direction changed."
◇ Even after ipchu, highs stay in the 30s C… typhoon Dolphin is a variable
The Korea Meteorological Administration expects this heat to continue for a while. Even on the 7th, "ipchu" (the start of autumn on the seasonal calendar), nationwide highs are forecast to climb to around 35 C.
According to the medium-range forecast for the next 10 days, nationwide temperatures are expected to reach the low to mid-30s C on the 15th. Forecast highs by region are 34 C in Seoul, 34 C in Cheongju, North Chungcheong, 34 C in Jeonju, North Jeolla, 33 C in Daejeon, 33 C in Daegu, 33 C in Jeju, 32 C in Chuncheon, Gangwon, 32 C in Gwangju, and 32 C in Busan. The heat will not break even after ipchu.
If Typhoon Dolphin, No. 13, now moving west over the sea east of Okinawa, Japan, approaches the Korean Peninsula, rain could fall and ease the heat. However, Dolphin's strength is "3 (strong)" on a scale from "1 (weak)" to "5 (super strong)," and if it makes landfall on the Korean Peninsula, typhoon damage could occur.
If Dolphin moves not over the Korean Peninsula but across southern Japan into inland China, the heat over the Korean Peninsula could intensify. That is because air converges around a typhoon, strengthening easterly winds over the Korean Peninsula, and those winds can cross the Taebaek Mountains and trigger the foehn effect.
An official with the Korea Meteorological Administration said, "Temperatures will not ease until the influence of the North Pacific and Tibetan highs on Korea abates, and the chance of that happening is low in the agency's forecast models, so the heat is likely to continue for a while."