BUSAN — East Asia's summer rain front is no longer retreating southward in September and has shifted north compared with the past, raising the risk of both late-season heat and torrential rain at the same time, according to a new study.
A team led by Moon Su-yeon, a researcher at the APEC Climate Center (APCC), and Kim Hyung-jun, a professor at the Korea Advanced Institute of Science and Technology (KAIST), said it tracked changes in the position of the East Asian summer front by analyzing reanalysis data from the European Centre for Medium-Range Weather Forecasts (ECMWF) covering 1979 through 2025. The findings were published in npj Climate and Atmospheric Science, an international journal on climate and atmospheric science.
During the summer monsoon period from late June to mid-July, the front's position in 2006-2025 had moved about 1.32 degrees of latitude, or roughly 146 kilometers, farther north than in earlier years, the researchers said. Between Sept. 10 and 30, the front's central axis rose about 1.92 degrees, or roughly 213 kilometers — a distance comparable to the straight-line gap between Seoul and Daegu, and a larger shift than during the monsoon season.
A front is the boundary where air masses with different properties meet. When warm, humid air flows in from the south, it can form heavy rain clouds and bring torrential downpours. The hot, humid air around a front also hampers the evaporation of sweat, increasing perceived heat and heat stress. The front typically retreats southward in September, but the researchers said it has recently shown a stronger tendency to remain in the north.
The team also analyzed wet-bulb globe temperature (WBGT), a measure that reflects humidity and sunlight as well as air temperature, and said the northward shift of the September front has increased the risk of heat and heavy rain occurring simultaneously.
"We identified changes in the front's position as the cause of more frequent late-season heat and heavy rain in September," Moon said. "Because summer disasters now extend later in the year, we need an integrated disaster-prevention strategy that prepares for heat waves and torrential rain together."