South Korea's Monsoon Season Shortened, Rainfall Polarized: Drought in South, Floods in North

South Korea's monsoon season this year was notably shorter and less intense than usual, with nationwide rainfall falling to 61.1% of the average. The season began later and ended earlier across the country, with Jeju Island experiencing only 20 days of monsoon, and the southern and central regions lasting 26 days each. This brevity is the sixth shortest for Jeju, tenth for the southern regions, and twelfth for the central regions since records began in 1973.

The distribution of rainfall, however, created stark regional disparities. While areas like Daejeon, Sejong, and Chungnam received rainfall close to the average, the Gwangju-Jeonnam region received only 43.6% of its typical monsoon rainfall, and the Busan-Ulsan-Gyeongnam region suffered from severe drought, receiving a mere 17.7% of the average. Conversely, the Seoul metropolitan area and parts of Gangwon province experienced torrential downpours, with some locations receiving over 500mm of rain, leading to significant flooding. Yeoncheon in Gyeonggi Province recorded the highest cumulative rainfall at 700.5mm.

Meteorologists attribute this unusual pattern to several factors. A blocking phenomenon in the high latitudes of the Northern Hemisphere likely hindered the northward expansion of the Pacific anticyclone. Additionally, weakened convective activity in the western tropical Pacific, potentially linked to El Niño, reduced the supply of heat and moisture to the anticyclone. The paths of Typhoons Koinu and Bualoi also played a role by pushing the Pacific anticyclone eastward.

The concentrated rainfall in specific areas, with over half of the total precipitation in some regions falling within just one to three days, exacerbated the flooding risks. For instance, in the Seoul metropolitan area, 51.3% of the total rainfall occurred over just three days. Similarly, in the Daejeon-Sejong-Chungnam region, 52.6% of the rain fell within three days. This intense, short-duration rainfall overwhelmed drainage systems, leading to widespread damage in the hardest-hit areas.