Fewer Typhoons Leave Korea’s Coastal Waters Vulnerable to Heat and Oxygen Loss
A three-year decline in typhoon activity around South Korea has weakened ocean mixing, allowing warm surface water and oxygen-poor bottom water to persist and increasing risks for aquaculture and coastal ecosystems.
A sustained decline in typhoon activity around South Korea is weakening the natural mixing of coastal waters, potentially extending periods of high sea temperatures and oxygen depletion, according to new findings from the Korea Institute of Ocean Science and Technology.
The institute analyzed storms passing near the Korean Peninsula between 2024 and 2026. It found that an average of 2.33 typhoons a year passed within 200 nautical miles of South Korea’s coast during that period, 28.6 percent below the 1991–2020 climate average of 3.27.
The change was sharper when measured by how long and how strongly storms affected nearby seas. Average typhoon residence time within 200 nautical miles fell from 66.44 hours to 31.26 hours, a decline of 53 percent. Accumulated cyclone energy, which combines wind speed and duration, was 67.2 percent below the climate norm. Average storm intensity in the area also dropped from 55.71 knots to 45.80 knots.
Closer to shore, the decline was even more pronounced. Within 100 nautical miles of the coastline, the number of passing typhoons decreased by 55.2 percent, residence time fell by 71.7 percent and accumulated cyclone energy dropped by 88.6 percent. Activity was generally below normal across the East Sea, South Sea, Yellow Sea and the northern East China Sea.
Typhoons can be destructive on land, but at sea they help mix warm surface water with colder water below. When that mixing is reduced, layers of different temperatures and salinity can remain separated for longer. Oxygen then has more difficulty reaching deeper water while decomposing organic matter continues to consume it, allowing hypoxic conditions to persist or worsen. Shallow coastal areas and enclosed bays with limited water exchange are particularly exposed.
Those conditions can put farmed marine species under greater stress, raise the danger of mass mortality at aquaculture facilities and damage seafloor ecosystems and water quality. The institute cautioned that continued monitoring is needed before firm conclusions can be drawn about the long-term relationship between lower typhoon activity and marine environmental changes.
Researchers also warned that accumulated ocean heat can influence later storms. A typhoon usually brings colder deep water upward, cooling the surface and limiting further intensification. If a deep warm layer or low-salinity surface layer prevents effective mixing, however, an approaching storm may continue drawing energy from the sea.
The findings suggest that storm counts alone are not enough to assess marine stability. Scientists said monitoring should also account for the intensity of ocean mixing and the strength and duration of seasonal stratification, especially as coastal communities and aquaculture operators adapt to changing conditions.