Fewer typhoons, bigger ocean problems: Korea's seas face prolonged heat and oxygen depletion

Contrary to the common assumption that fewer typhoons mean safer seas, a new analysis warns that a decline in typhoon activity can actually prolong high sea-surface temperatures, low-salinity surface water and oxygen-depleted bottom water. Typhoons play a critical role in mixing the ocean's upper and lower layers, and as fewer storms pass through waters around the Korean Peninsula, that vertical mixing is weakening.

The Korea Institute of Ocean Science and Technology (KIOST) released findings Monday on recent typhoon activity and marine environmental changes in seas surrounding South Korea. An average of 2.33 typhoons per year passed within 200 nautical miles of the Korean coastline from 2024 through this year — down 28.6% from the climate normal of 3.27 per year recorded between 1991 and 2020.

The time typhoons spent in those waters shrank even more sharply. Average residence time within 200 nautical miles fell 53.0%, from 66.44 hours under the climate normal to 31.26 hours over the past three years. Accumulated cyclone energy (ACE) — a measure combining wind speed and duration — was 67.2% below the climate normal. Average intensity as storms passed through also weakened, dropping from 55.71 knots to 45.80 knots.

Changes in typhoon activity in seas around South Korea Provided by the Korea Institute of Ocean Science and Technology

Category | Climate normal (1991–2020) | Past 3 years (2024–2026) | Change Annual average passing within 200 nautical miles | 3.27 | 2.33 | −28.6% Average residence time within 200 nautical miles | 66.44 hrs | 31.26 hrs | −53.0% ACE within 200 nautical miles | — | — | −67.2% Average intensity within 200 nautical miles | 55.71 kt | 45.80 kt | Weakened Number passing within 100 nautical miles | — | — | −55.2% Residence time within 100 nautical miles | — | — | −71.7% ACE within 100 nautical miles | — | — | −88.6%

The "typhoon gap" grew more pronounced closer to shore. Within 100 nautical miles of the coastline, the number of typhoons passing through fell 55.2% from the climate normal over the past three years, while residence time dropped 71.7% and ACE fell 88.6%. Typhoon frequency, residence time and ACE were all generally below normal in the East Sea, the South Sea, the Yellow Sea and the northern East China Sea as well.

When the ocean's upper and lower layers remain unmixed for extended periods, the effects reach all the way to the seafloor. Even as oxygen transfer from the surface to deeper water becomes increasingly difficult, organic matter continues to decompose and consume what oxygen remains. The result is that hypoxic — or oxygen-depleted — conditions can persist longer or intensify. Coastal areas and bays with shallow depths and limited water exchange are particularly vulnerable.

Prolonged high sea-surface temperatures, low-salinity surface water and hypoxic conditions can heighten stress and mortality risk for farmed marine species, and can also affect bottom-layer ecosystems and water quality. However, KIOST said continuous monitoring is needed before drawing conclusions about the long-term relationship between reduced typhoon activity and these marine environmental changes.

"When fewer typhoons pass through the seas around South Korea, as has been the case recently, the force that vigorously mixes the summer ocean can also diminish," said Kim Seong-hun, head of KIOST's Ocean Climate Prediction Center. "If the temperature and salinity stratification is not sufficiently broken down, high sea-surface temperatures and low-salinity surface water can persist for a relatively long time."

Heat that accumulates in the ocean during typhoon-scarce periods can in turn affect the development of storms that approach later. Normally, a passing typhoon mixes the water column, drawing cold water up from below and cooling the sea surface in a way that limits further storm intensification. But when the warm layer extends deep or a low-salinity surface layer blocks mixing, the sea surface does not cool adequately, allowing a typhoon to keep drawing on the ocean's thermal energy.

"Fewer typhoons does not mean the ocean is stable," said Kim Gyeong-ok, head of KIOST's Ocean Circulation and Climate Research Division. "We need to look not only at how many typhoons pass through, but also at the actual degree of ocean mixing and the intensity and duration of stratification."