Fewer typhoons, bigger ocean threat: warm water, low-salinity layers put Korean seas at risk

Does a quieter typhoon season mean safer seas? Researchers warn the opposite may be true. As fewer typhoons pass through Korean waters, the ocean loses a critical mixing force — and that could allow high water temperatures, low-salinity layers and oxygen depletion to persist far longer than usual.

According to the Korea Institute of Ocean Science and Technology (KIOST), an average of 2.33 typhoons per year passed within 200 nautical miles (about 370 kilometers) of the Korean coastline over the past three years through 2024 — down 28.6 percent from the 1991–2020 climate normal of 3.27.

The drop in duration and energy was even steeper. Average residence time fell 53 percent, from 66.44 hours to 31.26 hours. Accumulated cyclone energy, which accounts for both wind speed and duration, came in 67.2 percent below the long-term average. Mean intensity also weakened, from 55.71 knots to 45.80 knots.

Within 100 nautical miles (about 185 kilometers) of the coast, the numbers were more dramatic: typhoon frequency fell 55.2 percent, residence time dropped 71.7 percent, and accumulated cyclone energy plunged 88.6 percent.

The concern is that fewer typhoons may be triggering unforeseen consequences for the marine environment.

Typhoons normally churn the ocean, mixing surface and deep water with powerful winds and waves. That process cools the overheated surface layer and breaks down "stratification" — the tendency of seawater to separate into distinct layers based on temperature and salinity differences.

When typhoons become less frequent, however, that mixing opportunity diminishes. High-temperature, low-salinity surface layers formed by summer heat waves, heavy rainfall and river runoff are then more likely to persist for extended periods.

"When the number of typhoon passages and their residence time decrease, so do opportunities for vertical mixing," said Kim Seong-hun, director of the Ocean Climate Prediction Center at KIOST. "High-temperature, low-salinity surface water can then persist for a relatively longer time."

Prolonged stratification also cuts off oxygen supply to deeper waters. As organic matter continues to decompose and consume oxygen at lower depths, hypoxic conditions — where dissolved oxygen falls to dangerously low levels — can intensify.

Shallow coastal areas and bays with limited seawater exchange face the greatest risk. Analysts say a prolonged combination of high water temperatures, low salinity and hypoxic conditions would raise stress and mortality rates among farmed marine species while degrading bottom-layer ecosystems and water quality.

The decline in typhoons is also affecting how low-salinity water discharged from China's Yangtze River moves through the region.

Freshwater flowing from the Yangtze in summer is lighter than seawater and forms a low-salinity surface layer. As it spreads along seasonal winds and currents toward Jeju and the southern coast, it can damage aquaculture operations with abnormally low salinity levels.

"When typhoons decrease, the low-salinity layer is not diluted sufficiently and can spread more widely and persist longer," said Noh Su-yeon, a senior researcher at KIOST.

This year, however, the low-salinity water shifted toward the Yellow Sea rather than Jeju and the southern coast, driven by the cyclonic circulation of a typhoon that affected the East China Sea and stronger-than-normal easterly winds — and no significant low-salinity damage was observed. The episode illustrates that even a typhoon that does not make a direct pass can still alter wind patterns and currents enough to redirect low-salinity water.

There are also concerns that ocean heat accumulated during typhoon-free periods could fuel more powerful storms when they do arrive.

Normally, a passing typhoon draws up cold water from the deep, lowering sea surface temperatures and limiting further storm development. But when the warm water layer runs deep or a low-salinity surface layer impedes mixing, the sea surface does not cool adequately — allowing a typhoon to draw on a sustained supply of thermal energy.

"Fewer typhoons does not mean the ocean is stable," said Kim Gyeong-ok, head of the Ocean Circulation and Climate Research Division at KIOST. "The heat and changes that accumulate in the ocean during typhoon-free periods can pose a threat to the next typhoon and to coastal waters."