NIFS said that the average sea surface temperature (SST) in East Asian waters around S. Korea reached 20.84℃ in 2025, the second-highest level since the records began in 2000. Korean waters also averaged 17.66℃, higher than usual. 

The monthly data showed water temperature broadly in line with the 2001-2020 average from January to May, but from June to October the average jumped to 26.44℃, setting a new seasonal record. 

The primary driver of the unprecedented high SST in East Asian waters including S. Korea last year was: the early expansion of the North Pacific high pressure; prolonged heat linked to the Tibetan high from late July; and increased inflows of warm, low-latitude waters from the Tsushima Current extending from summer into autumn.