A group of experts in Japan says an abnormally warm sea off northern Japan last summer led to fewer clouds and less fog, which likely resulted in record-breaking heat.

The Japan Meteorological Agency says the average sea surface temperature near the country for this June hit a record high for the month. It also says the temperature was 6 degrees Celsius higher than usual in some places off southeastern Hokkaido as of Thursday.

Such a state is called a "marine heatwave." The phenomenon has continued since last year amid an extreme northerly advance of the Kuroshio Current.

A panel formed by the agency said the phenomenon may have been a factor behind the extreme heat in northern Japan last summer.

A study team of the panel has carried out further analysis. The experts say it is likely that the heatwave raised the temperature of the air through multiple factors.

They say clouds and fog usually form when the atmosphere is cooled by seawater. But they say the smaller temperature difference between the sea and air resulted in fewer clouds and less fog than usual. They say more direct exposure to sunlight led to further warming.

Observational data apparently backs up the analysis. Sendai City in northeastern Japan saw fog on only a single day last summer -- the lowest number since recordkeeping began in 1931.

The team also says evaporation of seawater led to a stronger greenhouse effect, trapping more heat in the atmosphere. It adds that a hotter sea also directly warmed the air.

Team member and University of Tokyo professor Nakamura Hisashi says an extremely strong marine heatwave continues, and it will undoubtedly raise temperatures again this summer in northern Japan.