An "El Nino" weather phenomenon is presumed to be under way in the tropical western Pacific and is expected to last until winter, Japan's meteorological agency said Friday.

The governmental agency said the phenomenon usually causes Japan's weather to be relatively cooler in summer and warmer in winter, but such a pattern might not become evident until October this year.

El Nino can lead to severe disruption in the world's weather patterns with widespread effects including drought in countries such as Australia, Indonesia and India and heavy rainfall on the US Gulf Coast and in parts of South America.

The phenomenon is caused by an invasion of warm water into the surface of the Pacific Ocean off the coast of Peru and Ecuador every four to seven years.

There was no significant anomaly in July in weather over the Japanese islands, the meteorological agency said in a statement.

"It is highly possible that the El Nino phenomenon will be maintained until winter," the agency said after analysing data on the temperatures of sea surfaces in the Pacific.