Under it, the surface temperature of the sea drops sharply in the southeastern part of the Indian Ocean, off Indonesia and the northern coast of Australia.

At the same time, the temperature rises in the western part of the Ocean, off the eastern coast of tropical Africa.

The result is very disruptive, bringing droughts to Indonesia and parts of Australia, and heavy rain to semi-arid parts of Africa.

Unlike the Pacific's El Nino effect, which can last a couple of years, the IOD usually eases within months as temperatures even out on both sides of the ocean.

As the IOD was only discovered in 1999, very little is known about how it works, especially the climate mechanisms that cause it to be unleashed.

Researchers in Britain, led by Nerilie Abram of the British Antarctic Survey (BAS) in Cambridge, used records from ancient Indian Ocean coral, whose growth is affected by sea temperatures and rainfall, as an indicator for IOD events stretching back into the distant past.

Looking at such records going back 6,500 years, they found that the big driver for the IOD is the monsoon. Years with severe monsoons triggered an IOD.

The finding is worrying, they say.

Monsoons are also linked to the El Nino. When an El Nino happens, monsoons tend to be weaker -- which thus diminishes the risk of an IOD event.

But man-made global warming is breaking down this link, and the trends are towards stronger monsoons, said Abram.

That, as a result, suggest Dipole events will become more frequent, she said in an interview.

"The monsoon is affected by El Nino but with global warming, that relationship is breaking down and the monsoon is strengthening independently of El Nino," said Abram.

"In Indonesia, droughts are likely to be shifted to the time of year when they normally expect the most rainfall, so the impact of that could be quite severe for the people who are trying to live in that area," she said.

"At the moment, Indian Ocean Dipole events already have a dramatic effect on the climate in this area. They already have droughts and wildfires in Indonesia that are a threat to human health and the environment."

The last Dipole event occurred last year, and its predecessor was in the last 1990s.

"Normally the Dipole events peak in October and November, then after the monsoon winds change direction, then it dissipates and things start to go back to normal again," she explained.

The paper appears in the weekly British science journal, Nature.

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