AN EVOLVING El Nino event is set to cool the east Indian Ocean delivering a double whammy of drought-inducing events to southern Australia.

That's the early prognosis from Japanese and Australian climatic computer models.

An evolving El Nino is primed to drive the Indian Ocean into a cycle of events that strengthens the subtropical high pressure ridge across northern Australia.

If it develops, the ridge will in turn block major rainfall events reaching southeast Australia in the next six months.

The El Nino and Indian Ocean Dipole events are primed to pump dry cold air from a strengthening subtropical high-pressure ridge down across Australia.

Climate scientists have only recently started exploring El Nino's links to the Indian Ocean.

Centre for Australian Weather and Climate Research scientist Harry Hendon said most large El Nino events in the past 50 years were associated with development of a positive IOD.

"If El Nino conditions get established early enough in the year (before winter), then El Nino will tend to promote development of a IOD event," Dr Hendon said.

But understanding the link first requires an understanding of what triggers an El Nino event.

It all starts with a wave of warm water, called a Kelvin wave, moving from the western to central Pacific.

CAWCR scientists have already tracked this wave of warm water, triggering early forecasts of an El Nino this year.

But CAWCR Oceanographer Oscar Alves said they were yet to see crucial changes in the atmospheric conditions associated with an El Nino event.

A full blown El Nino results in moist air rising off the abnormally warm central Pacific, which then dumps rain on the Pacific Islands of Fiji and the Northern Cook Islands.

The same air then travels to the west and sinks back down to the surface of northern Australia, creating unusually dry high-pressure systems.

Climatologists now believe this downward air flow induces another Walker circulation cell along the equator that spreads out across the Indian Ocean.

This cell intensifies the easterly trade winds across the south equatorial Indian Ocean, which cool the east ocean from Java to northwest Australia.

The cooler ocean leads to a cooler atmosphere, creating a feedback loop that strengthens the high-pressure systems across the region and northern Australia.

The end result is the development of a positive Indian Ocean Dipole event.

The high-pressure system over the cold waters to the north and west of Australia also extends southward to subtropical latitudes, thereby acting to block weather systems from delivering rain to southern parts of Australia during winter and spring.

The easterlies that evolve during an IOD also cause warming in the western Indian Ocean, where warm surface waters are piled up leading to higher rainfall in eastern Africa.

IOD events tend to rapidly disappear in early summer, although El Nino conditions in the Pacific tend to last through summer.