In its so-called positive phase, tropical waters off Australia's north-west are relatively cool - compared with those near Africa - strengthening easterly winds and reducing the potential convection that typically supplies much of south-eastern Australia's critical winter and spring rains. A negative IOD has the opposite effect. “They used to think the Indian Ocean was a slave to the Pacific," says Cai Wenju, a senior climate researcher at the CSIRO, adding this year's IOD figures are "gigantic". “The biggest clue" that the Indian Ocean could influence Australia independently came in 2007 and 2008 when the Pacific was in its La Nina phase, which should have raised the odds for good rains, Dr Cai said. Instead, the Millenium Drought was still playing out, and there were positive-phase IODs three years in a row. "Sometimes, the El-Nino Southern Oscillation has copped a bad rap when it should have been the IOD," Andrew Watkins, head of long-range forecasting at the Bureau of Meteorology, says.

Australian researchers from the 1980s had started examining how relative warm or cool waters off Western Australia could affect rainfall over the continent. However, it took two papers published in Nature in 1999 by Japanese and North American scientists - including Australian Peter Webster - to tease out the potential of an independent IOD. Average Australian rainfall for the first nine months of the year has been the lowest since 1965, made worse by poor cool-season rains. Credit:Nick Moir Scientists including England and Cai will gather in China next month to mark that 20th anniversary, with the IOD now a key component of Australia's and global weather and climate predictions. Scientists caution that reliable observation data only goes back a couple of decades but it is clear this year's positive-IOD is already one of the strongest of record. So-called "reanalysis" using a combination of observations and modelling suggests the event is also notable over the past 150 years.

Nerilie Abram, an associate professor at the Australian National University, published work in 2009 that used coral cores among other data to push IOD estimates back to the mid-1800s. Research awaiting publication will look back 1000 years. While the current event is significant, her study suggests “perhaps the instrumental record doesn’t tell us the full range what’s actually possible in the Indian Ocean”. The magnitude of an IOD appears to matter more for rainfall over south-eastern Australia than the El Nino-La Nina flux, the Bureau of Meteorology's Watkins said: "The stronger the IOD, the stronger the impacts ... for Australia, and maybe for Africa." Another difference is that Indian Ocean conditions are more regulated by the seasonal cycle than the Pacific. Positive or negative IODs typically take form by May or June, peak around September and October, and break down in November to December as the monsoon shifts south, disrupting the easterly winds. Poor winter and spring rains from positive IODs are not just bad for farmers. Those rains also supply much of the run-off that let our rivers run and fill the dams. Heatwaves are more severe and prolonged as soils dry out, removing the cooling function from evaporation, and setting up a busy bushfire season. Australia's year-to-date daytime temperatures are already running at a record high, the bureau says. "It’s not a great precursor for the summer ahead when we’ve had a strong positive IOD," Abram says. “We’ve tended to have very severe summer bushfires particularly in the Victoria area."

While researchers are yet to settle on how much of a role climate change is already playing in big El Ninos or IODs, "we’re seeing extreme events become more common”, Abram says. Dr Cai says that while the Indian Ocean is warming - along with others around the world - “the west is warming faster”. Under such conditions, "it’s easier to have an extreme positive IOD event", he said. Bad bushfire seasons in Victoria and elsewhere in Australia's south-east have often coincided with positive IOD events, such as in 2009. Credit:AAP Such a future would be bad news for farmers, and raise doubts about the effectiveness of policies proclaimed to be "drought-proofing". “We change the average climate by having these events more frequently or more strongly," Abram says. "It has an effect of changing our average rainfall.”