Indian Ocean Dipole (IOD): ENSO's Partner in Climate Chaos

Published: June 3, 2026 · 10 min read

Everyone talks about El Niño and La Niña, but there's another climate oscillation happening right next door that gets way less attention — even though for a billion people living around the Indian Ocean basin, it matters just as much. The Indian Ocean Dipole (IOD) is basically the Indian Ocean's version of ENSO, and when it lines up with El Niño or La Niña, the results can be catastrophic.

IOD doesn't get the same media coverage as ENSO, partly because it was discovered much later (the 1999 paper by Saji et al. finally gave it a name) and partly because its effects are more regional — concentrated on the countries surrounding the Indian Ocean. But for those regions, a strong IOD event can mean the difference between a good harvest and a famine.

What Is the IOD?

The IOD measures the difference in sea surface temperature between the western Indian Ocean (near the coast of Africa) and the eastern Indian Ocean (near Indonesia and Australia). Yes, it's that simple — a temperature gradient across the basin. But that gradient drives massive shifts in rainfall, wind patterns, and weather across the region.

The IOD has three phases:

Positive IOD — warmer water in the western Indian Ocean, cooler in the east. This suppresses convection near Indonesia and Australia, bringing drought to both. Meanwhile, East Africa gets more rain than usual. A positive IOD is the scary one — it's strongly associated with East African floods and Australian bushfire seasons.

Negative IOD — the reverse: warmer water in the east, cooler in the west. This tends to bring more rain to Australia and Indonesia, and drier conditions to East Africa. Negative IOD events are generally less damaging than positive ones, which is probably why they get less press.

Neutral IOD — no significant gradient. This is the default state most of the time.

The IOD is measured by the Dipole Mode Index (DMI), which is literally the SST difference between the western and eastern tropical Indian Ocean. A DMI above +0.4°C sustained for several weeks qualifies as a positive IOD event.

IOD and ENSO: How They Dance Together

Here's the pattern that matters: El Niño tends to trigger positive IOD events. The mechanism is kind of indirect — El Niño weakens the Walker Circulation, which reduces convection over the Maritime Continent, which in turn allows cooler water to upwell in the eastern Indian Ocean while the western Indian Ocean stays warm. That's a positive IOD.

The 1997-98 super El Niño produced the strongest positive IOD on record. The 2015 super El Niño also triggered a positive IOD that contributed to severe drought in Indonesia and massive wildfires. The 2023-24 El Niño produced a moderate positive IOD that was partly responsible for the devastating flooding in East Africa between October and December 2023.

But IOD can also act independently of ENSO. The 2019 positive IOD happened during a neutral ENSO year — meaning El Niño wasn't present — yet it produced one of Australia's worst bushfire seasons in history. So you can't just look at ENSO and assume you know what the Indian Ocean is doing.

Regional Impacts

Australia

Positive IOD = less winter-spring rainfall for southern and eastern Australia. This is a bigger deal than most people realize because Australia's wheat crop depends on April-October rainfall. The 2006, 2015, and 2019 positive IOD events all coincided with significant drops in Australian wheat yields. The 2019 event pushed much of New South Wales and Queensland into severe drought conditions that set the stage for the devastating 2019-20 bushfire season — the Black Summer that killed billions of animals and destroyed thousands of homes.

East Africa

Positive IOD brings heavy rains to East Africa — Somalia, Kenya, Ethiopia, Tanzania. In principle, more rain sounds good for a dry region. In practice, it comes as intense downpours that cause flash flooding, landslides, and displacement. The 2019 positive IOD contributed to the East African floods that affected over a million people. The 2023 positive IOD set rainfall records in parts of Kenya and Somalia that had been in drought just a year earlier — the whiplash was brutal.

There's a cruel irony here: the same IOD phase that dries out Australia brings floods to East Africa, and vice versa. Nobody wins.

India Monsoon

The relationship between IOD and the Indian monsoon is complex. A positive IOD can actually strengthen the monsoon — the warmer western Indian Ocean drives more moisture toward the subcontinent. But a negative IOD, especially when combined with El Niño, can suppress monsoon rainfall. The 2015 El Niño + negative IOD gave India one of its driest monsoons in recent memory. The 2023 El Niño would have been worse for the monsoon if not for a concurrent positive IOD that partly offset the drying effect.

How to Read the IOD

The DMI is updated weekly by the Bureau of Meteorology Australia (they're the main source, along with NOAA). If the DMI stays above +0.4°C for at least 8 weeks, a positive IOD event is officially declared. The BOM is generally the authority because Australia gets hit harder by IOD than almost any other country.

Historical positive IOD events include: 1961, 1994, 1997, 2006, 2015, 2018, 2019, and 2023. Notice how some of these line up with El Niño years (1997, 2015, 2023) and some don't (2019).

For 2026, IOD forecasts currently hover around neutral — the ENSO models don't show a clear IOD response yet. But if a strong El Niño develops by summer as the subsurface data suggests, a positive IOD is statistically likely to follow. The two events tend to peak at slightly different times — El Niño peaks in boreal winter, IOD peaks in boreal fall — which means the combined impact window is September through January. That's when we'd see the most extreme outcomes if both events fire simultaneously.

Further Reading

• Modoki El Niño — the central Pacific variant
• 2026 El Niño Watch — current data and forecasts
• 2023-2024 El Niño review

References: Saji, N.H. et al. "A dipole mode in the tropical Indian Ocean" (Nature, 1999); Webster, P.J. et al. "Coupled ocean-atmosphere dynamics in the Indian Ocean" (GRL, 1999); Cai, W. et al. "Increased frequency of extreme Indian Ocean Dipole events due to greenhouse warming" (Nature, 2014); BoM Australia IOD monitoring and forecasts; NOAA CPC IOD tracking; WMO state of the climate reports.