ENSO and IOD: the climate signals that move Australian grain prices

Most farmers know that La Niña means more rain and El Niño means drought. That is true but incomplete, and the incomplete version leads to poor decisions. The full picture, including the Indian Ocean Dipole which most farmers have never heard of, is considerably more useful and considerably more specific about which parts of Australia get wet and which stay dry.

The Southern Oscillation: what ENSO actually measures

ENSO stands for El Niño-Southern Oscillation. It describes the interaction between sea surface temperatures in the tropical Pacific and atmospheric pressure patterns across the Indo-Pacific region.

The key number is the Oceanic Niño Index (ONI), a three-month rolling average of sea surface temperature anomalies in the central and eastern Pacific. The Bureau of Meteorology declares an El Niño or La Niña event when the ONI stays above +0.5°C or below -0.5°C for at least five consecutive overlapping seasons.

• La Niña (ONI below -0.5°C): cooler-than-average Pacific waters. Associated with increased rainfall across eastern and northern Australia, wetter springs, and above-average crop yields in most years.
• El Niño (ONI above +0.5°C): warmer-than-average Pacific waters. Associated with below-average rainfall, drier winter-spring, and elevated bushfire risk. Historically the most reliable warning signal for drought.
• Neutral: neither event in place. Rainfall probabilities revert to climatological averages.

Research from the Bureau of Meteorology and CSIRO consistently shows that ENSO accounts for roughly 20-30% of the year-to-year variance in Australian rainfall, a meaningful signal rather than a guarantee.

The Indian Ocean Dipole: the signal most farmers ignore

The Indian Ocean Dipole (IOD) describes the difference in sea surface temperatures between the western and eastern tropical Indian Ocean. It was formally identified as a distinct climate driver in 1999 by Saji et al. in Nature, and subsequent research, including work by Ummenhofer et al. (2009) published in Geophysical Research Letters, has linked it specifically to southeastern Australia's most severe droughts.

The IOD index is the Dipole Mode Index (DMI), measured in degrees Celsius.

• Positive IOD (DMI above +0.4°C): warmer Indian Ocean in the west, cooler in the east near Australia. This suppresses moisture transport into southern and eastern Australia. The 2019 positive IOD, one of the strongest on record, was a major contributor to the conditions preceding the Black Summer fires.
• Negative IOD (DMI below -0.4°C): cooler western Indian Ocean, warmer waters near Australia. Associated with increased rainfall and flooding risk across southern Australia. The negative IOD events of 2010 and 2011 contributed to two of the largest eastern Australian wheat harvests in recorded history.
• Neutral IOD: minimal influence from the Indian Ocean.

The IOD typically develops between May and October and collapses at the end of the year. This makes it most relevant for grain farmers during the winter and spring window when Australian crops are establishing and filling.

Risbey et al. (2009), published in Monthly Weather Review, found the IOD to be the dominant driver of winter and spring rainfall across southeastern Australia. For grain farmers in the Riverina, Mallee, and Eyre Peninsula, it may matter more than ENSO during the growing season.

How ENSO and IOD interact

The two systems are partially correlated: positive IOD events tend to co-occur with El Niño, and negative IOD events with La Niña. But they are not the same thing and they do not always move together. Research from the Australian Bureau of Meteorology has documented that when both systems are in the same phase, their effects on Australian rainfall are amplified.

The worst outcomes for Australian grain production typically involve both systems working against farmers simultaneously: El Niño plus positive IOD. The 2002-03 drought, the 2006-07 drought, and the 2018-19 sequence all had both signals present.

Conversely, the bumper harvests of 2010-11, 2016-17, and 2020-22 all featured La Niña and/or negative IOD conditions.

What this means for marketing decisions

A farmer who understands that they are heading into a La Niña with a developing negative IOD has materially different information than a farmer who is watching weather apps.

Specifically:

If La Niña and/or negative IOD are developing (October through December):

• Yield potential across eastern and southern Australia is likely above average
• National crop size may exceed ABARES early forecasts as the season develops
• Additional supply typically weighs on local cash prices through harvest (November-February)
• Holding grain into a flush supply environment requires conviction that demand will absorb it

If El Niño and/or positive IOD are developing:

• Below-average production risk increases; crops in dry years often miss yield targets by 20-40%
• Australian export volumes may fall, tightening port-zone basis
• International supply shortfalls in the same year (US and European droughts often correlate with El Niño) can lift global benchmarks even as local conditions are difficult
• This is the scenario where Australian prices can still be strong despite a poor domestic season

The timing window

Both ENSO and IOD signals are typically clearest from June through October, exactly when you are making canola establishment and early wheat decisions, and when first-crop marketing begins. The BOM ENSO Outlook and IOD status are updated weekly.

The GRDC Seasonal Climate Outlook, produced jointly with the Bureau of Meteorology, translates climate indices into probability statements for rainfall and temperature that are specific to grain growing regions. It is one of the most practically useful documents produced for Australian grain farmers and is available free.