Ag Weather Forum

Has The Pacific Southern Oscillation Index Started Moving to La Nina?

Bryce Anderson
By  Bryce Anderson , Ag Meteorologist Emeritus
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Southern Oscillation Index (SOI) 30-day average values have steadily risen since late April, from -6.75 on Monday, April 29, to +3.02 on Tuesday, May 28. (Queensland, Australia Government graphic)

The closely watched atmospheric pressure side of the Pacific Ocean El Nino-Southern Oscillation (ENSO) is showing a move toward the La Nina side of the spectrum. Values of the Southern Oscillation Index (SOI), tracked by the Australian government, show that the 30-day average has steadily risen since late April, from -6.75, close to the El Nino threshold, on Monday, April 29, to the positive side of neutral at +3.02 on Tuesday, May 28. The SOI is calculated by comparing the barometric readings of Darwin, Australia, and the island of Tahiti in the central Pacific; SOI data tracking has been done by the Australian government for almost 150 years, beginning in 1876.

SOI La Nina values have a +7.0 benchmark. Daily SOI values have shown readings that are over that threshold. In the last week, the daily SOI values were logged at +9.44 on Sunday, May 26; +11.20 on Monday, May 27; +15.95 on Tuesday, May 28; and +11.59 on Wednesday, May 29.

The evolution of the SOI is important because of its relationship to longer-term atmospheric features during the following seasons. When the SOI has either been noted moving toward La Nina or into the La Nina category during the U.S. summer season, corn yields have been affected at times. The year 2020, a key analog year for DTN long-range forecasters, is a good example. In 2020, the SOI moved into the positive side of neutral during July and into the La Nina category from August through the rest of the year. Production, yield and even acreage were affected by late-summer heat and dryness, along with a devastating derecho across Iowa in mid-August. Final 2020 corn production came in at 14.182 billion bushels with the final average yield indicated by USDA at 172.0 bushels per acre. Both production and yield were less than expected record values when the first official corn estimate was posted in August of 2020.

The Pacific SOI trend by itself is no surprise to the weather and climate community. Cooler conditions over the last few months in the Pacific Ocean are well documented. Forecasts have for months pointed to La Nina developing in the Pacific basin during the last half of this year. But the pace and timing of that change are features that could play a big part in how conditions develop for the grain-filling stage of this year's corn crop.

Bryce Anderson can be reached at


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