Ag Weather Forum

SOI Update

Bryce Anderson
By  Bryce Anderson , DTN Senior Ag Meteorologist
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SOI values are just below the plus 8.0 threshold for La Nina on the 30-day running average scale. (BOM graphic by Nick Scalise)

There was a great deal of interest on the part of producers at the Farm Progress Show this past week in the potential for the Pacific Ocean temperature pattern to move into a La Nina phase.

In a La Nina event, the water temperatures in the equator region of the Pacific are at a constant below-normal level, and if the event is strong enough, the jet stream over the U.S. can get reconfigured to focus over the northern states, leaving central areas much drier. (La Nina potential was a big part of some well-publicized forecasts last winter that called for sub-par crop yields in 2016. We were not participants in that interpretation of the Pacific's evolution.)

A look at the Pacific temperatures shows that cooling is taking place. There are several pockets of the equatorial ocean where the water temperatures have cooled to minus 1.0 Celsius. That value is definitely in La Nina territory. However, such cooling has not encompassed the entire equatorial basin east of the International Date Line, though the trend is moving that way toward cooling.

In a similar fashion, the barometric measurement of the El Nino-La Nina relationship known as the Southern Oscillation Index (SOI) is showing a steady but slow trend toward La Nina values. The SOI monthly average for August was plus 4.67, following plus 3.72 in July and plus 3.70 in June. And, 30-day running averages of the SOI are in the range of plus 5.15 -- so this facet of ENSO (El Nino Southern Oscillation) is showing an inclination toward the La Nina threshold value of plus 8.0 on a 30-day running average. But it's going to take a while to get there.

Will La Nina have a big impact on harvest weather in the U.S. this fall? At this point it does not look like it. The biggest impact that La Nina could have in the relatively short term would be in bringing drier conditions to Argentina and southern Brazil during the last quarter of the year and into the early portion of 2017. There is a very high relationship between La Nina setting in and a resulting drier trend in that southern portion of the South America crop belt. That will get plenty of attention.

Farther down the road, if La Nina holds together through the majority of the spring season next year, it could possibly have some harsh effects on growing-season weather here in the U.S.

Saying it could and will, of course, are two far different things. For now, the consensus in forecast model depiction of the Pacific temperature pattern for next year is that the water temperatures will be neutral during fieldwork season in the Northern Hemisphere next spring.

Bryce Anderson can be reached at

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10/23/2018 | 4:23 PM CDT
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Bryce Anderson
9/6/2016 | 6:05 AM CDT
The 30-day SOI is now at +7.0--reached that value Monday September 5.