Pacific Shows Signs of La Nina Return

La Nina Development Has Potential to Affect Crops in US, South America

Bryce Anderson
By  Bryce Anderson , Ag Meteorologist Emeritus
Connect with Bryce:
Both the 30- and 90-day Southern Oscillation Index (SOI) values are above the La Nina threshold. (Queensland, Australia Government graphic)

As the calendar moves into the beginning of the fall season, the closely watched barometric pressure indicator of the El Nino-Southern Oscillation (ENSO) situation in the Pacific Ocean is showing signs of La Nina development. The Sept. 8 30-day running average of the Southern Oscillation Index (SOI) was a positive 7.68, and the 90-day SOI running average was logged at positive 8.48. Both values are above the La Nina threshold value of positive 7.00.

According to the NOAA Pacific Marine Environmental Laboratory (PMEL): "La Nina is defined as cooler-than-normal sea-surface temperatures in the central and eastern tropical Pacific Ocean that impact global weather patterns. La Nina conditions recur every few years and can persist for as long as two years." In very general terms, the SOI relates to a comparison of the barometer readings on the island of Tahiti in the central Pacific Ocean and the far northern Australia weather station at Darwin, Australia. The positive SOI numbers point to a higher-pressure value at the Tahiti location than the Darwin location. The SOI has been tracked for more than 100 years, going back to the late 1800s.

That pattern going on more than 5,000 miles from the central United States will get plenty of attention during this season. La Nina in the September-December time period has the potential to contribute to such large-scale features in the U.S. as: below-normal precipitation in the Southern Plains and Midwest; enhanced tropical storm development in the Atlantic Basin; drier conditions in Argentina and southern Brazil; and a wetter pattern in eastern Australia.

Crop impact from these features presents itself as follows:

-- The below-normal precipitation (drier) trend in the Southern Plains suggests reduced soil moisture for the 2021-22 winter wheat crop that will be planted during the next couple months. Most of the Southern Plains is drought-free in early September; however, more than half the key wheat state of Kansas has some phase of dryness or drought noted.

-- A drier pattern in the Midwest suggests generally favorable conditions for row crop ripening and harvest. A wild card is how any tropical storms or hurricanes that form in the Atlantic Basin would track through the region. The damaging heavy rain and wind from Hurricane Ida in late August came close to tracking into the southeastern Midwest.

-- Tropical storm and hurricane formation was off the charts a year ago in 2020, when La Nina set in during the last half of the Northern Hemisphere summer season and continued into Northern Hemisphere fall. As of early September, the Atlantic Basin has already had 13 named storms, with a new potential system forming in the Gulf of Mexico. Hurricane Ida and its remnants have already entered the record books for winds, heavy rain and flooding.

-- In South America, drought damage to Brazil corn, coffee and sugarcane crops was widely noted in news coverage and in market trends during 2021. Argentina has also struggled with drought impact; the key Parana River had a 50-year low flow in 2021. La Nina-related dryness implies no relief from the drought and a threat to crop output for the 2022 harvest in Argentina. In Brazil, southern and south-central areas have the highest relationship to La Nina production impact. Already, early corn planting in south-central areas is off to a slow start because of low soil moisture.

-- On the other wide of the Pacific, La Nina formation promises to be a rain maker for eastern Australia. The Australia Bureau of Meteorology three-month forecast notes an 80% probability of above-media precipitation for the entire eastern half of the Australia continent. The suggestion is that wheat in all but Western Australia will have favorable moisture to finish its production cycle. Official forecasts point to the third-highest winter crop production figure on record for Australia.

Bryce Anderson can be reached at

Follow him on Twitter @BAndersonDTN

Bryce Anderson

Bryce Anderson
Connect with Bryce: