Ag Weather Forum

La Nina Settles In

Bryce Anderson
By  Bryce Anderson , Ag Meteorologist Emeritus
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Equatorial Pacific Ocean temperatures show widespread cooling of 1 to 2.5 degrees Celsius below normal from South America to the International Date Line. (NOAA graphic)

La Nina shows more signs of settling into the equatorial Pacific Ocean sea surface temperature and atmospheric dynamic scene. Sea surface temperatures in the Pacific equatorial region were first noted moving into La Nina categories during September by the National Oceanic and Atmospheric Administration (NOAA) Climate Prediction Center (CPC).

"Equatorial subsurface temperature anomalies averaged across 180 degrees West longitude to minus 100 deg W were negative, with the largest departures observed in the east-central Pacific from the surface to 200 meters depth," the CPC said in a summary Sept. 10. Those values continue to cool; readings in the equatorial Pacific as of Oct. 3 were as much as 2.0 degrees Celsius (3.6 deg Fahrenheit) below normal in the eastern equatorial Pacific.

Along with the temperatures, the atmospheric circulation continues to move into a phase consistent with La Nina. The Pacific Ocean barometric indicator known as the Southern Oscillation Index (SOI) and tracked closely by the Australia Bureau of Meteorology (BOM) showed readings on Oct. 5 of plus 9.90 on the 30-day average and plus 8.05 on the 90-day average. Both readings are above the established SOI La Nina threshold of plus 8.0. Research done by USDA Midwest Climate Hub director Dennis Todey at Iowa State University shows that the 90-day SOI of plus 8.0 or higher is related to La Nina's having an impact on central U.S. weather patterns.

Following are some of the primary crop weather impact features of La Nina:

-- Dryness and drought in the Southern Plains. This, of course, simply reinforces and intensifies drought already in effect.

-- An active tropical storm-hurricane season. This year is already at a record level.

-- Prospects for a cold and stormy winter in the Canadian Prairies, Northern Plains and northern Midwest.

-- Stormy conditions in the Ohio Valley and eastern Midwest. This includes ice storm risk. In addition, the Delta, Midsouth and Southeast are prone to have more winter and spring severe weather occurrences.

-- Drier conditions in southern Brazil and Argentina. Southern Brazil and central Argentina have a strongly identified relationship to La Nina events. This relationship suggests the prospect of drought and unfavorable crop conditions during the Southern Hemisphere growing season.

-- More rain in eastern Australia. This has already been happening; wheat crop prospects are much improved compared with the last two years in Australia.

A PowerPoint summary of the CPC update on the Pacific situation is available at this link:…

Bryce Anderson can be reached at

Follow him on Twitter @BAndersonDTN


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