Ag Weather Forum

Hints of El Nino in Early Winter

Bryce Anderson
By  Bryce Anderson , Ag Meteorologist Emeritus
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The Pacific Ocean atmospheric measurement known as the Southern Oscillation Index enters December with El Nino values on both the 30-day and 90-day averages. (Australia Bureau of Meteorology graphic)

Just a couple weeks ago, this space noted how the Pacific Ocean was officially absent of either El Nino (warm) or La Nina (cool) sea surface temperature and barometric pressure conditions. And, the prospects for the winter 2019-20 season would more likely be dependent on more intermediate-term features.

That scenario may indeed play out. However, recent measurements of both temperature and barometric pressure in the Pacific basin are at least worth noting for strong resemblances to a weak El Nino event. Sea surface temperatures at the equator in the Pacific Ocean show several pools of water with between 1 degree Celsius and 2 degrees Celsius above normal, going across the ocean from the coast of Ecuador and Peru in South America to the East Indies. When the water temperature is a sustained 0.8 degree C above normal, an El Nino event is indicated.

At the same time, the atmospheric fingerprint of either El Nino or La Nina, the Southern Oscillation Index (SOI), is around a minus 10.0 reading on the 30-day moving average, and has a 90-day moving average of around minus 9.0. A value of the SOI at minus 8.0 is the threshold for El Nino on this measurement -- and research done on the influence of El Nino on weather patterns in the U.S. Midwest by Iowa State University shows that the 90-day SOI reading of minus 8.0 or lower indicates that a given El Nino event is robust enough to affect the U.S. Midwest weather pattern.

This set of conditions will be interesting to keep track of through the winter season. When El Nino is in effect during the winter, the weather pattern over the northern U.S., Western Canada, and Alaska, tends to be warm and dry. In contrast, the southern tier of the U.S. has a cooler and wetter pattern. If the northern warmer and drier trend indeed develops, even for at least part of the season, there may be an opportunity for the final leg of the much-delayed 2019 corn harvest to get done. It would also, perhaps, dial back the prospect of winter moisture loading up to the point of enhancing spring flood threat quite as much as feared during the late-fall time frame.

At the same time, higher chances for precipitation in the southern U.S. could improve drought conditions in the Southwest, along with offering better moisture for crop areas of the southwestern Plains ahead of spring 2020.

So far, the winter forecast setup of a warm first segment of the season is verifying. There, of course, are still 11 weeks to go through the rest of the season.

Bryce Anderson can be reached at

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