Ag Weather Forum

La Nina Recedes at Month's End

Bryce Anderson
By  Bryce Anderson , DTN Senior Ag Meteorologist
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At the end of October, the Pacific Ocean Southern Oscillation Index (SOI)--a key feature of El Nino or La Nina--shows a retreat from La Nina levels to almost a flat zero or Neutral. (Australia Bureau of Meteorology graphic by Nick Scalise)

Official forecasts call for a La Nina temperature and barometric pressure pattern to be a primary feature over the next six months. But, both indicators at the end of October are falling well short of such an entity.

La Nina is the term used to describe the situation when Pacific Ocean equatorial-region temperatures are one-half degree Celsius (about 2 degrees Fahrenheit) below normal for at least a sustained three-month period. In conjunction with the temperature pattern, the barometric pressure relationship between the island of Tahiti and Darwin, Australia, measured by the Southern Oscillation Index (SOI), shows a sustained greater value in Tahiti versus Darwin, at has a +8.0 or higher number.

During late summer, the equatorial Pacific did seem to be moving toward La Nina values. Water temperatures, notably in the central equator sector, were getting cooler and moving toward the below-normal threshold of 0.5 Celsius below normal. And, the SOI was steadily moving into positive territory. In fact, the Australia SOI 30-day moving average was in the +10.0 or higher category throughout much of September.

However, since the first week of October, the SOI has retreated from the strong positive numbers -- in fact, the 30-day moving average as of Friday, October 28, was near zero. Also, the Pacific temperature pattern has shown a split personality of sorts; central temperatures remain below normal, but eastern Pacific values (off the South America coast) have shown an above-normal tendency.

The message in this inconsistency is simply this: La Nina does not seem to be a major, long-lasting feature in the Pacific Ocean over the next six to twelve months. It may form, but is showing some definite weakness.

And, that prospect means that weather conditions may play out to give better rain chances to southern Brazil and Argentina crop areas over the next few months as the South America growing season moves along. It also may indicate at least slightly more favorable precipitation chances for the Far West, southern Plains, and the southeastern U.S. during the rest of this fall season.

The bottom line is this: La Nina is a more questionable entity for the Pacific Ocean than we would have thought just four weeks ago.


Twitter @BAndersonDTN



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