Ag Weather Forum

La Nina May Be Peaking

Bryce Anderson
By  Bryce Anderson , Ag Meteorologist Emeritus
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The Australia Bureau of Meteorology Pacific Ocean forecast calls for Pacific temperatures to rise above La Nina cool-water levels by mid to late January 2021. (BOM graphic)

The latest Australia Bureau of Meteorology (BOM) analysis of conditions in the equatorial Pacific Ocean suggests that the La Nina event now in place may be close to its highest intensity, with ocean conditions possibly moving away from La Nina to the classification of ENSO-neutral by March 2021.

"All international climate models surveyed by the Bureau indicate the current La?Nina is at or near its peak and will likely persist until at least January 2021," the Dec. 22, 2020 Pacific Ocean update reads on the BOM website. "All but two of the models indicate a return to neutral conditions (neither El Nino nor La Nina) by the end of the southern autumn in 2021." The southern autumn is, of course, the Northern Hemisphere spring, which takes in the months of March, April and May.

And in a key statement, the Australia analysis makes this statement: "La? Nina conditions are weaker now than they were at the same point in 2010." This is an important point. La Nina in 2010 had a life span of almost two years. Its effect was prominent in U.S. corn production, which hit a three-year run of below-trendline yields: 151.2 bushels per acre in 2010; 147.2 bpa in 2011; and then dropped off to 123.4 bpa in 2012.

The Pacific Southern Oscillation Index (SOI), which is faithfully tracked by the BOM, had a 30-day average value of plus 15.16 on Dec. 23, 2020. But it is much less than the December SOI value back in 2010.

In fact, the SOI moved to strong La Nina levels almost immediately, with the following monthly values in the last half of the year 2010: July 20.08; August 18.22; September 25.52; October 18.20; November 16.42; and December 27.03.

2020 shows a much lower intensity of this ENSO metric. Monthly SOI values for the last half of 2020 are: July 4.02 (below the La Nina 8.0 threshold); August 9.39; September 9.96; October 4.18 (slipping back below the La Nina threshold); November 9.14; and the December 22 30-day value at 15.16.

If the forecast of Pacific sea surface temperatures reaching ENSO neutral levels by possibly late January verifies, the suggestion is that the SOI atmospheric feature will also start relaxing and the La Nina re-constituting of the jet stream patterns will have a short shelf life during early 2021. This feature is obviously a key item in projecting crop weather impact.

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Bryce Anderson
12/25/2020 | 7:05 AM CST
The 30-day SOI in late December is plus 15.2 and is the highest value of December. That is well below the highest value that the SOI had in December 2010 of plus 27.1.