Ag Weather Forum

The long-anticipated development of La Nina in the equatorial Pacific Ocean may be underway, just in time to bring concern over weather prospects for the final weeks of U.S. corn and soybean crops.

La Nina -- the cool-water phase of the Pacific Ocean El Nino Southern Oscillation (ENSO) climate feature -- has taken its time to form this summer. Some forecast models earlier this summer called for La Nina to possibly be in effect by mid-August.

However, that did not occur. The equatorial region of the Pacific maintained a neutral sea surface temperature and barometric pressure identity throughout June and July. In fact, the Southern Oscillation Index (SOI), which analyzes the relationship between the barometric pressure readings on the island of Tahiti and in Darwin, Australia, had 30-day average values of minus 8 or lower from Wednesday, July 31, through Friday, Aug. 16 -- values which are decidedly in the category of El Nino, not La Nina.

That trend has dramatically changed. The 30-day average of the SOI on Monday, Aug. 19, was minus 2.29 -- well within the neutral category of the SOI. And that may be a sign atmospheric circulation is responding to sustained temperature changes in the eastern equatorial Pacific. NOAA communications specialist Tom DiLiberto noted in a U.S. Climate.gov blog post on Thursday, Aug. 8, that " ... below-average water temperatures in the subsurface (the surface to 300 meters below the surface) of the tropical Pacific Ocean strengthened in the past month, expanding across more of the central and eastern Pacific Ocean." That building pool of cool water is seen as a "driving force" behind La Nina forming later this year.

La Nina's formation brings a note of caution when it comes to how the U.S. crop weather scene will evolve during these last two weeks of August and into September. Hot and dry high pressure over the central U.S. has frequently occurred when La Nina is in effect. That development could lead to U.S. corn and soybeans getting pushed toward maturity. In that case, corn kernels and soybeans in the pod would not be able to completely fill out, which leads to lower kernel and bean weight and in turn reduces yields. This scenario, of course, is going on just a week after public and private forecasts suggested either above-trendline or record corn and soybean yields.

The Pacific Ocean may have gone on a midyear holiday, so to speak, but it seems to be back in business and affecting commodity outlooks marketwide through the balance of the 2024 growing season.

Bryce Anderson can be reached at Bryce.Anderson@dtn.com

(c) Copyright 2024 DTN, LLC. All rights reserved.