Ag Weather Forum

A look at the Pacific Ocean Southern Oscillation Index (SOI) in early August shows that the expected change from El Nino earlier this year to La Nina by now has not happened. That slowdown in this big climate driver could be a reason why row crops in the United States had a favorable July.

The SOI is part of the big Pacific climate-driver system known as the El Nino Southern Oscillation (ENSO). ENSO describes the relationship between sea surface temperatures and large-scale barometric pressure conditions in the equatorial region of the Pacific. The SOI measures the atmospheric state of this relationship and is based on a comparison of the barometer readings at Darwin, Australia and on the island of Tahiti. Index readings of plus 7.0 or higher indicate La Nina, and index readings of minus 7.0 or lower point to El Nino. The SOI has been tracked by the Australia Bureau of Meteorology (BOM) since the late 1800s.

So far at this point in the Northern Hemisphere summer, the SOI values suggest that El Nino has not completely faded away. The monthly SOI value for July 2024 was minus 5.83, very close to the El Nino benchmark. The 30-day running average of the SOI dipped to minus 17.1 on Aug. 3, definitely in the El Nino category. And the daily SOI value Aug. 5 was a neutral reading at plus 3.09.

Those numbers are much different than in 2020, which is the top year in DTN's analog year ENSO analysis. In 2020, the SOI July monthly value was already moving toward La Nina at plus 4.02. And on Aug. 5, 2020, the daily SOI value was a robust plus 27.17 -- far different than the plus 3.09 on Aug. 5, 2024.

This slowdown in transition toward La Nina suggests that prevailing wind patterns across the U.S. also have not taken on a La Nina characteristic.

La Nina in 2020 brought on stressful hotter and drier conditions for filling crops, which resulted in lower yields at harvest. (August 2020 also featured a devastating Midwest derecho windstorm, which significantly lowered corn harvested acreage.)

Pinning down the reason for this year's slowdown in the Pacific's transition to La Nina is difficult. However, the Australia BOM noted in a July 23 comment that "Global SSTs (sea surface temperatures) have been the warmest on record for each month between April 2023 and June 2024. July 2024 SSTs are comparable with 2023 but are much warmer than for any other July on record. The current global pattern of warmth differs to historical patterns of SSTs associated with ENSO and IOD (Indian Ocean Dipole). This means future predictions based on SSTs during past ENSO or IOD events may not be reliable."

The bottom line is: La Nina is still expected to develop, but the timeline has been pushed back into this fall compared with late this summer. And that delayed onset likely benefited the U.S. corn and soybean crops.

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

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