Ag Weather Forum

Pacific SOI Goes to La Nina

Bryce Anderson
By  Bryce Anderson , Ag Meteorologist Emeritus
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The Pacific Ocean Southern Oscillation Index (SOI) 30-day average reached the La Nina threshold July 10 and has steadily strengthened in the last two weeks. (Queensland Government, Australia graphic)

Pacific Ocean El Nino-Southern Oscillation (ENSO) indicators are showing the prospect of La Nina developing as we move into the last half of July. ENSO is comprised of two components -- sea surface temperatures and the air pressure relationship between values in Darwin, Australia and the island of Tahiti in the tropical Pacific. In the past 30 days, the Southern Oscillation Index (SOI) has had a strong move into values that indicate sustained higher values in the central Pacific compared with the western portion of the Pacific Basin.

The SOI surge began in a benign fashion on June 30 with a daily value of positive 10.75. However, that value was then followed by daily readings during the first two-thirds of July which were strongly positive; only two daily readings, July 5 and 6, were not in the double-digit positive category. In a similar trend, the 30-day SOI value crossed the threshold into La Nina territory (plus 7.0 or higher) on July 10 at positive 7.41. The 10 days since have seen that 30-day value continue to work higher; as of July 19, the 30-day SOI was at positive 12.32.

The SOI jump during July is occurring at almost the same point in the summer as a year ago, in July 2020. That move indicated a robust La Nina presence for late summer. La Nina alters the jet stream track and blocks off moisture inflow into the central U.S. That effect enhanced drought during the filling phase of corn and soybeans contributed to lower yields than early to mid-summer projections. Those losses were enhanced, of course, by the devastating Midwest derecho on Aug. 10, 2020.

SOI patterns are closely tracked by the Australia Bureau of Meteorology. In its latest discussion about ocean conditions, the Bureau cited temporary features as responsible for the sharply higher trend in the SOI:

"The El Nino -- Southern Oscillation (ENSO) remains neutral with most oceanic and atmospheric indicators within the neutral range. The past fortnight has seen the Southern Oscillation Index (SOI) increase to plus 13.9 for the 30 days ending 18 July 2021, although this is mostly due to local sea surface pressure variations at Darwin and Tahiti. Additionally, trade winds are stronger than average in the western half of the Pacific, which may encourage some temporary cooling of the ocean surface. Most climate model outlooks indicate the central tropical Pacific Ocean is likely to cool over the coming months, with three of seven models surveyed by the Bureau indicating this cooling will be enough to reach La Nina thresholds in (Southern Hemisphere) spring, with the remaining four models staying neutral. This forecast cooling may also be contributing to the outlook for above median rainfall for much of Australia in the coming months."

In addition to the 30-day SOI reading reaching into the La Nina range, the 90-day SOI value is also creeping closer to the positive 7.0 threshold for La Nina values. (As of July 19, the 90-day SOI value was positive 5.74.)

Bryce Anderson can be reached at

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