Ag Weather Forum

La Nina SOI Update

Bryce Anderson
By  Bryce Anderson , Ag Meteorologist Emeritus
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The 30-day Southern Oscillation Index (SOI) value for Oct. 11, plus 11.7, indicates a growing but not intense La Nina. (Australia Bureau of Meteorology graphic)

The combination of sea surface temperatures and barometric pressure features in the Pacific Ocean is definitely showing the presence of La Nina. The water temperatures between the South America coast and the International Date Line around the equator are running from 1 to 1.5 degrees Celsius below average. And in the atmosphere, the barometric pressure metric known as the Southern Oscillation Index (SOI) is also pointing to the establishment of La Nina.

The SOI has a long tracking history by the Australia Bureau of Meteorology (BOM). The SOI archives on the BOM website go back almost 140 years to the early 1880s. The SOI has daily numbers; 30-day average calculations; 90-day average calculations; and monthly averages. Its comparison in that time frame has been very consistent; using a calculation based on comparing the barometer readings on the island of Tahiti and at Darwin in the northernmost tip of the Australian continent. An SOI value of positive 7.0 or higher over a 30-day period or over a month or sustained period of months.

As of Oct. 11, the 30-day SOI value was at plus 11.7. The 90-day value on Oct. 11 was slightly more than plus 8.0. Both those values point to the presence of La Nina. These numbers coordinate well with sea surface temperatures in that minus 1 to minus 1.5 deg C range, showing cooler than normal sea surface temperatures in the equatorial Pacific. Since July, the SOI has steadily moved from El Nino levels at minus 10 to the La Nina category at plus 11.7 on the monthly scale.

Forecast details continue to point to this La Nina having a moderate intensity and losing its identity during the end of the first quarter or early in the second quarter of 2021. The onset of this particular La Nina appears to be supporting that call. A comparison of the September monthly SOI values for 2020 with September of 2010 and September 2008 shows a lower reading on the SOI scale. The 2020 September SOI value was plus 9.93. In contrast, September 2008 had a monthly SOI value of plus 13.68; and the September 2010 SOI calculation was plus 25.79. So, this year's SOI suggests that La Nina is not showing the intensity early on that La Nina events in 2008 and 2010 did. It does bear pointing out that the SOI reading for September 2010 indicated the strongest La Nina in more than 50 years. That event lasted all the way through 2011 and into early 2012.

This 2020 La Nina is already getting much attention. We can expect continued close scrutiny of the equatorial Pacific waters for quite a few weeks to come.

Bryce Anderson can be reached at

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