Southern Oscillation Index

What this is

The Southern Oscillation Index measures the difference in sea level pressure between the central Pacific and northern Australia. That pressure seesaw is the atmospheric half of the El Niño / La Niña cycle, so the index is one of the earliest measurable signals of which way the cycle is leaning, often before the ocean temperature numbers catch up.

The standard index compares Tahiti with Darwin, Australia. David's version swaps in Nuku Hiva in the Marquesas Islands as the Pacific station and runs the calculation daily, which is why the numbers here won't line up exactly with the official Bureau of Meteorology index. The daily contribution is that day's input; the 30-day and 90-day averages smooth the day-to-day noise into a usable signal.

Why it matters

Which phase the Pacific settles into shapes the weather patterns this site covers for months at a time:

• Hurricane season. La Niña tends to reduce wind shear over the Atlantic, which favors a more active season. El Niño tends to do the opposite and suppress Atlantic activity.
• Winter storms. El Niño winters tend to push a stronger, wetter storm track across the southern United States. La Niña winters tend to run drier in the South and colder and snowier across the northern tier.
• Severe weather. The phase influences where the jet stream sets up in spring, and with it where the more active severe weather corridors tend to land.

Watching the index move day by day is a way of seeing that shift as it happens, weeks before it shows up in a seasonal outlook.

How to read it

• Sustained positive values mean pressure is running higher on the Pacific side than at Darwin, the pattern that goes with La Niña.
• Sustained negative values mean the opposite, the pattern that goes with El Niño.
• The 30-day average is the number to watch. Single days swing a lot; a run of 30-day values holding on one side of zero is what carries forecast meaning.

Intensity scale

Each reading above is graded on this scale, most negative at the top. The markers show where today's readings sit.

1. Exceptionally Negative-36 and below
2. Extremely Negative-36 to -28Daily
3. Strongly Negative-28 to -2130-day
4. Moderately Negative-21 to -1490-day
5. Weakly Negative-14 to -7
6. Neutral-7 to +7
7. Weakly Positive+7 to +14
8. Moderately Positive+14 to +21
9. Strongly Positive+21 to +28
10. Extremely Positive+28 to +36
11. Exceptionally Positive+36 and up

Readings are entered by David and pulled from his spreadsheet each time this page loads. As with everything on this site, this is independent analysis, not an official product. See the Terms & Conditions.