---
title: "Southern Oscillation Index (SOI): ENSO Tracker"
description: "The Southern Oscillation Index gauges Tahiti-minus-Darwin pressure to flag El Niño and La Niña. Sustained negative SOI signals El Niño and weak Indian monsoons."
url: "https://modeldiplomat.com/learn/glossary/southern-oscillation-index"
---

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Model Diplomat Research Team · Last reviewed June 22, 2026

TL;DR

- The Southern Oscillation Index measures the standardized atmospheric pressure difference between Tahiti and Darwin to track the El Niño-Southern Oscillation phase.
- How does a negative SOI relate to the Indian monsoon?
- What is the difference between the SOI and the Oceanic Niño Index?
- What SOI values indicate El Niño versus La Niña?
- Why are Tahiti and Darwin the chosen stations?

[Civil Service & Diplomatic Exams](https://modeldiplomat.com/learn/civil-services/glossary)Updated June 22, 2026

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In 2015, Australia's Bureau of Meteorology recorded sustained deeply negative SOI values confirming a powerful El Niño, which coincided with a deficient Indian southwest monsoon and eastern Australian drought.

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