Explanation – ENSO is measured by two indices- Oceanic Niño Index and Southern Oscillation Index. Oceanic Nino Index (ONI) is the primary indicator for monitoring the ocean part of El Niño-Southern Oscillation, or “ENSO”. ONI is a widely used index that primarily measures sea surface temperature anomalies in the equatorial Pacific Ocean. Southern Oscillation Index (SOI) is the primary indicator for monitoring the atmospheric part of ENSO. The ONI tracks the running 3-month average sea surface temperatures in the east-central tropical Pacific between 120°-170°W, near the International Dateline. It also determines whether they are warmer or cooler than average.

Source: Forum IAS