The Composite El Niño. Due October 15, 2019 The Oceanic Nino Index (ONI) measures SST variability in the eastern equatorial Pacific, providing a proxy for the El Niño Southern Oscillation (ENSO). The ONI is the 3-month running mean of the Niño 3.4 index, which is an area average of monthly sea surface temperature (SST) anomalies in the region between 5°S-5°N and 170°-120°W (Figure 1)
El Niño events are defined when the ONI exceeds 0.5°C for five consecutive months, La
Niña when the ONI falls below -0.5°C (Figure 2).
Figure 2. The Oceanic Nino Index measures El Nino Southern Oscillation (ENSO) variability,