The dataset contains data on El Niño and La Niña, phenomena in the equatorial Pacific Ocean defined by a five consecutive 3-month running mean of sea surface temperature (SST) anomalies in the Niño 3.4 region. El Niño occurs when the anomalies exceed +0.5°C, while La Niña occurs when they fall below -0.5°C. This measure is known as the Oceanic Niño Index (ONI).