The intensity of El Niño events varies from weak temperature increases (about 4–5° F) with only moderate local effects on weather and climate to very strong increases (14–18° F) associated with worldwide climatic changes.
Scientists use the Oceanic Nino Index (ONI) to measure deviations from normal sea-surface temperatures. El Niño events are indicated by sea surface temperature increases of more than 0.9° Fahrenheit for at least five successive three-month seasons. The intensity of El Niño events varies from weak temperature increases (about 4–5° F) with only moderate local effects on weather and climate to very strong increases (14–18° F) associated with worldwide climatic changes.