While El Niño events have been occurring for thousands of years, they were not studied in depth until the late 20th century. Sea surface temperature data is most often used to represent when the El Niño Southern Oscillation (ENSO) is occurring. For this reason, most ENSO climatologies are based on data from 1950 to present, consistent with when sea surface temperature data are available.
Many different indices have been created to track ENSO, but the most commonly used index is the Oceanic Niño Index (ONI). The ONI uses a three month mean of sea surface temperature anomalies in the Niño 3.4 Region. This region is east of the international dateline along the equator and is about as long as the distance between Hawaii and California. For the purpose of this climatology, the ONI will be used.
About 20 El Niño events have been recorded since 1950. Around half of these events were considered moderate to strong, with six or seven considered strong. While the 1987-88 El Niño was considered strong, the ONI peaked in the summer to early fall of 1987, unlike the other six strong El Niño events which peaked in the winter. The strongest El Niño event was in the winter of 1997-98, where the ONI peaked at 2.3.
Oceanic Niño Index since 1950. Noted years represent the seven El Niño events where the ONI exceeded a value of 1.5,
the definition of a strong El Niño.
While an El Niño event occurs every three years on average, strong El Niño events typically occur every 6-10 years. The five strongest El Niño events since 1950 were in the winters of 1957-58, 1965-66, 1972-73, 1982-83 and 1997-98. It has been 17 years since the last strong El Niño event, the longest such stretch without a strong El Niño in this 65 year period. Should the El Niño of 2015-2016 continue its current forecasted trajectory, it will become one of the strongest El Niño events on record.
There are other phases in ENSO that are also represented by the ONI and other indices. The cold phase of ENSO, La Niña, has a similar frequency of about 20 events. About ten of the La Nina events were categorized as moderate or strong with four to five categorized as strong. The last phase, Neutral, makes up the remaining third of ENSO events. For more information on how El Niño events can differ, see the Diversity of El Niño page.