Because ENSO events differ in their strength, coverage, and seasonality, there isn't unanimous agreement on what constitutes and ENSO event. But, there are broad agreements and you can find some of the lists researchers have used along with how they chose those years. The list in the right column represents ENSO years based on PSD's Extended Multivariate ENSO Index (MEI.ext).
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Temporal evolution of El Niño and La Niña events indicated by the MEI.v2. Shown are the top 5 warm and cold non-overlapping events since 1979. Year 0 corresponds to the year of the event in the legends. Click on image to enlarge.