Applicable publications, involving five languages, have been reviewed to obtain information on El Niños that occurred over the past four and a half centuries. Since this information refers strictly to El Niño occurrences, a regional manifestation of the large‐scale (El Niño‐Southern Oscillation (ENSO)) event, it is based primarily on evidence obtained from the west coast region of northern South America and its adjacent Pacific Ocean waters. Authored lists of events were not acceptable without referenced valid information sources. It was desirable to have cross‐correlated reports from independent sources. Relative strengths of events are based on such considerations as wind and current effects on travel times of ancient sailing ships, degree of physical damage and destruction, amounts of rainfall and flooding, mass mortality of endemic marine organisms and guano birds, extent of invasion by tropical nekton, rises in sea temperatures and sea levels, affects on coastal fisheries and fish meal production, etc. Emphasis is placed on strong and very strong events. For example, the 1940–1941, 1957–1958, and 1972–1973 events fall into the strong category, whereas the 1891, 1925–1926 and 1982–1983 events are considered very strong. Over our period of study, 47 El Niño events were placed in the strong or very strong categories. Over the period 1800‐present, we noted 32 El Niño events of moderate or near moderate intensity. Weak events are not included here. The approach used here caused us to revise many of our earlier evaluations concerning event occurrences and intensities. Our tropical Pacific thickness analyses and cumulative plots of Southern Oscillation index anomalies over the southeast Pacific trade wind zone showed additional evidence as to the unusual strength of the 1982–1983 event. Also, in our investigation we noted several periods of long‐term (near decadal or longer) climatic change.

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In October 1974 the occurrence of a weak El Niño event was predicted for early 1975 on the basis of the southern oscillation index. An expedition was organized to observe the event in the waters off Peru and Ecuador during two cruises in order to study its occurrence and its development with time. During the first cruise a massive transgression of warm low salinity water across the equator to 4 degrees S was observed, as well as a depression of the thermocline along the equator and off the coast of South America, indicating the start of El Niño development. During the second cruise the oceanographic situation had changed and conditions were returning to normal.

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