The most recent El Niño event began in the spring months of 1997. Instrumentation placed on Buoys in the Pacific Ocean after the 1982-1983 El Niño began recording abnormally high temperatures off the coast of Peru. Over the next couple of months, these strength of these anomalies grew. The anomalies grew so large by October 1997 that this El Niño had already become the strongest in the 50+ years of accurate data gathering.
The image below displays the Sea Surface Temperature (SST) Anomalies in degrees Celsius for the middle of September, 1997. By this time, the classic El Niño pattern has almost fully ripened, with maxima above +4 degrees Celsius.
Image by: CPC ENSO Main Page