[Narrator] There was a very large El Nino in 1997, the largest on record at the time, and scientists thought that 2014 was going to be a repeat performance with an El Nino of similar strength, but it didn't happen. Why? Dr. Michael McPhaden explains. [McPhaden] This is a great mystery. And 2014 started out actually a lot like 1997. We had some pretty strong westerly wind bursts. We had some really big Kelvin waves, and we had some warming in the eastern Pacific. If you look at the magnitude of the sub-surface anomalies in temperature, indicative of that Kelvin wave. This is for 2014 and this is for 1997. 1997 got underway a little bit later than 2014. But you see the magnitude of the anomalies, or effectively the strength of the Kelvin wave, was comparable in 2014, at what we thought was the beginning of a big El Nino, comparable to that of April 1997, at onset of the largest El Nino on record. So, there was a lot of excitement in the community, quite a stir, and the popular press was abuzz, with headlines like this... "Unusually intense El Nino may lie ahead" "El Nino could grow into a monster" "The world is unprepared for a major El Nino" Of course, we know it didn't work out that way. The monster El Nino turned to be quite a midget after all. These conditions in December 1997, anomalies of wind and sea surface temperature. These are sea level anomalies, over here, from altimetry. These are about a foot and a half of elevated sea level associated with the warm ocean. Compared with 2014, very weak anomalies across the basin. In fact there is an argument or debate going on whether we should even call 2014 an El Nino year. [Narrator] There are several possible reasons for the demise of the monster 2014 El Nino. [McPhaden] But I'll just mention one, because it is perhaps the most immediate cause, and that was the westerly wind bursts. Now we know these westerly wind bursts are an important part of the evolution of all El Nino events. And we did start out with a couple strong ones in 2014, but unlike 1997, for whatever reason, there was no real strong subsequent development of these westerly wind bursts. And the warm waters favor the occurrence of these wind bursts, so we had the initial warming, you might have thought we would get more of these wind bursts, but there is also a very random nature to these. I mean, they're related to weather events. It may be that it was just dumb luck that we didn't get the kinds of weather events in 2014 that we got in '97 or this year.