NARRATOR: The eastern Pacific, July, 2010: Ocean buoys record unusually cold sea-surface temperatures. This is called "La Niña," and for centuries, Peruvian fishermen have been aware that it not only affects their fishing but also the weather.

Scientists can now measure the effect. Here's the way a La Niña looks on a satellite's thermal imaging camera, showing the cooler sea temperature off the coast of Peru in green. Scientists discovered that the huge expanse of cool La Niña water could affect the surrounding atmosphere and the jet stream, shifting severe weather into new areas, and intensifying it in places like the southern United States.

ROGER A. PIELKE, SR. (University of Colorado): We had a very strong La Niña in the wintertime that set up a strong jet stream that provided the wind speed energy that was necessary to generate thunderstorms, but we also, then, had very humid and moist air in the southeastern part of the United States that provided the fuel for these thunderstorms. And the combination of those two provided an environment that was more conducive to large tornadic outbreaks than you might have in other years.

NARRATOR: By spring, 2011, the newly intensified jet stream was already contributing to rainfall and floods across the south, plus droughts and raging wildfires in Texas, warning signs of historic weather extremes.

ROGER A. PIELKE, SR.: We were worried about it, because when you have a La Niña, as our research has shown, there tends to be more family outbreaks of these tornadoes in the southeast of the United States.

NARRATOR: In fact, one of the worst tornado events in history, the "Super Outbreak" of April 1974, also took place in a La Niña year.

A hundred-forty-eight twisters touched down, in 13 states, from Mississippi all the way up to New York, killing 330 people and injuring thousands.