Yet both winters began the same way – with La Niña reigning in the tropical Pacific. La Niña is the cooler half of what's called the El Niño-Southern Oscillation. El Niño brings warmer-than-normal waters to the equatorial eastern Pacific, where it piles up against the coasts of Central and South America. La Niña brings colder-than-normal waters to the same region. Both alter atmospheric-circulation patterns in ways that are felt far beyond the tropics.
Typically, La Niña pushes the eastward-flowing jet stream – which serves as a kind of superhighway for storms – farther north than usual. That pattern appeared last year in a relatively stark boundary between a very wet northern half of the country and a parched southern tier, stretching from Arizona to northern Florida and up into the Carolinas.
This year, even with a somewhat weaker La Niña, the average path of the jet stream has moved farther north still, leaving the northern US drier than normal. Without extensive snow cover to help keep a lid on winter temperatures, the stage was set for a warmer-than-normal winter, weather and climate specialists say.
The back-to-back La Niñas have a marked effect on rivers in the Southwest and Southeast, notes Klaus Wolter, a researcher with the National Oceanic and Atmospheric Administration (NOAA) and the University of Colorado.
"We've had 10 cases in the last century of double-dip La Niña events," he says. If the initial event is strong – last year was one of the Top 3 La Niñas in the past 50 years – the second, weaker one tends to bring drier conditions to the Southwest and southern tier. The difference shows up strikingly in river flows, he says. They tend to be even lower coming out of the second event than they were at the end of the first event.