The winter of 2011-12 might well earn the title of "the winter that wasn't" in many parts of the United States.
The season has entered the books as the fourth warmest on record for the Lower 48, according to an analysis by the National Climatic Data Center in Asheville, N.C.
In January, for instance, daily high-temperature records were broken across the northern Plains. Minot, N.D., posted a 61 degree F. high on Jan. 5, eclipsing the previous record of 58 degrees set in 1906.
Despite several powerful snowstorms that crossed the continent during the season, the extent of the country blanketed with snow was the third smallest since satellites began keeping track 46 years ago. The amount of rain was also below normal.
What a contrast with the winter before. Who could forget the seemingly endless conga line of storms that traversed the country? That winter also was somewhat colder than normal, which meant the snow didn't melt significantly between storms, explains Jeff Weber, a meteorologist at the National Center for Atmospheric Research (NCAR) in Boulder, Colo.
"This year was dramatically different," he says.
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.
"That's what we're looking at now for the Colorado River, and it's also what we're looking at for parts of the Southeast – Florida, Georgia, places like that," Dr. Wolter says.
One glaring exception this winter was Texas, where several storms helped moderate the state's severe drought.
Elsewhere, the reduced blanket of snow is likely to give areas ravaged by last year's floods along the Mississippi River a much-needed break. A year ago, runoff from heavy snows, combined with intense spring storms, brought record floods in many parts of the Midwest.
"We're in better shape with regard to flooding this year than we were at this time last year," says Jim Pogue, spokesman for the US Army Corps of Engineers office in Memphis, Tenn. "That's very encouraging for us." Yet it remains to be seen what the lack of runoff may hold for river levels in the summer, he adds. If water levels fall considerably, low water can hamper barge traffic.
As consequential as La Niña is, particularly in the Western US, it isn't the only player in the country's winter weather patterns. A seesaw feature over the North Atlantic, dubbed the North Atlantic Oscillation (NAO), has played a role, as has the Arctic Oscillation, a varying wind pattern over the polar region.
As a storm track cuts across the continent, the NAO can in effect impede that flow when it's strong or stay out of the way during its weak phase. The oscillation can flip-flop in strength quickly. During the winter of 2010-11, the NAO on average remained strong, forcing the jet stream to form deep dips to the south as it crossed the continent. This allowed winter storms to track farther south than usual, bringing snow and cold to parts of the country where snowfall is rare.
The Arctic Oscillation, which can keep Arctic air bottled up near the poles, has been generally strong this winter, Dr. Weber of NCAR says. This has kept the country free of too many spells of frigid air, compared with last year, when the oscillation was weaker.
Still, these features over the Atlantic and Arctic have occasionally shifted this season to allow a few powerful storm systems to reach into the Central and Southern US, triggering fatal tornado outbreaks on Jan. 22 and again the week of Feb. 27.
The contrasts between the winters of 2010-11 and 2011-12 occur against a backdrop of a global long-term warming trend, says Martin Hoerling, a researcher at NOAA's Earth System Research Laboratory in Boulder, Colo. But efforts to explain those two differing winters in terms of global warming fall short, he adds.
Both started off with La Niñas of relatively comparable strength. Both occurred under similar conditions of summer Arctic sea ice melt-back, which some researchers have offered as a key factor for the snowy winters of 2009-10 and 2010-11.
If anything, the two contrasting winters, with their respective extremes and records, "remind us of how dynamic the weather patterns can be, unrelated to anything that is tickling them" over the long term, he says.