What's with these snowstorms? Natural patterns, plus randomness.

If you’re hunting for an explanation for this winter’s record-setting behavior across the United States, look no farther than the coins in your pocket.

Total up the range of usual suspects – El Niño, the North Atlantic Oscillation, the Arctic Oscillation, and other influential, natural swings in seasonal weather and climate patterns – and at most, they represent no more than half of the drivers behind the headline-generating conditions around the country this winter.

The other half?

“There’s a lot of stuff going on in the atmosphere that has nothing to do with these. It’s the equivalent of a random coin flip,” says Ron Stouffer, a researcher at the National Oceanic and Atmospheric Administration’s Geophysical Fluid Dynamics Laboratory. “Weather is chaotic.”

Linkages between natural modes of change in the atmosphere and weather events “aren’t as simple as people like to make them out to be,” he says.

Simple or complicated, it’s a winter to remember – one that is likely to trigger a host of studies that try to tease out details of the factors contributing to the season’s storms.

Snowball fights in Pensacola

Even as the mid-Atlantic states continued to dig out from their major back-to-back snowstorms, Georgia, Mississippi, Louisiana, and the Florida Panhandle saw white. Dallas is coping with more than 12 inches of snow, while snowball fights broke out in Pensacola, Fla., at least until temperatures warmed and snow became rain.

While at least half the drivers for this winter’s weather can be summed up as pure chaos, well-known patterns have played their roles as well.

Start with El Niño, a periodic shift in location of a vast deep pool of warm water that migrates from the western Pacific and piles up against the west coast of South America. As the warm water travels, so do the bands of convective thunderstorms that the warm water spawns. As these storms and their strong updrafts migrate, the change alters atmospheric circulation patterns globally.

The warm waters made their move during the fall, says Tony Barnston, the lead forecaster at the Lamont-Doherty Earth Observatory’s International Research Institute for Climate and Society in Palisades, N.Y. But only recently did stronger-than-normal thunderstorm activity move east of the international date line to begin rearranging atmospheric circulation patterns.

Jet stream a superhighway for storms

One result: The subtropical jet stream, a high-altitude river of air that snakes its way west to east across North America, is driven farther south than normal. The jet stream acts as a superhighway for storm systems that roar in off the Pacific.

The more-southerly average track allows storms to draw more moisture from the Gulf of Mexico as they move east, compared with storms crossing the continent farther to the north, adds Klaus Wolter, a researcher at NOAA’s Earth Systems Research Laboratory in Boulder, Colo.

In addition, El Niño has intensified the strength of the subtropical jet stream, pushing it to speeds of up to 200 miles an hour. That can translate into high wind speeds that the storms bring at ground level, he adds.

If El Niño’s atmospheric reach has enriched the moisture content of storms this winter, another atmospheric feature has helped ensure that more of that moisture falls as snow as the storms head east along the southern tier, then up the East Coast.

Seesaw pattern in atmospheric pressure

It’s dubbed the North Atlantic Oscillation – a seesaw pattern in atmospheric pressure between the northern and southern North Atlantic. It can vary on time scales as short as 10 day days, rather than remaining relatively settled over a season.

The NAO has been in a strong negative phase, which translates into colder-than-usual temperatures in much of the eastern half of the country, Mr. Barnston explains. That increases the temperature contrast between the chillier land and the Gulf Stream-warmed waters off the East Coast. That heightened contrast boosts the strength of the storm systems that travel up its length.

Others invoke the Arctic Oscillation, which is related to the NAO (indeed, some researchers hold that they are two names for the same phenomenon), as the engine driving colder temperatures deeper south this winter.

Either way, these two in concert have combined with El Niño – and a strong dash of random atmospheric actions – to make this a winter to remember.

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