The causes of massive snow storms

Schubert and his team isolated the role that sea surface temperatures played in changing the storminess across the Northern Hemisphere. By initializing the model with the early December 2009 atmospheric conditions and the higher sea surface temperatures from that time, Schubert and his colleagues were able to reproduce many of the subsequent changes in winter storminess.

“El Niño is predictable on monthly and seasonal time scales. But we know that sea surface temperatures don’t control everything about the atmosphere,” Schubert said. “Storms develop in the atmosphere whenever they decide to as a result of instabilities. Models can’t replicate the actual sequence of events in predictions extending beyond a few weeks, but they can predict whether or not there will be more or fewer storms, because of the sea surface temperatures.” Schubert and his team ran fifty different simulations, slightly changing the atmospheric conditions each time while keeping the actual sea surface temperatures the same. In the end, the data showed that the storms were influenced more by the sea surface temperatures, and less by the changing atmospheric conditions.

“The atmosphere is chaotic, but if we do this over and over again, slightly changing the initial conditions, we can average the runs, filter out all the random atmosphere variability and see the part that’s driven by sea surface temperatures,” Schubert said.

Getting snow instead of rain
The release notes that while El Niño tends to produce greater storminess, it does not necessarily lead to more snowstorms along the East Coast. Without colder temperatures, these storms bring just rain.

Cue the second culprit: a fluctuation of the atmospheric pressure differences in the Atlantic between the Icelandic low-pressure field and the Azores high-pressure field further south. The North Atlantic Oscillation, as it is called, controls the strength and direction of westerly winds, as well as storm tracks across the North Atlantic.

Scientists cannot predict these fluctuations very well. But it is known that in a positive phase, the north-south pressure difference is enhanced and the west-to-east winds are strong, effectively creating a wall that keeps cold air in the Arctic. In the negative phase, the north-south pressure difference is reduced, allowing cold Arctic wind to bear down across the North Atlantic.

“It’s a structure that tends to favor cold temperatures on the East Coast when it’s in the negative phase,” said Schubert. While the atmospheric pressure fields oscillate at daily and weekly time scales, the winter of 2009-10 saw the