The causes of massive snow storms

North Atlantic Oscillation in a strong extended-negative phase. Combine the resulting influx of Arctic air for an unusually long period of time with the moisture and storminess from El Niño, and the once fuzzy cause of these monster storms starts to come into focus. The research shows that the extreme weather over the Eastern United States in the winter of 2009-10 was part of a response mainly to El Niño and its associated Pacific Ocean sea surface temperatures.

The results were then compared with those of a winter (1999-2000) characterized by having completely opposite conditions: a La Niña and a positive phase of the North Atlantic Oscillation. When compared with Snowmaggedon, the winter of 1999-2000 showed less storminess and decreased chances of snow. This comparison helped Schubert and his team corroborate the hypothesis, confirming that El Niño-induced sea surface temperatures and an extended negative phase of the North Atlantic Oscillation caused the changes in 2009-10 winter storminess.

Predicting another Snowmaggedon
In order to improve snow predictions, scientists need to better understand how the North Atlantic Oscillation works and what causes it to stay in an extended period.

There is evidence that the extended negative mode is impacted by sea surface temperatures and maybe even snow cover in Asia. Scientists, however, have not directly linked any one weather variable to the North Atlantic Oscillation.

“People have done these historical studies before to come up with measures, and if you look at the record of major snow storms, some have occurred during El Niño winters and a negative North Atlantic Oscillation phase,” Schubert said. “But sea surface temperatures impacting storminess in the different ocean basins has never been quantified and it’s never been clear what is a relative contribution and in what way they are contributing,” he added.

Richard Seager, a research professor at the Lamont-Doherty Earth Observatory at Columbia University, also studies how El Niño and weakened atmospheric pressure contributes to snow anomalies.

“This model not only confirms that a negative North American Oscillation and El Niño conditions created the conditions that allowed these storms to form,” said Seager, who did not work with Schubert on this research. “But it is useful in showing how the atmosphere can act differently when combining El Niño with different sea surface temperatures. These models provide controlled conditions, which allow us to be sure about the exact causes,” Seager added.

Scientists have predicted that current La Niña conditions and below-average sea surface temperatures might be the cause for the mild 2012 winter on the Eastern U.S. Examples like this, Schubert said, is why it is important to better understand the relationship between sea surface temperatures and storminess. “People want to know whether it’s going to be a snowy winter. Snow prediction is developing but if we predict El Niño, we know it will be more likely stormier. Now whether those storms will be rain or snow depends on the North Atlantic oscillation, which is a big challenge for us because of its constant oscillations.” Schubert and his team’s extended findings on the role of sea surface temperatures in Snowmaggedon will be published in the Journal of Climate this spring.