Combing through historical records dating back to the late 1800s, the CSI team identified the separate “fingerprints” of wintertime climate conditions related to El Niño and the NAO. They deduced that NAO conditions alone could explain Europe’s extreme winter and the large-scale cold temperatures in the United States, but not the remarkable occurrence of record-setting snowstorms in the mid-Atlantic region. They also found that while El Niño conferred additional risk of storms for the mid-Atlantic, these conditions alone didn’t always result in snow. The team then compared the combination of these fingerprints with the 2009-10 conditions. Using a mathematical model to combine the characteristic climate patterns related to El Niño with those of a negative NAO, their reconstructed winter conditions agreed with real-world observations.

These maps are centered on the North Pole to show near-surface temperature anomalies in the Northern Hemisphere. Asia is at the top of the map and North America is at the bottom. The top pair of maps are long-term composites (from 1951-2010) illustrating typical winter-time influence of El Niño (top left) and the NAO (top right). The bottom pair of maps shows a composite of the combined influence of El Niño and the NAO (bottom left) compared to the real-world observation from December 2009-February 2010 (bottom right). (Maps courtesy NOAA Climate Prediction Center.)

Wintry Weather in a Climate Context
The CSI Team found abundant historical evidence of heavy mid-Atlantic snowstorms whenever an El Niño and a negative NAO acted in concert, further supporting their conclusion that the record-setting snowstorms were the result of natural causes. But could global warming have elevated the potency of this dynamic duo? Again, the CSI Team didn’t find a connection.

While the U.S. shivered this winter, ranking 18th coldest since 1895, the planet’s average winter temperature ranked as the fifth warmest on record. Sea surface temperatures ranked second warmest this winter when averaged over the world oceans, according to preliminary data from NOAA’s National Climatic Data Center.

But the extreme blocked-NAO of this past winter was opposite to the trend toward more positive phases of the NAO since 1950, and also opposite to projections for a positive trend in the NAO during the 21st Century due to greenhouse gas increases.

The observed variations of winter precipitation in the Washington, D.C., area (left) have been very different from what models predict would occur if only human-induced emissions of greenhouse gas and aerosols (right) were taken into consideration. This evidence suggests the mid-Atlantic’s trends in precipitation are mainly due to natural variability, not human influence. (Graph courtesy of Marty Hoerling, NOAA ESRL.)

Attribution is often in high demand when climate behaves in unusual or extreme ways as it did this past winter. NOAA’s ability to respond with the best possible science is critical so that society can anticipate and respond to climate and its impact. For a more detailed science assessment of the causes for this winter’s snows, please see Understanding the Mid-Atlantic Snowstorms During the Winter of 2009-2010.

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Martin Hoerling is a senior climate scientist at NOAA’s Earth System Research Laboratory (ESRL), where he leads the Climate Scene Investigators team. Katy Human and Barb Deluisi are science writers at NOAA ESRL.