Evidence: Negative Phase of the North Atlantic Oscillation
By January, scientists worldwide were already abuzz about the extremely low values for the NAO index. The NAO, first discovered by British atmospheric scientist Sir Gilbert Walker in the 1920s, has been extensively studied, and its affect on U.S. snowfall is clear: When the NAO index is negative, or “blocked,” snow can pile up along the East Coast. At the atmospheric steering level for storms, high-pressure systems develop over Iceland and Greenland while low pressure builds over the central North Atlantic. This situation redirects the tracks of surface weather systems that are conducive for cold and snow toward the mid-Atlantic coast. Winter precipitation along the eastern seaboard is often in the form of rain, but in a blocked NAO pattern, temperatures can drop low enough to create snow instead.

This rendering shows climate conditions and weather events associated with extreme phases of the North Atlantic Oscillation. Based on scientific reanalysis, the NAO index is the difference of normalized surface pressure values between grid points closest to the stations Ponta Delgada, in the Azores, and Stykkisholmur/Reykjavik, Iceland. During a negative phase (left), surface pressure in the Azores is much greater than in Iceland (data from May 1992 shown). During a positive phase, that difference is much weaker (data from July 1993 shown), resulting in different circulation patterns (Schematic adapted from AIRMAP by Ned Gardiner and David Herring, NOAA.).

Historical snowstorms affirm the link. In Baltimore and Washington, D.C., thirteen of the fifteen heaviest snowstorms since 1891 occurred when the NAO index was negative. And case studies of infamous Northeast U.S. storms over the last century, summarized in a Monograph of the American Meteorological Society, have discovered a link to blocked NAO conditions.

The CSI team took its analysis a step further, mapping out historical climate conditions associated with the ten snowiest Decembers, Januaries, and Februaries since 1891. The 30-month composite map of jet stream level and surface conditions revealed a textbook picture of a blocked NAO pattern over the Atlantic Ocean.

Evidence: El Niño
But the negative NAO didn’t act alone. By fall of 2009, a NOAA network of ocean buoys in the tropical Pacific Ocean picked up a moderate El Niño, which strengthened a bit by winter. El Niños typically influence North American climate by displacing the track of wintertime storms across the Pacific Ocean southward, often delivering heavy precipitation into a belt from Southern California through Texas and into the Southeast. The weather pattern also generally cools the eastern seaboard, though not as significantly as a blocked NAO pattern can.

The top 10 heaviest snowstorms for Philadelphia, Baltimore, and Washington, D.C. Snowstorms occurring in years when there was a negative NAO and/or an El Niño are indicated. The shaded boxes highlight the storms from this past winter. (Table produced using data courtesy of Jeff Master, Weather Underground, and Klaus Wolter, NOAA ESRL.)

Historically, El Niños are associated with more winter snowfall along the East Coast. And many of the biggest snowstorms in mid-Atlantic cities occurred during El Niño years. Of the top ten storms in Washington, Philadelphia, and Baltimore, more than half have coincided with El Niño events. Yet not all El Niños yield heavy mid-Atlantic snowstorms. Notably, there was no statistically significant increase in snowfall during 1997-98, when one of the strongest El Niños of the century occurred.