The CSI team focused on two suspects known to be at large this winter — the negative phase of the North Atlantic Oscillation (NAO) and El Niño. El Niño, with its warming of tropical Pacific Ocean temperatures, may be best known for delivering heavy rains across the southern United States. El Niño events can trigger mudslides in California, floods along the Gulf Coast, and unusual warmth and drought in the Pacific Northwest. The latter should sound familiar: an unusually warm winter from Portland to Seattle was part of the same climate pattern affecting the venue of the Winter Olympics. The CSI Team suspected that El Niño was a conspirator in the United States’ unusual winter weather, and that it had an accomplice.

False-color map showing El Niño pattern of sea-surface height anomalies in the equatorial Pacific Ocean on February 15, 2010. Higher areas, shown in red, are warmer than average, and lower areas, shown in blue, are cooler than average. White areas show average heights and temperatures. Image courtesy of NASA Earth Observatory.

The North Atlantic Oscillation is a fluctuating air-pressure pattern that alternatively enhances or blocks the storm-steering jet stream over North America. So the NAO is particularly relevant in understanding eastern U.S. wintertime climate variations. The NAO describes the contrast in surface air pressure between Iceland and the Azores as well as the vigor of the jet stream that normally flows between them.

The winter of 2009-10 witnessed the most extreme negative (blocked) NAO phase since at least 1950. (Graph courtesy of Marty Hoerling, NOAA Earth System Research Lab.)

This winter the NAO was in its negative phase and the jet stream flowed further south than usual, pushed toward the Azores by a massive “block” of high surface pressure over Greenland. It’s an unusual atmospheric circulation pattern, but one that has been implicated before. For example, remarkably cold winters persisted over Europe and Russia in the early 1940s, helping to turn the tide of World War II. The NAO, in a blocked phase, was one conspirator in those cold events. Likewise, the CSI Team suspected the pattern was a co-conspirator in the extreme winter weather conditions this year in the mid-Atlantic region. But could they find the evidence they would need to finger El Niño and NAO?

[Editor's note: There is ongoing discussion among scientists as to which of the climate patterns is a more meaningful description of real-world conditions — the Arctic Oscillation or the North Atlantic Oscillation? While these phenomena are measured using different indexes, their values are so highly correlated that some scientists use the terms interchangeably, referring to them together simply as “AO/NAO.” Both AO and NAO were at record lows this winter. (For more information about the Arctic Oscillation this winter, see Can Record Snowstorms and Global Warming Coexist?)]