The heavy snowstorms in the northern hemisphere this winter have been blamed on global warming. Al Gore says so; the New York Times says so; ABC news says so; the Washington Post says so; even Science Daily News said so.

But, the CSI team (climate scene investigators) at the National Oceanic and Atmospheric Administration (NOAA) says not so.

NOAA says:

They found no evidence — no human ‘fingerprints’ — to implicate our involvement in the snowstorms. If global warming was the culprit, the team would have expected to find a gradual increase in heavy snowstorms in the mid-Atlantic region as temperatures rose during the past century. But historical analysis revealed no such increase in snowfall. Nor did the CSI team find any indication of an upward trend in winter precipitation along the eastern seaboard.”

The CSI team turned its attention to natural factors that control the ordinary ups and downs of weather. Many extreme weather events are due to cyclical, large-scale anomalies in air pressure and sea surface temperature across large tracts of ocean. Such fluctuations spawn weather systems that can cause droughts, floods, and massive snowstorms. While El Niño is the most famous, scientists have identified other climate anomalies throughout Earth’s climate system as well. Their names may seem unimpressive — the Arctic Oscillation, North Atlantic Oscillation, Pacific Decadal Oscillation, to name a few — but they can pack quite a punch!

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.

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 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?

See here for the rest of NOAAs story.

See also the Rutgers University global snow lab. There you can click on a series of graphs showing snowfall data for the northern hemisphere since 1967. You will see that snowfall in 1978 exceeded the current winter so far.

This entry was posted on Tuesday, February 22nd, 2011 at 9:14 am and is filed under Climate change. Tags for this post: Climate change, CSI, El Nino, global warming, NAO, NOAA, PDO, winter snowstorms. You can follow any responses to this entry through the RSS 2.0 feed. Both comments and pings are currently closed.