When scientists run computer models to simulate climate events, they often add elements, such as effect of volcanic eruptions, adding ice sheets, or altering the concentrations of greenhouse gases in the model's atmosphere. One NOAA scientist and his colleague discovered a large, abrupt climate event without the additions.

"When I first saw the results, I thought that I had bad data," said Ronald Stouffer, a meteorologist at NOAA's Geophysical Fluid Dynamics Laboratory in Princeton, N.J. "I would never would have guessed the ‘bad data' was a very interesting event."

The "interesting event" is a severe and abrupt cooling of the North Atlantic Ocean near Greenland. The event will be described by Stouffer and Alex Hall of the Lamont-Doherty Earth Observatory in Palisades, N.Y. in the Jan. 11 issue of the science journal Nature.

Stouffer and Hall found that an unusually long-lived atmospheric wind deviation leads to the northern North Atlantic Ocean and surrounding regions becoming very cold for 30- to 40- years.

"We have a 15,000-year simulation," Stouffer said. "About the model year 3,100, there is a dramatic drop in the surface air temperature. It just happened spontaneously, which is what makes the event so interesting."

Stouffer said that those researchers trying to detect human-influenced signals in climate change will be interested to know that the climate may generate more variability on its own than previously thought. He added that climate modelers will be interested to know that their models can produce large, abrupt climate events, without forcing changes. And, he noted that researchers studying paleoclimatology – the climates of the past - may have a new explanation for some of the oscillations seen in ice core records. Some of these oscillations have no widely accepted explanation. The cooling seen in the ice core record 8,200 years ago is one example and resembles the event in the model.

Stouffer and Hall's work traces the cooling to an abnormally intense East Greenland current caused by the wind deviation. As it flows southward along Greenland's east coast, it transports relatively cold, fresh Arctic water into the North Atlantic. The fresh Arctic water effectively shuts down the oceanic convection, which normally warms surface waters in the region.

"Since the ocean surface is no longer being warmed by the deeper waters, it cools. As the waters cool, the overlaying atmosphere also gets colder," Stouffer said.

Stouffer and Hall note in their paper that the probability of such an event happening in the near future seems low – only one such event was found in the 15,000-year integration of the model. However, they note that the probability may be increasing.

"First, nearly all coupled ocean-atmosphere models show a decrease in the high latitude surface salinity when greenhouse gases increase as has been the case for some time. The decreased salinity may make the region more susceptible to a large event," Stouffer said. "Second, consistent with the observed 20- to 30- year increase in the real climate's North Atlantic Oscillation index is a strengthening of the northwesterly winds blowing across the East Greenland current. If this wind anomaly continues, it could set the stage for a dramatic climate event in the North Atlantic."