Greenland ice cores offer glimpse of weather system history

12/13/04



The image above shows the locations of the eight sites on the Greenland Ice Sheet where cores were drilled to retrieve climate records which researchers hoped would help trace the history of the North Atlantic Oscillation. The image on the left shows the extent of coverage over the ice sheet. Courtesy Ellen Mosley Thompson.

Full size image available here

COLUMBUS, Ohio The recent analyses of eight ice cores drilled from the massive Greenland Ice Sheet may paint a map researchers can use to uncover the history of a massive weather machine controlling the climate around the North Atlantic basin.

The boundary between two major pressure systems the Icelandic Low and the Azores High -- controls whether storms reaching Europe are strong or weak, and whether the seasons are wetter or dryer.

This phenomenon called the North Atlantic Oscillation, or NAO, was only fully recognized a few decades ago. But available meteorological records can only trace its behavior back into the mid-1800s. That period is too short for scientists to really determine if variations they've seen this century might be linked to some larger, global climate change.

What scientists need to address that question is a much longer record than is now available. Those eight cores may provide the key, said Ellen Mosley Thompson, a professor of geography at Ohio State University and researcher with the Byrd Polar Research Center. She reported her findings today at the meeting of the American Geophysical Union in San Francisco.



The image above shows the locations of the eight sites on the Greenland Ice Sheet where cores were drilled to retrieve climate records which researchers hoped would help trace the history of the North Atlantic Oscillation. The image on the right, compiled from satellite imagery shows the summit ridge bisecting the ice sheet, as well as additional ridges. Courtesy Ellen Mosley Thompson.

Full size image available here

When the low-pressure region around Iceland and the high-pressure region off the coast of Portugal are both strong, the increased pressure difference between them funnels more strong winter storms into northern Europe bringing warmer and wetter winters. Winters in the eastern U.S. are also warmer and wetter. When both pressure systems are weak, northern Europe tends to be cold and the eastern U.S. experiences more cold air outbreaks and more snowfall.

In the first scenario, the NAO is considered "positive" and Greenland is drier and fairly isolated from its effects. A "negative" NAO allows more storms to reach Greenland, bringing additional precipitation and warmer temperatures.

And those patterns of more or less moisture lie trapped in Greenland's ice sheet.

"The big question in climatology," Thompson says, "is that everything we know with confidence is in historic records dating back only to the mid-1800s. We have very little data before that." She hopes the Greenland ice cores could lengthen that record but so far, earlier analyses haven't shown a close tie between the ice record and historic data.

However, earlier studies were limited to a few cores, mainly from the summit of the ice sheet, and still other cores were analyzed for only a single climate indicator. Most current core analyses use several, redundant parameters to insure their accuracy.

Thompson and her team turned to eight ice cores, six drilled as part of a NASA-sponsored project called PARCA during the late 1990s. These drill sites were spread across the massive ice sheet, the second-largest ice body in the world. The researchers analyzed the cores looking for annual layers based on dust, nitrates, oxygen isotope ratios and hydrogen peroxide levels.

"We are focusing on the top part of the cores that might match the historical record," Thompson said. "We must 'calibrate' that with what we already know to prove that the cores contain a valid record of the NAO."

If that part of the core matched the historic record, then evidence drawn from deeper in the ice could provide a picture of NAO behavior centuries earlier.

"We found that how well the historic record agreed with the ice core depended on where it was drilled, and what part of the 1865-to-1994 period we were considering," she explained. But one site, dubbed NASA-U, in west central Greenland most closely matches historic records.

"That's the 'sweet spot," she said. "If you could drill just one core to reconstruct the NAO, then that's the place to do it." The study also showed:

Cores retrieved from the northwest quadrant of Greenland provided a better matching record before 1925, after which the Arctic warmed abruptly;

After the 1925 warming, the cores from southeastern Greenland more closely matched the strength of the NAO;

Cores drilled from the central summit of the Greenland ice sheet provided a poor match to the historical NAO record.

Thompson's group has begun to examine whether another weather system the Pacific Decadal Oscillation, or PDO might have left evidence in the Greenland cores. Researchers have assumed that the PDO only influences climate over North America and have not considered that its effects might be felt further away over Greenland.

"I think we have a reasonable indication that the PDO leaves some imprint in Greenland and that imprint appears in both the temperature and precipitation records," she said. The strongest evidence of the PDO showed up in cores drilled from the southern part of the ice cap.

Source: Eurekalert & others

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