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. Originally posted on ScienceDaily 2004-12-27
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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.
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.
The National Aeronautics and Space Administration and the National Science
Foundation sponsored this research.
