Dartmouth researchers learn that North America's wind patterns have shifted
significantly in the past 30,000 years
Dartmouth researchers have learned that the prevailing winds in the
mid-latitudes of North America, which now blow from the west, once blew from
the east. They reached this conclusion by analyzing 14,000- to 30,000-year-old
wood samples from areas in the mid-latitudes of North America (40-50¡N), which
represent the region north of Denver, Co., and Philadelphia, Penn., and south
of Winnipeg and Vancouver, Canada.
A team of Dartmouth researchers studied ancient wood samples to reconstruct
past changes in continental wind patterns. From left: Yong Shu, earth sciences
Ph.D. student; Eric Posmentier, adjunct professor of earth sciences; Xiahong
Feng, professor of earth sciences and Frederick Hall Professor in Mineralogy
and Geology; and Anthony Faiia, research associate. (Photo by Joseph Mehling
'69)
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The researchers reported their findings online on Jan. 23 in the journal
Geology, published by the Geological Society of
America.
"Today in the mid-latitude zone of North America, marine moisture is
transported either from the West Coast by westerly winds, or from both the West
and East Coasts by storms," says Xiahong Feng,
the paper's lead author and a professor of earth sciences. "In this study,
we found evidence that during the last glacial period, about 14,000 to 36,000
years ago, the prevailing wind in this zone was easterly, and marine moisture
came predominantly from the East Coast."
Feng explains that global climate change is often manifested by changes in
general atmospheric circulation, i.e. winds, and this results in changing
temperature and precipitation patterns. Clues of past climates usually hint at
temperature and precipitation changes, but this is the first time that changing
continental wind patterns have been reconstructed.
The researchers gathered their evidence using oxygen and hydrogen isotopic
compositions of cellulose extracted from ancient wood. Feng and her team
interpret the historic prevailing easterlies to be a result of a growing and
intensifying northern circumpolar vortex, which was influenced by the powerful
Laurentide Ice Sheet, an enormous mass of ice that covered a great deal of
northern North America. Under this circulation regime, the jet stream shifted
southward, and as a result, the Pacific Northwest received much less marine
moisture from the Pacific. This is consistent with earlier studies of
vegetation in the Pacific Northwest, indicating that the region was
significantly drier during the last glaciation.
"This study is likely to open up new avenues of research based on
oxygen and hydrogen isotopes in old wood," says Feng. "Climate change
involves interactions among temperature, precipitation, and wind, but until now
research has rarely been able to observe or confirm prehistoric winds and their
continental-scale patterns. In the future, studies using this methodology will
be able to look into ancient climates through a new window, and test hypotheses
about climate change mechanisms. Such studies can potentially lead to more
realistic formulations of future climate scenarios and better evaluations of
their plausibility."
In addition to Xiahong Feng, who also holds the Frederick Hall Professorship
in Mineralogy and Geology, other authors on the paper include: Allison L.
Reddington '04; Anthony M. Faiia, research associate; Eric S.
Posmentier, adjunct professor of earth sciences; Yong Shu, earth
sciences Ph.D. student; and Xiaomei Xu of the University of California,
Irvine.
"This study began as Allison Reddington's undergraduate honors
thesis," says Feng. "It exemplifies the extraordinary opportunities
that undergraduates at Dartmouth have to become integral parts of research
groups."
By SUSAN KNAPP
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