NORTH ATLANTIC - ASIAN MONSOONS INFLUENCE EACH OTHER'S PATTERNS,
NOAA REPORTS
Like ballroom dancers on a crowded floor, aspects of the climate like El Niño,
the Asian monsoons, and the North Atlantic influence each other's
patterns, according to David M. Anderson, a scientist with the Commerce
Department's National Oceanic and Atmospheric Administration (NOAA).
Anderson, along with Anil K. Gupta from the Indian Institute of Technology in
Kharagpur and Jonathan Overpeck, a professor of geosciences and director of the
University of Arizona's Institute for the Study of Planet Earth, used sediments from the
floor of the Arabian sea near Oman to reconstruct monsoon strength in the region for
the past 10,000 years. Their work appears in the January 23 issue of Nature.
The authors suggest that the link between the North Atlantic climate and the
Asian monsoon is a persistent aspect of global climate. The link was demonstrated
previously by various researchers, but the new research examines a much longer time
period - the past 10,000 years. The new study reveals substantial natural variation in
climate and the monsoon in a time prior to any significant human influence. The new
information may lead to improved predictions of the monsoon in the coming decades.
"The significance of these results lies in demonstrating a pattern of persistent
variability in the monsoon throughout the Holocene (past 10,000 years to the present)
that may be linked with episodic warming and cooling of the North Atlantic," said
Anderson, who is with NOAA's Paleoclimatology program in Boulder, Colo.
"The results highlight the need to improve our understanding of abrupt -- and
difficult to predict -- weakening in monsoon strength that could accompany abrupt
climate shifts in the North Atlantic that may occur in the future," Anderson said.
The researchers used fossils of the plankton G. bulloides to estimate wind
intensity. During a monsoon, the seasonal reversal of winds brings moisture from the
ocean onto land. The winds also blow surface waters off shore, Overpeck said, causing
an upwelling of colder, nutrient-rich water where the microscopic marine animals can
thrive.
By counting the amount of G. bulloides present in different layers of the sediment
and using radiocarbon dating, the scientists were able to approximate monsoon
strength from 10,500 years ago on up to the present. The resulting record showed a
natural variation in the monsoon from one century to the next.
"We have new evidence that the strength of Asian monsoon varies substantially
on century to millennial time scales," the authors say. "We need to understand this if
we're going to ensure human and ecological sustainability in Tibet, China, India and the
rest of Southeast Asia."
"While researchers aren't sure of the exact causes of the link between the North
Atlantic and the Asian monsoon, earlier research showed the amount of snow on the
Tibetan plateau may play a critical role," Overpeck said. As the land warms in the
spring, the air rises above the land causing a pressure gradient that drives the
monsoon.
"More snow on the plateau in spring or early summer uses up all the sun's
heating because it has to be melted and evaporated before the land can warm,"
Overpeck said. "So the more snow you have in winter, the weaker the monsoon the
following summer."
The authors speculate that when the North Atlantic is cold, areas downwind like
the Tibetan plateau stay cold longer, allowing more snow to persist and setting up a
weakened monsoon.
"The monsoon snow-cover link may lead to a stronger or more variable monsoon
in the coming century as the northern hemisphere continues to warm faster than the
tropics," said Anderson.
Other studies show that changes in the amount of sunlight correlate to variations
in both the North Atlantic climate and the Asian monsoon. The researchers aren't
certain if the sun affects each system directly or if solar radiation influences the North
Atlantic circulation, which in turn impacts the monsoon.
"More research is needed to identify the role of solar variability, and the remote
influence of the north Atlantic climate," Anderson said.
In an earlier study, the authors found evidence from sediments in the same
region showing an increase in monsoon strength in the past 400 years. Their work was
published in the July 26, 2002 issue of Science. The new Nature paper is titled "Abrupt
changes in the Asian southwest monsoon during the Holocene and their links to the
North Atlantic Ocean."
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