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NOAA image of fossils of the plankton G. bulloides.January 23, 2003 — 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 NOAA scientist. 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. (Click NOAA image for larger view of fossils of the plankton G. bulloides. Please credit “NOAA.”)

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 the NOAA 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.

NOAA image of fossils of the plankton G. bulloides.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. (Click NOAA image for larger view of fossils of the plankton G. bulloides. Please credit “NOAA.”)

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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