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Monday, May 16, 2011

"A 2,300-year-long annually resolved record of the South American summer monsoon from the Peruvian Andes" by Broxton W. Bird et al., PNAS (May 9, 2011)

PNAS May 9, 2011

  1. Broxton W. Birda,1,*
  2. Mark B. Abbotta
  3. Mathias Vuilleb
  4. Donald T. Rodbellc
  5. Nathan D. Stansella,1 and 
  6. Michael F. Rosenmeiera
+Author Affiliations
  1. aDepartment of Geology and Planetary Science, University of Pittsburgh, 4107 O’Hara Street, Pittsburgh, PA 15260;
  2. bDepartment of Atmospheric and Environmental Sciences, University at Albany, State University of New York, 1400 Washington Avenue, Albany, NY 12222; and
  3. cGeology Department, Union College, 807 Union Street, Schenectady, NY 12308
  1. Edited by Paul A. Baker, Duke University, Durham, NC, and accepted by the Editorial Board April 12, 2011 (received for review March 20, 2010).


Decadal and centennial mean state changes in South American summer monsoon (SASM) precipitation during the last 2,300 years are detailed using an annually resolved authigenic calcite record of precipitation δ18O from a varved lake in the Central Peruvian Andes. This unique sediment record shows that δ18O peaked during the Medieval Climate Anomaly (MCA) from A.D. 900 to 1100, providing evidence that the SASM weakened considerably during this period. Minimum δ18O values occurred during the Little Ice Age (LIA) between A.D. 1400 and 1820, reflecting a prolonged intensification of the SASM that was regionally synchronous. After the LIA, δ18O increased rapidly, particularly during the current warm period (CWP; A.D. 1900 to present), indicating a return to reduced SASM precipitation that was more abrupt and sustained than the onset of the MCA. Diminished SASM precipitation during the MCA and CWP tracks reconstructed Northern Hemisphere and North Atlantic warming and a northward displacement of the Intertropical Convergence Zone (ITCZ) over the Atlantic, and likely the Pacific. Intensified SASM precipitation during the LIA follows reconstructed Northern Hemisphere and North Atlantic cooling, El Niño-like warming in the Pacific, and a southward displacement of the ITCZ over both oceans. These results suggest that SASM mean state changes are sensitive to ITCZ variability as mediated by Western Hemisphere tropical sea surface temperatures, particularly in the Atlantic. Continued Northern Hemisphere and North Atlantic warming may therefore help perpetuate the recent reductions in SASM precipitation that characterize the last 100 years, which would negatively impact Andean water resources.
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