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David W. Stahle
,
Edward R. Cook
,
Dorian J. Burnette
,
Max C. A. Torbenson
,
Ian M. Howard
,
Daniel Griffin
,
Jose Villanueva Diaz
,
Benjamin I. Cook
,
A. Park Williams
,
Emma Watson
,
David J. Sauchyn
,
Neil Pederson
,
Connie A. Woodhouse
,
Gregory T. Pederson
,
David Meko
,
Bethany Coulthard
, and
Christopher J. Crawford

Abstract

Cool- and warm-season precipitation totals have been reconstructed on a gridded basis for North America using 439 tree-ring chronologies correlated with December–April totals and 547 different chronologies correlated with May–July totals. These discrete seasonal chronologies are not significantly correlated with the alternate season; the December–April reconstructions are skillful over most of the southern and western United States and north-central Mexico, and the May–July estimates have skill over most of the United States, southwestern Canada, and northeastern Mexico. Both the strong continent-wide El Niño–Southern Oscillation (ENSO) signal embedded in the cool-season reconstructions and the Arctic Oscillation signal registered by the warm-season estimates faithfully reproduce the sign, intensity, and spatial patterns of these ocean–atmospheric influences on North American precipitation as recorded with instrumental data. The reconstructions are included in the North American Seasonal Precipitation Atlas (NASPA) and provide insight into decadal droughts and pluvials. They indicate that the sixteenth-century megadrought, the most severe and sustained North American drought of the past 500 years, was the combined result of three distinct seasonal droughts, each bearing unique spatial patterns potentially associated with seasonal forcing from ENSO, the Arctic Oscillation, and the Atlantic multidecadal oscillation. Significant 200–500-yr-long trends toward increased precipitation have been detected in the cool- and warm-season reconstructions for eastern North America. These seasonal precipitation changes appear to be part of the positive moisture trend measured in other paleoclimate proxies for the eastern area that began as a result of natural forcing before the industrial revolution and may have recently been enhanced by anthropogenic climate change.

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Thomas C. Peterson
,
Richard R. Heim Jr.
,
Robert Hirsch
,
Dale P. Kaiser
,
Harold Brooks
,
Noah S. Diffenbaugh
,
Randall M. Dole
,
Jason P. Giovannettone
,
Kristen Guirguis
,
Thomas R. Karl
,
Richard W. Katz
,
Kenneth Kunkel
,
Dennis Lettenmaier
,
Gregory J. McCabe
,
Christopher J. Paciorek
,
Karen R. Ryberg
,
Siegfried Schubert
,
Viviane B. S. Silva
,
Brooke C. Stewart
,
Aldo V. Vecchia
,
Gabriele Villarini
,
Russell S. Vose
,
John Walsh
,
Michael Wehner
,
David Wolock
,
Klaus Wolter
,
Connie A. Woodhouse
, and
Donald Wuebbles

Weather and climate extremes have been varying and changing on many different time scales. In recent decades, heat waves have generally become more frequent across the United States, while cold waves have been decreasing. While this is in keeping with expectations in a warming climate, it turns out that decadal variations in the number of U.S. heat and cold waves do not correlate well with the observed U.S. warming during the last century. Annual peak flow data reveal that river flooding trends on the century scale do not show uniform changes across the country. While flood magnitudes in the Southwest have been decreasing, flood magnitudes in the Northeast and north-central United States have been increasing. Confounding the analysis of trends in river flooding is multiyear and even multidecadal variability likely caused by both large-scale atmospheric circulation changes and basin-scale “memory” in the form of soil moisture. Droughts also have long-term trends as well as multiyear and decadal variability. Instrumental data indicate that the Dust Bowl of the 1930s and the drought in the 1950s were the most significant twentieth-century droughts in the United States, while tree ring data indicate that the megadroughts over the twelfth century exceeded anything in the twentieth century in both spatial extent and duration. The state of knowledge of the factors that cause heat waves, cold waves, floods, and drought to change is fairly good with heat waves being the best understood.

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