The Tree-Ring Record of Monsoon Climate in the American Southwest
April 24th, 2013
Bio Sci East, Room 225
Climate in the American Southwest is characterized by two distinct seasonal precipitation regimes. Cool-season frontal storms are the primary contributor to surface water supplies. The summer monsoon provides critical moisture, modulating social and environmental systems across the region. The North American monsoon is a focus for modern and paleoclimate research, but relatively little is known about interannual monsoon variability and its relationship to cool-seasonal climate in the pre-instrumental era.
This presentation describes results from the first systematic tree-ring study of the summer monsoon in the southwestern U.S. Specimens from 53 sites were analyzed for width variations of “earlywood” and “latewood,” the light- and dark-colored sub-annual components of conifer tree rings that form in spring and summer, respectively. This network of sub-annual chronologies reflects temporal variability and spatial patterns evident in instrumental records of season-specific precipitation from the region. A 470-year reconstruction of monsoon precipitation was developed for a large area in the southwestern U.S. and northwestern Mexico (area 2 in figure). Comparison with an independent reconstruction of cool-season precipitation indicates that decadal droughts of the last five centuries were characterized not just by cool-season precipitation deficits, but also by concurrent failure of the summer monsoon. Reconstructed monsoon drought events were more severe and persistent than any of the instrumental era. Results indicate that the inverse relationship between winter and summer precipitation, identified in several modern climate studies, was stronger during the mid- to late-20th century than any other period in the last 470 years.
Daniel Griffin is doctoral candidate in the School of Geography at the University of Arizona in Tucson. He is an EPA STAR Graduate Research Fellow at the Laboratory of Tree-Ring Research and is affiliated with the Climate Assessment for the Southwest, a project designed to provide climate information to regional stakeholders.